THE NECK, THORAX, AND THORACIC LIMB

Superficial Vessels and Nerves of the Thoracic Wall

Before dissecting the thoracic nerves and vessels, study Figs. 3-4 through 3-7, which show the pattern of distribution of these structures. Notice that the artery and nerve of each intercostal space divide into dorsal and ventral branches. The dorsal branches enter the epaxial muscles. The ventral branches descend in the intercostal spaces along the caudal border of each rib. The dorsal and ventral arterial branches are derived from the dorsal intercostal arteries (Figs. 3-4, 3-5). The first three dorsal intercostal arteries come from a branch of the costocervical trunk; the remaining nine come from the aorta. The dorsal intercostal arteries and veins have lateral cutaneous branches that perforate the intercostal and adjacent muscles to supply cutaneous structures, including the thoracic mammary glands. The dorsal intercostal artery and vein pass ventrally, where they anastomose with ventral intercostal branches from the internal thoracic artery and vein. At the ventral aspect of each intercostal space, perforating branches of the internal thoracic vessels emerge and supply cutaneous structures and the thoracic mammary glands. The dorsal and ventral nerve branches are derived from the spinal nerve as it emerges from the intervertebral foramen (Fig. 3-6). The ventral branches of the first 12 thoracic spinal nerves are intercostal nerves and have lateral and ventral cutaneous branches and branches medial to these that go largely to muscles.

Dorsal and lateral rows of lateral cutaneous branches of intercostal nerves, arteries, and veins emerge at regular intervals between the ribs and supply the cutaneous muscle, subcutaneous tissue, and skin (Fig. 3-6). The nerves of the dorsal row arise from the dorsal branches of the thoracic spinal nerves. A row of ventral cutaneous branches emerge through the origin of the deep pectoral muscle after having penetrated the ventral ends of the intercostal spaces. These vessels are perforating branches of the internal thoracic artery and vein. The nerves are terminal branches of the intercostal nerves. Although these emerge at regular intervals, not all will be seen in the dissection.

The cranial thoracic mamma is supplied by the fourth, fifth, and sixth ventral and lateral cutaneous vessels and nerves and by branches of the lateral thoracic vessels. The latter are from the axillary vessels, which will be dissected later.

The caudal thoracic mamma is supplied in a similar manner from the sixth and seventh cutaneous nerves and vessels. In addition, mammary branches of the cranial superficial epigastric vessels supply this mamma.

The axilla is the space between the thoracic limb and the thoracic wall. It is bounded ventrally by the pectoral muscles and dorsally by the attachment of the serratus ventralis to the scapula. Cranially, it extends under the muscles that extend from the arm to the neck. Caudally, a similar extension is found under the latissimus dorsi and cutaneus trunci.

The lateral thoracic artery, vein, and nerve emerge from the axilla between the latissimus dorsi and deep pectoral muscles. The nerve is motor to the cutaneus trunci and may be found on its ventral border. It consists of fibers from the ventral branches of the eighth cervical and first thoracic spinal nerves. The vessels are branches of the axillary artery and vein that supply the muscle, skin, and subcutaneous tissues, including the cranial thoracic mamma. If these vessels and this nerve are not readily identified in your dissection, you may find them later when the axillary vessels and the brachial plexus are dissected.

Transect the pectoral muscles close to the sternum. Reflect them toward the forelimb to expose the axilla.

The axillary lymph node lies dorsal to the deep pectoral muscle and caudal to the large axillary vein coming from the arm. Most of the afferent lymph vessels of the thoracic wall and deep structures of the limb drain into this node.

LIVE DOG

Palpate the structures in the neck ventral to the cervical vertebrae. The larynx and trachea are readily palpable. The esophagus is usually too soft to feel but should be on the left of the trachea in the middle and caudal cervical region. Try to palpate a pulse in the carotid artery. It usually courses along the dorsolateral side of the trachea but is too deep to allow a pulse to be felt regularly. Cranially, feel the firm oval mandibular salivary gland and the smaller, looser mandibular lymph nodes. The latter can be felt subcutaneously at the angle of the mandible. Caudally in the neck, feel the superficial cervical lymph nodes cranial to the shoulder and deep to the omotransversarius or cleidocephalicus muscle. Extend the neck and compress the vessels that enter the thorax at the thoracic inlet to try to distend the external jugular vein so that it is visible. This is more difficult to observe in long-haired breeds without removing the hair.

Deep Vessels and Nerves of the Thoracic Wall

Expose the lumbar and costal origins of the external abdominal oblique and detach them. Reflect the muscle ventrally to the rectus abdominis. Reflect the mammae if necessary. Free the aponeurotic thoracic attachment of the rectus abdominis close to the sternum and first costal cartilage. Reflect the rectus abdominis caudally, noting and cutting any nerves or vessels that enter the deep face of the muscle from any of the intercostal spaces.

The cranial epigastric artery is a terminal branch of the internal thoracic artery that emerges from the thorax in the angle between the costal arch and the sternum. It passes caudally on the deep surface of the rectus abdominis. The cranial epigastric artery gives rise to the cranial superficial epigastric (Figs. 3-7, 4-2, 4-32), which perforates the muscle and runs caudally on its external surface. This artery supplies the skin over the rectus abdominis and the caudal thoracic and cranial abdominal mammae. The cranial epigastric vessels continue on the deep surface of the rectus abdominis. Most of their branches terminate in this muscle.

Make a sagittal incision completely through the thoracic wall 1 cm from the ventral median plane on each side. These incisions should extend from the thoracic inlet through the ninth costal cartilage. The transversus thoracis muscle is a flat, fleshy muscle on the medial surface of the costal cartilages of ribs 2 though 8 (Figs. 3-4, 3-6). Its fascicles extend from the costochondral junctions to the sternum. Connect the caudal ends of the right and left sagittal incisions and free the sternum, except for the wide, thin fold of mediastinum that is now its only attachment.

On the right half of the thorax, clean and transect the origin of the latissimus dorsi and reflect it toward the forelimb. Locate and transect the caudal portion of the origin of the serratus ventralis, exposing the ribs. Starting at the costal arch and using bone cutters, cut only the ribs, close to their vertebral articulation within the thorax, without damaging the sympathetic trunk. Reflect the thoracic wall without removing it. As this is done, cut the attachments of the internal abdominal oblique, transversus abdominis, and diaphragm from the ribs along the costal arch. If this is done carefully, the peritoneal cavity will not be opened. Reflect the left thoracic wall in a similar manner.

On the internal surface of the thoracic wall, notice the intercostal vessels and nerves coursing along the caudal border of the ribs. Ventrally, the vessels bifurcate and anastomose with the ventral intercostal branches of the internal thoracic artery and vein. The intercostal nerves supply the intercostal musculature. Their sensory branches were seen as lateral and ventral cutaneous branches.

The pleurae (Figs. 3-8, 3-9) are serous membranes that cover the lungs and line the walls of the thorax. These form right and left sacs that enclose the pleural cavities. Each consists of visceral and parietal parts, depending on their location.

The pulmonary or visceral pleura closely attaches to the surfaces of the lungs, following all their small irregularities as well as the fissures that separate the two lobes.

The parietal pleura is attached to the thoracic wall by the endothoracic fascia. This pleura may be divided into costal, diaphragmatic, and mediastinal parts. Each of these is named after the region or surface it covers, and all are continuous, one with another. The costal pleura covers the inner surfaces of the ribs and their associated intercostal and transversus thoracis muscles. The diaphragmatic pleura covers the cranial surface of the diaphragm. The mediastinal pleurae are the layers that cover the sides of the partition between the two pleural cavities. The mediastinum includes the two mediastinal pleurae and the space between them. Enclosed in the mediastinum are the thymus, the lymph nodes, the heart, the aorta, the trachea, the esophagus, the vagus nerves, and other nerves and vessels. The pericardial mediastinal pleura is that portion covering the heart.

The mediastinum can be divided into a cranial part, that lying cranial to the heart; a middle part, that containing the heart; a dorsal portion dorsal to the heart; a ventral portion, ventral to the heart; and a caudal part, lying caudal to the heart. The caudal mediastinum is thin. It attaches to the diaphragm far to the left of the median plane. Cranially, it is continuous with the middle mediastinum.

Note the passage of the esophagus through the mediastinum and the esophageal hiatus of the diaphragm. At the esophageal hiatus, a thin layer of pleura, peritoneum, and enclosed connective tissue attaches the esophagus to the muscle of the diaphragm.

The plica venae cavae is a loose fold of pleura derived from the right caudal mediastinal portion of the pleural sac that surrounds the caudal vena cava. The root of the lung is composed of pleura and the bronchi, vessels, and nerves entering the lung. Here the mediastinal parietal pleura is continuous with the pulmonary pleura. Caudal to the hilus this connection forms a free border, known as the pulmonary ligament (Figs. 3-9, 3-10), between the caudal lobe of the lung and the mediastinum at the level of the esophagus. Observe this ligament. In thoracic surgery this must be cut to reflect the caudal lung lobe cranially.

The thymus (Figs. 3-8, 3-11, 3-12, 3-14, 3-16, 3-20) is a bilobed, compressed structure situated in the cranial mediastinum. It is largest in the young dog and usually atrophies with age until only a trace remains. When maximally developed, the caudal part of the thymus is molded on the cranial surface of the pericardium.

The internal thoracic artery (Figs. 3-14, 3-16 through 3-20) leaves the subclavian artery, courses ventrocaudally in the cranial mediastinum, and disappears deep to the cranial border of the transversus thoracis muscle. It supplies many branches to surrounding structures—the phrenic nerve, the thymus, the mediastinal pleurae, and the dorsal intercostal spaces. The perforating branches to the superficial structures of the ventral third of the thorax have been seen. The anastomoses with the dorsal intercostal arteries on the medial side of the thoracic wall have been seen. Near the attachment of the costal arch with the sternum, the internal thoracic artery terminates in the musculophrenic artery and the larger cranial epigastric artery. The latter has been dissected along with its cranial superficial epigastric branch. The musculophrenic artery (Fig. 4-33) runs caudodorsally in the angle formed by the diaphragm and lateral thoracic wall. Dissect its origin. Cut the mediastinum near the sternum and reflect the sternum cranially.

Lungs

Each lung is divided into lobes based on the branching pattern of its principal bronchus into lobar bronchi (Fig. 3-13). The left lung (see Figs. 3-10, 3-11) is divided into cranial and caudal lobes by deep fissures. The cranial lobe is further divided into cranial and caudal parts. The right lung (Fig. 3-12) is divided into cranial, middle, caudal, and accessory lobes. A part of the accessory lobe can be seen from the left through the caudal mediastinum (Fig. 3-20) or from the right through the plica venae cavae, where it lies in the space between these two structures. Reflect the caudal lung lobes to observe this.

Examine the cardiac notch of the right lung at the fourth and fifth intercostal spaces. The apex of the notch is continuous with the fissure between cranial and middle lobes. A larger area of the ventral convexity of the heart is exposed on the right side. The right ventricle occupies this area of the heart and is accessible for cardiac puncture here.

Remove the lungs by transecting all structures that enter the hilus. On the right side, this will involve slipping the accessory lobe over the caudal vena cava. Make the transection far enough from the heart so that the vagal nerves crossing the heart are not severed but close enough so that the lobes are not removed individually.

Examine the structures that attach the lungs. The trachea bifurcates into left and right principal bronchi. The carina is the partition between them at their origin from the trachea. Each principal bronchus divides into lobar bronchi that supply the lobes of the lung. Find these on the lungs that were removed. They can be identified by the cartilage rings within their walls.

At the level of the carina, the right principal bronchus gives off the right cranial bronchus to the right cranial lobe of the lung, directly laterally. This is followed by the right middle bronchus that branches off ventrally. The principal bronchus terminates caudally as the accessory bronchus medially and right caudal bronchus laterally. On the left, the left principal bronchus gives off the left cranial bronchus laterally. This divides into cranial and caudal parts, and the left principal bronchus continues caudally as the left caudal bronchus. Within each lobe segmental bronchi branch off the lobar bronchus dorsally and ventrally except for the right middle lobe, where they are cranial and caudal (Fig. 3-13).

Notice that there is usually a single pulmonary vein from each lobe that drains directly into the left atrium of the heart. (The pulmonary veins contain red latex because the specimen was prepared by injecting the latex into the carotid artery. Moving in a retrograde direction, the latex in turn filled the aorta, left ventricle, left atrium, and pulmonary veins. Because latex does not cross capillary beds, there is usually no latex in the pulmonary arteries. Occasionally, the pressure of injection ruptures the interatrial or interventricular septum in the heart, flooding the right chambers with the latex and thus filling the pulmonary arteries as well as the veins.)

The pulmonary trunk supplies each lung with a pulmonary artery. At the root of the lung, the left pulmonary artery usually lies cranial to the left principal bronchus. The right pulmonary artery is ventral to the right principal bronchus. The artery and bronchus are at a more dorsal level than the veins. Using a scissors or scalpel, open a few of the major bronchi to observe the lumen.

Note the tracheobronchial lymph nodes located at the bifurcation of the trachea and also farther out on the bronchi.

Determine which structures form the various grooves and impressions by replacing the lungs in the thorax. Observe the long aortic impression of the left lung. The most marked impressions on the right lung are on the accessory lobe. This lobe is interposed between the caudal vena cava on one side and the esophagus on the other, and both leave impressions on it. Observe the vascular impressions on the cranial lobes of the lungs and the costal impressions on each lung.

Veins Cranial to the Heart

Carefully expose the larger veins cranial to the heart. Reflect the sternum to one side to facilitate this exposure.

The cranial vena cava (Figs. 3-15, 3-17, 3-20, 3-22, 3-35) drains into the right atrium after its formation by the union of the right and left brachiocephalic veins at the thoracic inlet. The brachiocephalic vein is formed on each side by the external jugular and subclavian veins. Usually the last branch entering the cranial vena cava is the right azygos vein (Fig. 3-14). Only the right azygos vein develops in the dog. The azygos vein may enter the right atrium directly. It is seen from the right in the mediastinal space winding ventrocranially around the root of the right lung. It originates dorsally in the abdomen and collects all of the dorsal intercostal veins on each side as far cranially as the third or fourth intercostal space.

The thoracic duct is the chief channel for the return of lymph from lymphatic capillaries and ducts to the venous system. It begins in the sublumbar region between the crura of the diaphragm as a cranial continuation of the cisterna chyli. The latter is a dilated structure that receives the lymph drainage from abdominal and pelvic viscera and the pelvic limbs. The thoracic duct runs cranially on the right dorsal border of the thoracic aorta and the ventral border of the azygos vein to the level of the sixth thoracic vertebra. (It may not be visible.) Here it crosses the ventral surface of the fifth thoracic vertebra and courses on the left side of the middle mediastinal pleura. It continues cranioventrally through the cranial mediastinum to the left brachiocephalic vein, where it usually terminates (Fig. 3-17). The thoracic duct also receives the lymph drainage from the left thoracic limb and the left tracheal trunk (from the left side of the head and neck). The lymph drainage from the right thoracic limb and the right tracheal trunk (from the right side of the head and neck) form a right lymphatic duct that enters the venous system in the vicinity of the right brachiocephalic vein. There are often multiple terminations of a complicated nature, which may include swellings or anastomoses. All lymphatic channels will be difficult to see unless they are congested with lymph or refluxed blood. They are frequently double.

Look for the thoracic duct. It is not always visible, but it may be identified by the reddish brown or straw color of its contents and the numerous random constrictions in its wall. The tracheal trunks may be found in each carotid sheath or parallel to the sheath and its contents.

Arteries Cranial to the Heart

The aorta (Figs. 3-8, 3-9, 3-14, 3-16, 3-17, 3-20) is the large, unpaired vessel that emerges from the left ventricle medial to the pulmonary trunk. As the ascending aorta, it extends cranially, covered by the pericardium; it makes a sharp bend dorsally and to the left as the aortic arch; it runs caudally as the descending aorta located ventral to the vertebrae. The part cranial to the diaphragm is the thoracic aorta, and the caudal part is the abdominal aorta. Cranial to the heart are several branches of the aorta. Reflect the veins that were dissected cranial to the heart to observe these arteries.

The right and left coronary arteries are branches of the ascending aorta that supply the heart muscle. They will be studied with the heart.

The brachiocephalic trunk (Figs. 3-14, 3-16 through 3-20), the first branch from the aortic arch, passes obliquely to the right across the ventral surface of the trachea. It gives rise to the left common carotid artery and terminates as the right common carotid artery and the right subclavian artery.

The left subclavian artery (Figs. 3-16 through 3-20) originates from the aortic arch beyond the level of the brachiocephalic trunk and passes obliquely to the left across the ventral surface of the esophagus.

The branches of the right and left subclavian arteries are similar; only the right subclavian artery will be described. For each artery described, there is a comparable vein with a similar area of distribution. The terminations of the veins are variable, and they will not be dissected. Remove them when necessary to expose the arteries. The right subclavian artery has four branches that arise medial to the first rib or intercostal space. They are the vertebral artery, the costocervical trunk, the superficial cervical artery, and the internal thoracic artery. Do not sever the nerves or arteries.

The vertebral artery (Figs. 3-14, 3-16 through 3-20) crosses the medial surface of the first rib and disappears dorsally between the longus colli and the scalenus muscles. It enters the transverse foramen of the sixth cervical vertebra and passes through the transverse foramina of the first six cervical vertebrae. It supplies both muscular branches to the cervical muscles and also spinal branches at each intervertebral foramen to the spinal cord and its coverings. At the level of the atlas, it terminates by entering the vertebral canal through the lateral vertebral foramen and contributes to the ventral spinal and basilar arteries. These will be seen later in the dissection of the nervous system.

The costocervical trunk (Figs. 3-14, 3-16 through 3-20) arises distal to the vertebral artery, crosses its lateral side, and extends dorsally as far as the vertebral end of the first rib. By its various branches it supplies the structures of the first, second, and third intercostal spaces; the muscles at the base of the neck; and the muscles dorsal to the first few thoracic vertebrae. These need not be dissected.

The superficial cervical artery (Figs. 3-14, 3-16 through 3-20, 3-26) arises from the subclavian opposite the origin of the internal thoracic artery, medial to the first rib. It emerges from the thoracic inlet to supply the base of the neck and the adjacent scapular region.

Branches of the Thoracic Aorta

The esophageal and bronchial arteries vary in number and origin. Usually the small bronchoesophageal artery (Figs. 3-14, 3-16) leaves the right fifth intercostal artery close to its origin and crosses the left face of the esophagus, which it supplies. It terminates shortly afterward in the bronchial arteries, which supply the lung.

There are eight to nine pairs of dorsal intercostal arteries that leave the aorta (Figs. 3-4, 3-5, 3-16, 3-19, 3-20). These start with either the fourth or the fifth intercostal artery and continue caudally, there being an artery in each of the remaining intercostal spaces. Each lies close to the caudal border of the rib. The costocervical trunk supplies the first three or four intercostal spaces (Fig. 3-19). The dorsal costoabdominal artery courses ventrally, caudal to the last rib.

The phrenic nerves (Figs. 3-14, 3-16, 3-20) supply the diaphragm. Find each nerve as it passes through the thoracic inlet. The nerve arises from the ventral branches of the fifth, the sixth, and usually the seventh cervical nerves. Follow the phrenic nerves through the mediastinum to the diaphragm. Each is both motor and sensory to the corresponding half of the diaphragm except at its periphery. This part of the muscle receives sensory fibers from the caudal intercostal nerves.

Dissection

Selected portions of the autonomic nervous system will be dissected as they are exposed in the regions being studied.

Examine the dorsal aspect of the interior of the pleural cavities (Fig. 3-20). Notice the sympathetic trunks coursing longitudinally across the ventral surface of the necks of the ribs. The small enlargements in these trunks at each intercostal space are sympathetic trunk ganglia. Dissect a portion of the trunk and a few ganglia on either side. Notice the fine filaments that run dorsally between the vertebrae to join the spinal nerve of that space. These are the rami communicantes of the sympathetic trunk.

Follow the thoracic portion of the sympathetic trunk cranially. Notice the irregular enlargement of the trunk medial to the dorsal end of the first intercostal space on the lateral side of the longus colli. This is the cervicothoracic ganglion. It is formed by a collection of cell bodies from a fusion of the caudal cervical ganglion and the first two or three thoracic ganglia. Locate this ganglion on both sides.

Many branches leave the cervicothoracic ganglion. Rami communicantes connect to the ventral branches of the first and second thoracic spinal nerves and to the ventral branches of the seventh and eighth cervical spinal nerves. These spinal nerves contribute to the formation of the brachial plexus, which provides a pathway for postganglionic axons to reach the thoracic limb. A branch or plexus from the cervicothoracic ganglion follows the vertebral artery through the transverse foramina—the vertebral nerve. This is a source of postganglionic axons for the remaining cervical spinal nerves via branches at each intervertebral space from the vertebral nerve to each cervical spinal nerve. Postganglionic axons may leave the cervicothoracic ganglion and course directly to the heart.

Cranial to the cervicothoracic ganglion, the sympathetic trunk divides to form a loop, the ansa subclavia, around the subclavian artery. The two branches of the loop unite at the middle cervical ganglion. This ganglion lies at the junction of the ansa and the vagosympathetic trunk and appears as a swelling of the combined structures. Locate these structures on both sides. Numerous branches, cardiac nerves, leave the ansa and middle cervical ganglion and course to the heart.

The vagosympathetic trunk in the neck lies in the carotid sheath. Its sympathetic portion carries preganglionic and postganglionic sympathetic axons cranially to structures in the head. The cranial cervical ganglion is located at its most cranial end. This is at the level of the base of the ear, just caudomedial to the tympanic bulla. It will be dissected later. The tenth cranial or vagus nerve contains parasympathetic preganglionic axons that course caudally down the neck to thoracic and abdominal organs.

At the level of the middle cervical ganglion, notice the vagus nerve as it leaves the vagosympathetic trunk to continue its course caudally. Cardiac nerves leave the vagus to innervate the heart. Study the caudal course of the vagus nerve on each side.

On the left side, at the level of the aortic arch, the left recurrent laryngeal nerve leaves the vagus, curves medially around the ligamentum arteriosum and the arch of the aorta, and becomes related to the ventrolateral aspect of the trachea, and the ventromedial edge of the esophagus. In this position it courses the neck to reach the larynx. As it ascends, it reaches the dorsolateral aspect of the trachea. On the right side, at the middle cervical ganglion, or slightly caudal to it, the right recurrent laryngeal nerve leaves the vagus, curves dorsocranially around the right subclavian artery, reaches the dorsolateral surface of the trachea, and courses cranially to the larynx. It may be found in the angle between the trachea and the longus colli. Each recurrent nerve sends branches to the heart, trachea, and esophagus before terminating in the laryngeal muscles as the caudal laryngeal nerve. The laryngeal nerves will be dissected later.

Follow each vagus nerve as it courses over the base of the heart (Fig. 3-20) and supplies cardiac nerves to it. Branches are supplied to the bronchi as the vagus passes over the roots of the lungs. Between the azygos vein and the right bronchus on the right and the area just caudal to the base of the heart on the left, each vagus divides into dorsal and ventral branches. The right and left ventral branches soon unite with each other to form the ventral vagal trunk on the esophagus. The dorsal branches of each vagus do not unite until farther caudally near the diaphragm, where they form the dorsal vagal trunk, which lies dorsal to the esophagus. The termination of these trunks in the abdomen will be studied later.

LIVE DOG

Observe the thorax and watch it expand and contract with each inspiration and expiration, respectively. Place the middle finger of one hand over the dorsal aspect of the ninth or tenth intercostal space. Tap the distal end of this finger just behind the nail with the middle finger of the other hand. Listen for the sound produced by this method of percussion. Compare this with an area over the epaxial muscles or abdomen. A resonating sound will result where normal lung is beneath the region percussed. This method of physical examination can be used to define the extent of normal lung tissue in the thorax.

Place your hands over the ventral thorax and feel for the heart beat. It should be more evident on the left, where the apex of the heart is directed.

It is important to know the relationship of the cardiac chambers and valve areas for auscultation. A simple hand rule may be helpful. Make a fist with your left hand and extend your thumb at the proximal interphalangeal joint. Your fist represents the left ventricle, and your thumb is the aorta arising from it. The metacarpophalangeal joint of your thumb is at the position of the left atrioventricular valve. Hold your right hand with the fingers extended. Place the palm of your right hand against the closed fingers of your left fist. Wrap the fingers of your right hand around the front of your left fist and curve the second digit of your right hand around your left thumb. Your right hand is in the position of the right ventricle (right and cranial sides of the heart). Your right second digit represents the pulmonary valve and trunk on the left craniodorsal aspect of the heart to the left of the aortic valve. Your right thumb is in the position of the right atrioventricular valve. The paraconal interventricular groove is between the ends of your right fingers and the metacarpophalangeal joints of your left fist. The subsinuosal interventricular groove is between the base of the palms of your two hands.

Primary Vessels of the Thoracic Limb

The main artery to the thoracic limb arises within the thorax as a terminal branch of the brachiocephalic on the right side and directly from the aorta on the left side. It is divided into three parts. That which extends from its origin to the first rib is the subclavian artery (Fig. 3-20). From the first rib to the conjoined tendons of the teres major and latissimus dorsi, the vessel is the axillary artery. The vessel from there to the terminal median artery distal to the elbow is called the brachial artery.

The superficial cervical artery (Figs. 3-11, 3-12, 3-14, 3-16, 3-19, 3-26) arises from the subclavian just inside the thoracic inlet. It runs dorsocranially between the scapula and the neck. It supplies the superficial muscles of the base of the neck, the superficial cervical lymph nodes, the muscles of the scapula, and the shoulder.

There are usually two superficial cervical lymph nodes (Fig. 3-26), which lie on the serratus ventralis and scalenus cranial to the supraspinatus, covered by the omotransversarius and the cleidocephalicus. These nodes receive the afferent lymph vessels from the superficial part of the lateral surface of the neck, the caudal surface of the head including the ear and pharynx, and the thoracic limb.

Brachial Plexus

The brachial plexus (Figs. 3-20, 3-27) is formed by the ventral branches of the sixth, seventh, and eighth cervical and the first and second thoracic spinal nerves. In some dogs there is a small contribution from the ventral branch of the fifth cervical spinal nerve. These branches arise from their spinal nerve just lateral to their respective intervertebral foramen. They emerge along the ventral border of the scalenus and extend across the axillary space to the thoracic limb. In the axilla numerous branches of these nerves communicate to form the brachial plexus. From the plexus arise nerves of mixed origin that supply the structures of the thoracic limb and adjacent muscles and skin (Fig. 3-27).

The pattern of interchange in the brachial plexus is variable, but the specific spinal nerve composition of the named nerves that continue into the thoracic limb is consistent. These nerves include the suprascapular, subscapular, axillary, musculocutaneous, radial, median, ulnar, thoracodorsal, lateral thoracic, and pectoral nerves. Expose the brachial plexus in the axilla. It will be studied later.

Reflect the superficial and deep pectoral muscles toward their insertions to expose the vessels and nerves on the medial aspect of the arm.

Axillary Artery

The axillary artery (Figs. 3-28, 3-29) is the continuation of the subclavian and extends from the first rib to the conjoined tendons of the teres major and latissimus dorsi. It has four branches: the external thoracic, the lateral thoracic, the subscapular, and the cranial circumflex humeral.

In the following dissection some variability may be encountered in the origin of specific blood vessels and nerves. However, although the origin may vary, the area or structures supplied are usually consistent.

The thoracodorsal artery (Figs. 3-28, 3-29) leaves the dorsal surface of the subscapular near its origin. It supplies a part of the teres major and latissimus dorsi and ends in the skin. It is readily seen on the deep surface of the latissimus dorsi. Transect the teres major and reflect both ends to expose the subscapular artery.

The caudal circumflex humeral artery (Figs. 3-3, 3-28, 3-29) leaves the subscapular opposite the thoracodorsal and courses laterally between the head of the humerus and the teres major. Pull the subscapular artery medially to see this branch coursing laterally. Expose the caudal circumflex humeral artery from the lateral side, where branches become superficial at the dorsal part of the lateral head of the triceps. Transect the insertion of the deltoideus. Reflect the deltoideus proximally and observe the axillary nerve and caudal circumflex humeral artery entering the deep surface of the muscle. These are located caudal to the shoulder between the adjacent origins of the long and lateral heads of the triceps. Notice the large branch of the axillobrachial vein that travels with the artery and nerve. The caudal circumflex humeral supplies the triceps, deltoideus, coracobrachialis, and infraspinatus muscles and the shoulder joint capsule. Transect the long head of the triceps at its origin. Reflect it and follow the continuation of the subscapular artery caudodorsally along the caudal surface of the scapula. Numerous branches supply the adjacent musculature and bone.

Brachial Artery

The brachial artery (Figs. 3-27 through 3-30) is a continuation of the axillary from the conjoined tendons of the teres major and latissimus dorsi. It courses distally across the body of the humerus to reach the craniomedial surface of the elbow, where it gives rise to several branches. The deep brachial and bicipital arteries are muscular branches of the brachial in the arm. Other branches are the collateral ulnar, superficial brachial, and transverse cubital arteries. The brachial artery continues into the proximal forearm and gives off its largest branch, the common interosseus, followed by a smaller deep antebrachial artery given off by the median artery.

Dissect the following branches of the brachial artery:

The transverse cubital artery supplies the elbow and adjacent muscles and need not be dissected. The brachial artery courses deep to the pronator teres and flexor carpi radialis. Its branches in the forearm will be dissected later.

Nerves of the Scapular Region and Arm

All of the following 10 nerves contain somatic efferent neurons to striated muscles and afferent neurons from these muscles. Cutaneous somatic afferents are found only in the musculocutaneous, axillary, radial, median, and ulnar nerves.

Transect the supraspinatus at its insertion and reflect the distal end. Trace the branches of the suprascapular nerve to this muscle. Note the continuation of the nerve distal to the scapular spine, where it enters the infraspinatus muscle.

The cranial lateral cutaneous brachial nerve (Figs. 3-3, 3-31) appears subcutaneously on the lateral surface of the brachium just caudal to the deltoideus. It supplies the skin on the lateral surface of the brachium and caudal scapular region. There are cranial cutaneous antebrachial branches of this nerve that supply the skin on the cranial surface of the forearm. The latter overlap with cutaneous antebrachial branches of the superficial branch of the radial nerve laterally and the musculocutaneous nerve medially.

The ulnar nerve, the caudal division of the common trunk, separates from the median nerve in the distal arm and crosses the elbow caudal to the medial epicondyle of the humerus. Trace it with the collateral ulnar artery to the cut edge of the skin. The caudal cutaneous antebrachial nerve (Figs. 3-3, 3-27, 3-30, 3-31) leaves the ulnar near the middle of the arm and runs caudodistally across the medial surface of the triceps and olecranon. This supplies the skin of the distal medial aspect of the brachium and the caudal aspect of the antebrachium.

Incise the skin from the olecranon to the palmar surface of the third digit. Pass through the carpal, metacarpal, and third digital pads. Remove the skin from the forearm and paw, leaving the vessels and nerves (cephalic vein and superficial branch, radial nerves) on the specimen wherever possible.

The cephalic vein (Figs. 3-3, 3-15, 3-17, 3-26, 3-30, 3-35 through 3-38) begins on the palmar side of the paw from the superficial palmar venous arch. This need not be dissected. The accessory cephalic vein arises from small veins on the dorsum of the paw and joins the cephalic on the cranial surface of the distal third of the forearm. At the flexor surface of the elbow, the median cubital vein forms a connection between the cephalic and brachial veins. From this connection the cephalic continues proximally on the craniolateral surface of the arm in a furrow between the brachiocephalicus cranially and the origin of the lateral head of the triceps caudally. In the middle of the arm, the axillobrachial vein leaves the caudal aspect of the cephalic vein. The cephalic vein runs deep to the brachiocephalicus and enters the external jugular near the thoracic inlet. The axillobrachial vein continues proximally and passes deep to the caudal border of the deltoideus to join the axillary vein at this site. The omobrachial vein arises from the axillobrachial vein and continues subcutaneously across the cranial surface of the arm and shoulder and brachiocephalicus muscle before entering the external jugular vein cranial to the cephalic vein.

Arteries of the Forearm and Paw

Make a longitudinal incision through the medial part of the antebrachial fascia midway between the cranial and caudal borders. Extend this incision to the carpus. Remove the fascia from the forearm. Transect the pronator teres and flexor carpi radialis close to their origins and reflect them to uncover the brachial artery.

The brachial artery (Figs. 3-27, 3-29, 3-30) in the forearm gives rise to the common interosseous and continues as the median artery. The median extends to the superficial palmar arch in the paw.

The common interosseous artery (Figs. 3-29, 3-30) is short and passes to the proximal part of the interosseous space between the radius and ulna before dividing into three branches. Pull the brachial artery medially to see the branches of the common interosseous. The common interosseous and ulnar arteries may arise together from the brachial.

The ulnar artery courses caudally. Separate the muscles on the caudomedial side of the forearm to expose its course. A recurrent branch extends proximally between the humeral and ulnar heads of the deep digital flexor. The ulnar artery continues distally with the ulnar nerve between the humeral head of the deep digital flexor and the flexor carpi ulnaris. It supplies the ulnar and humeral heads of the deep digital flexor and the flexor carpi ulnaris.

The caudal interosseous artery lies between the apposed surfaces of the radius and ulna. On the medial side of the forearm expose the pronator quadratus muscle between the radius and ulna. Cut the attachments of this muscle between the two bones and scrape it with the scalpel handle to remove it from the interosseous space. The caudal interosseous artery lies deep in this space. In its course down the forearm, the artery supplies many small branches to adjacent structures. It passes through the lateral side of the carpal canal (Figs. 2-22, 3-29, 3-38) and in the carpometacarpal region it joins with branches of the radial and median arteries to form arches that supply the palmar surface of the forepaw (Figs. 3-29, 3-38). These will not be dissected.

The cranial interosseous artery (Figs. 3-29 through 3-31) passes through the proximal part of the interosseous space cranially to supply the muscles lying laterally and cranially in the forearm. This artery will not be dissected.

The median artery (Figs. 3-29, 3-30, 3-38) is the continuation of the brachial artery beyond the origin of the common interosseous. It gives off the deep antebrachial and the radial artery in the forearm and continues into the paw deep to the flexor carpi radialis. It is the principal source of blood to the paw. It is accompanied by the median nerve along the humeral head of the deep digital flexor. The median artery and nerve pass through the carpal canal between the superficial and deep digital flexor tendons. Transect the flexor retinaculum and superficial digital flexor tendon. Reflect these and follow this artery through the carpal canal to the proximal end of the metacarpus, where it forms the superficial palmar arch with a branch of the caudal interosseous artery. This arch gives rise to the palmar common digital arteries, which course to the palmar surface of the forepaw (these need not be dissected).

The radial artery (Figs. 3-29, 3-30, 3-37, 3-38) arises from the medial side of the median artery in the middle of the forearm. It follows the medial border of the radius. At the carpus it divides into palmar and dorsal carpal branches that supply the deep vessels of the forepaw. These need not be dissected.

The deep antebrachial artery (Figs. 3-29, 3-30) is a caudal branch of the median artery. Follow it deep to the flexor carpi radialis and transect the humeral head of the deep digital flexor, which covers the vessel. The deep antebrachial artery supplies the flexor carpi radialis, the deep digital flexor, the flexor carpi ulnaris, and the superficial digital flexor.

Nerves of the Forearm and Paw

The skin of the antebrachium is innervated by four nerves: the cranial surface by the axillary and radial nerves; the lateral surface by the radial nerve; the caudal surface by the ulnar nerve; and the medial surface by the musculocutaneous nerve. There is considerable overlap in their cutaneous areas of distribution.

The superficial branch divides into a lateral cutaneous antebrachial nerve and medial and lateral branches (Figs. 3-31, 3-37). The small medial branch follows the medial ramus of the cranial superficial antebrachial artery and continues distally in the forearm on the medial side of the cephalic vein. The lateral branch becomes associated with the lateral side of the cephalic vein and enters the forearm with the lateral branches of the cranial superficial antebrachial artery. These branches continue to the paw on either side of the accessory cephalic vein (Fig. 2-22). These medial and lateral branches are sensory to the skin on the cranial and lateral surface of the forearm and the dorsal surface of the carpus, metacarpus, and digits (Fig. 3-39). They terminate in dorsal common digital nerves in the paw (Fig. 3-40).

Transect and reflect the pronator teres and reflect the flexor carpi radialis and the humeral head of the deep digital flexor to observe the course of the nerve. The median nerve passes through the carpal canal with the median artery and branches to supply sensory innervation to the palmar surface of the forepaw. Note the course of the nerve through the carpal canal (Figs. 2-22, 3-38).

Transect and reflect the ulnar and humeral heads of the flexor carpi ulnaris. The ulnar nerve lies on the caudal surface of the deep digital flexor deep to the humeral head of the flexor carpi ulnaris.

Trace the palmar branch of the ulnar nerve into the forepaw. The nerve lies on the deep surface of the flexor carpi ulnaris, just above the carpus. It then enters the lateral side of the carpal canal (Fig. 3-22), where it divides into a superficial and a deep branch (Fig. 3-38). Reflect the flexor carpi ulnaris and flexor retinaculum to expose the nerve in the carpal canal. In the metacarpus the superficial and deep branches further divide to supply sensory innervation to the palmar surface of the forepaw and motor innervation to the intrinsic muscles of the forepaw. Although the terminal cutaneous distribution of nerves in the forelimb is difficult to dissect, it is important to know both the cutaneous areas and the autonomous zones of these nerves for local anesthetic procedures and for the diagnosis of nerve lesions. The cutaneous area is the entire area of skin innervated by a peripheral nerve. The autonomous zone is that area of skin innervated solely by a specific peripheral nerve with no overlap from adjacent nerves. Fig. 3-39 shows the autonomous zones for the nerves of the forelimb and adjacent neck and trunk regions.

The distribution of vessels and nerves to the digits will not be dissected, but you should understand the pertinent terminology (Fig. 3-40). These are more important clinically in large animals that walk on one (horse) or two (most farm animals) digits. In the metacarpus there are superficial and deep branches on both the dorsal and palmar sides, but there are no deep nerves dorsally. Superficial branches are called dorsal or palmar common digital vessels or nerves and deep branches are called dorsal or palmar metacarpal vessels and nerves. These four branches are oriented in a sagittal plane between the metacarpal bones. At the metacarpophalangeal joint, the common digital and metacarpal branches unite to form a common trunk on the dorsal and palmar surfaces. Each common trunk then divides into medial and lateral branches to the digits. These digital branches are called axial or abaxial dorsal or palmar proper digital vessels or nerves. Axial or abaxial refers to whether they are on the surface of the digit that faces toward the axis (axial) or away from the axis (abaxial) of the paw. The axis of the paw passes between digits III and IV. As a rule, the largest digital vessels are the palmar axial proper digitals. (Table 3-2 shows the arrangement of these vessels and nerves in the forepaw.)

LIVE DOG

On the medial side of the arm, feel the vessels and nerves coursing distally between the biceps brachii cranially and the medial head of the triceps brachii caudally. Palpate a pulse in the brachial artery. Arterial injections can be made into this artery.

Palpate the medial epicondyle of the humerus. The brachial vessels and median nerve course just cranial to this and pass beneath the pronator teres. These cannot be felt here. The ulnar nerve courses caudal to the medial epicondyle and can be palpated by stroking the skin in a caudal to cranial direction toward the epicondyle. The ulnar nerve will be felt when it slips beneath your fingers.

On the lateral side of the distal arm, the radial nerve can be felt where it emerges from under the distal border of the lateral head of the triceps on the brachialis muscle and divides into superficial and deep branches. Press firmly on the skin over the brachialis muscle here and stroke from proximal to distal. The superficial branch will be felt as it slips out from under your fingers.

Place your fingers across the flexor surface of the elbow and compress this area. This will distend the cephalic vein in the forearm, which is commonly used for venipuncture. Remember that a small artery and sensory nerve accompany this vein on both sides. Repeated needle punctures may injure them and contribute to subcutaneous hemorrhage, as well as being a painful experience for the patient. In some short-haired breeds, the cephalic, axillobrachial, and omobrachial veins may be visible in the arm and shoulder region.

Place your fingers on the distal medial side of the forearm and extend the carpus. Feel the tendon of the flexor carpi radialis as it becomes taut. Palpation here may detect a pulse in the median artery where it passes deep to this tendon.

Review the location of the autonomous zones of the peripheral nerves in the forelimb of the dog.