Part 58

The =pulmonary artery= (A. pulmonalis) springs from the conus arteriosus at the left side of the base of the right ventricle. It curves upward, backward, and inward, and divides behind the arch of the aorta into right and left branches. It is related in front to the right auricle (appendix), behind to the left auricle (appendix), and internally to the aorta. It is enveloped with the latter in a common sheath of the visceral layer of the serous pericardium. Near the bifurcation it is connected with the arch of the aorta by a fibrous band about half an inch (ca. 1.2 cm.) in width; this is the =ligamentum arteriosum=, a remnant of the large =ductus arteriosus=, which conducts most of the blood from the pulmonary artery to the aorta in the fœtus. The artery is bulbous at its origin, and forms three =sinuses= or pouches, which correspond to the cusps of the semilunar valve. Beyond this it gradually diminishes in caliber.

In a horse of medium size the artery is about seven inches (ca. 17 to 18 cm.) long. At the origin it is about two and a half inches (ca. 6 to 6.5 cm.) in width; at the bifurcation its caliber is about one and a half inches (ca. 3.5 to 4 cm.). The wall is relatively thin, especially at the origin.

The =right branch= (Ramus dexter) of the pulmonary artery is a little longer and wider than the left one. It passes over the fore part of the left atrium and below the bifurcation of the trachea to the hilus of the right lung, and enters the latter below the right bronchus. In the lung it passes to the outer and lower side of the stem bronchus and accompanies it to the base of the organ. The branches correspond to the ramification of the bronchi. The =left branch= (Ramus sinister) is very short. It passes backward and enters the lung below the left bronchus. Its branches within the lung are arranged like that of the right one.

THE SYSTEMIC ARTERIES

The =aorta= is the main systemic arterial trunk. It begins at the base of the left ventricle and is practically median at its origin. It passes upward and slightly forward between the pulmonary artery on the left and right atrium on the right. It then curves sharply backward and upward and inclines somewhat to the left, forming the =arch of the aorta= (Arcus aortæ), and reaches the ventral surface of the spine at the eighth or ninth thoracic vertebra. After passing backward along the ventral aspect of the bodies of the vertebræ between the lungs it traverses the hiatus aorticus and enters the abdominal cavity, where it lies below the vertebral bodies and the psoas minor, just to the left of the median plane. It divides under the fifth lumbar vertebra into the two internal iliac or hypogastric arteries.

From the bifurcation a small vessel, the middle sacral artery (A. sacralis media), sometimes passes backward on the pelvic surface of the sacrum. It becomes lost in the periosteum or joins the coccygeal artery, or in exceptional cases is traceable to the sphincter ani externus.

The caliber of the aorta is greatest at its origin, which is termed the =bulbus aortæ=. Here it forms three pouch-like dilatations, the =sinuses of the aorta= (or of Valsalva). These correspond to the cusps of the aortic valve, and the coronary arteries arise from the left posterior and anterior sinuses. At the arch the diameter is about two inches (ca. 5 cm.), and beyond this it diminishes gradually in width.

[Illustration:

FIG. 426.—CARDIAC VESSELS OF HORSE, RIGHT SIDE.

Veins are black, arteries white. ]

It is convenient to divide the aorta into thoracic and abdominal parts. The =thoracic aorta= (Aorta thoracica) lies within the pericardium to the point of attachment of the ligamentum arteriosum, and is inclosed with the pulmonary artery in a prolongation of the epicardium. Beyond this it is between the two pleural sacs. It is crossed on the right by the œsophagus and trachea, on the left by the left vagus nerve. The left recurrent nerve winds around the concavity of the arch from left to right, and the vena azygos and thoracic duct lie along the dorsal part of its right face. The trachea causes it to deviate to the left; but beyond this it becomes median. The =abdominal aorta= (Aorta abdominalis) (Fig. 450) is related above to the lumbar vertebræ, the inferior common ligament, and the left psoas minor muscle; in the hiatus aorticus it is related to the cisterna chyli. On its right is the posterior vena cava, and on its left the left kidney and ureter.

BRANCHES OF THE THORACIC AORTA

1. CORONARY ARTERIES

The two coronary arteries, right and left, are distributed almost entirely to the heart, but send some small twigs to the origins of the great vessels.

The =right coronary artery= (A. coronaria dextra) arises from the anterior sinus of the aorta. It passes forward between the conus arteriosus and the right auricle (appendix) to the coronary groove, in which it curves around to the right and backward. It then descends in the right ventricular groove almost to the apex of the heart.

[Illustration:

FIG. 427.—CARDIAC VESSELS OF HORSE, LEFT SIDE.

The dotted lines indicate part of the left coronary artery which is concealed by the left auricle. ]

The =left coronary artery= (A. coronaria sinistra) arises from the left posterior sinus of the aorta, passes to the left behind the origin of the pulmonary artery, and divides into two branches. The =descending branch= (Ramus descendens) passes down the left ventricular groove toward the apex. The =circumflex branch= (Ramus circumflexus) runs backward in the coronary groove, in which it winds around to the right side.

2. COMMON BRACHIOCEPHALIC TRUNK OR ANTERIOR AORTA

The =common brachiocephalic trunk= or =anterior aorta= (Truncus brachiocephalicus communis) is a very large vessel which arises from the convexity of the arch of the aorta within the pericardium. It is directed forward and upward. Its length in horses of medium size is usually about two inches (ca. 5 to 6 cm.), but it is sometimes only half an inch or less (ca. 1 cm.). It is crossed on the left by the left vagus and cardiac nerves, and the left recurrent nerve runs between it and the trachea. It divides opposite to the second intercostal space or third rib into the brachiocephalic and left brachial arteries.

The =brachiocephalic artery= (A. brachiocephalica) is directed forward and a little upward in the anterior mediastinum, beneath the trachea. Opposite the first rib it gives off the bicarotid trunk and is continued as the =right brachial artery=. The latter (A. subclavia dextra) turns downward and bends around the anterior border of the first rib and the insertion of the scalenus muscle above the brachial vein. Its course and branches beyond this point will be described with the vessels of the thoracic limb.

[Illustration:

FIG. 428.—TOPOGRAPHY OF THORACIC CAVITY OF HORSE, LEFT SIDE, AFTER REMOVAL OF LEFT LUNG.

The ninth, fifteenth, and eighteenth ribs are retained. _a_, Pericardium; _b_, thoracic aorta; _c_, brachiocephalic trunk; _d_, _d″_, dorsal artery; _d′_, subcostal artery; _e_, _e′_, superior or deep cervical artery; _f_, vertebral artery; _g_, _g′_, left brachial artery; _h_, inferior cervical artery; _i_, internal thoracic artery; _k_, common carotid artery; _l_, jugular vein; _m_, brachial vein; _m′_, inferior cervical vein; _n_, anterior vena cava; _o_, thoracic duct; _p_, œsophagus; _q_, costal part of diaphragm; _q′_, tendinous center of same; _q″_, left crus of same; _r_, anterior mediastinal lymph glands; _s_, trachea; _t_, bronchial lymph glands; _u_, root of left lung; _v_, anterior mediastinum; _v′_, posterior mediastinum; _w_, longus colli; _x_, sterno-cephalicus; _y_, scalenus; _z_, transversalis costarum; _1_, longissimus; _2_, splenius; _3_, spinalis et semispinalis; _4_, _4′_, rhomboideus; _5_, left phrenic nerve; _6_, left vagus; _6′_, _6″_, œsophageal continuations of vagi; _7_, dorsal branch of right vagus; _8_, cardiac nerve; _9_, left recurrent nerve; _10_, cervical trunk of sympathetic; _11_, first thoracic ganglion of sympathetic; _12_, thoracic trunk of sympathetic; _13_, brachial plexus. (After Ellenberger-Baum, Top. Anat. d. Pferdes.) ]

The =left brachial artery= (A. subclavia sinistra) is longer than the right one and rises to a higher level. It forms an almost semicircular curve, the concavity being ventral. It is related internally to the œsophagus, trachea, and thoracic duct, and the left vagus, phrenic, and cardiac nerves cross under its origin. It emerges from the thorax like the artery of the right side. There is thus a difference at first between the trunks of opposite sides, but beyond this their course and distribution are similar.

The brachial and brachiocephalic arteries within the thorax give off the dorsal, superior cervical, vertebral, and internal thoracic arteries. At the first rib they give off the external thoracic and inferior cervical arteries.

1. The left =dorsal= or =dorso-intercostal artery= (A. costo-cervicalis) passes dorsally across the left face of the trachea and œsophagus toward the second intercostal space. The right artery arises usually by a common trunk with the superior cervical, crosses the right face of the trachea and has no contact with the œsophagus. Both detach small branches to the trachea, lymph glands, and pleura, and divide on reaching the longus colli into two branches. Of these the =subcostal artery= (A. intercostalis suprema) is the smaller. It passes backward under the costo-vertebral joints with the sympathetic trunk. It gives off the second, third, and fourth intercostal arteries, and ends at the fifth space, where it anastomoses with the first aortic intercostal artery, or constitutes the fifth intercostal artery, or dips into the longissimus muscle. It also gives off spinal branches and twigs to the longus colli and the pleura. The other branch (A. transversa colli) is the direct continuation of the trunk. It emerges through the upper end of the second intercostal space, passes across the transversalis costarum and longissimus toward the withers, and divides into several diverging branches. An anterior branch passes upward and forward between the splenius and complexus and anastomoses with branches of the superior cervical artery; the others are directed upward under the serratus magnus and rhomboideus to the withers, supplying the muscles and skin of this region.

[Illustration:

FIG. 429.—TOPOGRAPHY OF THORACIC CAVITY OF HORSE, RIGHT VIEW.

The ninth, fifteenth, and eighteenth ribs are retained. _a_, Pericardium; _b_, posterior vena cava; _c_, vena azygos; _c′_, œsophageal vein; _d_, _e_, anterior vena cava; _f_, brachial vein; _g_, inferior cervical vein; _h_, jugular vein; _i_, vertebral vein; _k_, superior cervical vein; _l_, dorsal vein; _m_, trachea; _n_, root of right lung; _o_, œsophagus; _p_, mediastinal lobe of right lung; _q_, mediastinum; _r_, bronchial lymph glands; _s_, mediastinal lymph glands; _t_, costal part of diaphragm; _t′_, tendinous center of same; _u_, intercostal muscle; _v_, longus colli; _w_, posterior deep pectoral muscle; _w′_, anterior deep pectoral muscle; _x_, superficial pectoral muscle; _y_, sterno-cephalicus; _z_, scalenus; _1_, _5_, right brachial artery; _2_, dorso-cervical trunk; _2′_, superior or deep cervical artery; _3_, dorsal artery (cut); _3′_, subcostal artery; _4_, internal thoracic artery; _6_, external thoracic artery; _7_, inferior cervical artery; _8_, bicarotid trunk; _10_, aorta; _11_, œsophageal artery; _12_, thoracic duct; _13_, right phrenic nerve; _14_, brachial plexus; _15_, right vagus; _15′_, _15″_, œsophageal continuations of vagi; _16_, right recurrent nerve; _17_, cervical trunk of sympathetic; _18_, thoracic trunk of sympathetic; _20_, inferior cervical ganglion; _21_, first thoracic ganglion of sympathetic; _22_, right cardiac nerve; _23_, longissimus; _24_, transversalis costarum; _25_, spinalis; _26_, multifidus; _27_, ligamentum nuchæ. (After Ellenberger-Baum, Top. Anat. d. Pferdes.) ]

The left dorsal artery sometimes arises with the superior cervical by a common trunk. On the right side there may be a common stem for the dorsal, superior cervical, and vertebral arteries. Occasionally the artery arises from the anterior aorta. Sometimes it emerges through the third space. The subcostal may arise independently behind the dorsal or from the superior cervical artery.

2. The =superior= or =deep cervical artery= (A. cervicalis profunda) arises in front of the dorsal or by a common trunk with it. It crosses the œsophagus (left side), the trachea (right side), and the longus colli, and emerges from the thoracic cavity by passing through the space behind the first costo-transverse articulation. In the thorax it gives off a small branch (A. mediastini cranialis) to the mediastinum and the pericardium; also the first intercostal artery (A. intercostalis prima), a very small vessel which passes down in the first intercostal space. After leaving the thorax the artery passes upward and forward on the spinalis muscle and the lamellar part of the ligamentum nuchæ, covered by the complexus. Its terminal branches anastomose with branches of the occipital and vertebral arteries in the region of the axis. Numerous collateral branches are detached to the extensor muscles of the head and neck, the ligamentum nuchæ, and the skin, and anastomoses occur with the dorsal artery above and the vertebral artery below.

The artery sometimes emerges through the second intercostal space.

[Illustration:

FIG. 430.—VERTEBRAL ARTERY OF HORSE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

3. The =vertebral artery= (A. vertebralis) arises from the brachial (or brachiocephalic) opposite the first intercostal space and passes upward and forward. On the left side it crosses the œsophagus, on the right the trachea. Emerging from the thorax it passes between the longus colli internally and the scalenus externally, under the transverse process of the seventh cervical vertebra, and continues along the neck through the series of foramina transversaria, between which it is covered by the intertransversales colli. Emerging from the foramen of the axis, it crosses the capsule of the atlanto-axial joint, and joins the retrograde branch of the occipital artery under cover of the great oblique muscle of the head. At each intervertebral foramen a =spinal branch= (Ramus spinalis) is given off which enters the vertebral canal and reinforces the ventral spinal artery. It also gives off series of dorsal and ventral =muscular branches= (Rami musculares). The dorsal branches are the larger; they supply the deep extensor muscles of the head and neck, and anastomose with the deep cervical and occipital arteries. The ventral branches supply chiefly the scalenus, longus colli, intertransversales, and rectus capitis anterior major. The artery is accompanied by the vertebral vein and a sympathetic nerve-trunk.

In some cases the last cervical transverse process has a foramen transversarium, through which the artery passes.

4. The =internal thoracic artery= (A. thoracica s. mammaria interna) is a large vessel which arises from the ventral side of the brachial opposite the first rib. It curves downward and backward, being at first on the inner surface of the rib, and then crosses the lower part of the first intercostal space and passes under the transversus thoracis muscle. It runs backward under cover of that muscle over the chondro-sternal joints to the eighth costal cartilage, where it divides into asternal and anterior abdominal branches. At each intercostal space two collateral branches are detached. The =intercostal branches= (Rami intercostales) pass upward in the intercostal spaces and anastomose with homonymous descending arteries. The lower branches detach small twigs to the transversus thoracis, pleura, and pericardium, and pass out between the costal cartilages as =perforating branches= (Rami perforantes) to supply the pectoral muscles and skin, anastomosing with the external thoracic artery. A very small pericardiaco-phrenic artery ascends in the mediastinum on the left side, in the caval fold of pleura on the right side; it supplies fine twigs to the pericardium and pleura and accompanies the phrenic nerve to the diaphragm. In the young subject it gives small branches (Aa. thymicæ) to the thymus gland. The =asternal artery= (A. musculophrenica) passes along the ninth costal cartilage and continues along the costal attachment of the transversus abdominis (Fig. 185). It gives off intercostal branches which anastomose with those descending from the thoracic aorta, and twigs to the diaphragm and transversus abdominis. The =anterior abdominal artery= (A. epigastrica cranialis) is the direct continuation of the internal thoracic. It passes between the ninth costal cartilage and the xiphoid cartilage, runs backward on the abdominal surface of the rectus abdominis and then becomes embedded in it (Fig. 466). It supplies the ventral wall of the abdomen and anastomoses with the posterior abdominal artery.

5. The =external thoracic artery= (A. thoracica externa) is given off from the ventral aspect of the brachial, usually at the inner surface or anterior border of the first rib. It turns around the first rib below or behind the brachial vein (when given off within the thorax) and passes backward under the deep pectoral muscle; it is continued as a small vessel in the panniculus carnosus, where it accompanies the external thoracic (“spur”) vein. It sends branches to the pectoral muscles and the axillary lymph glands.

This artery varies in origin and size. Not rarely it arises from the internal thoracic or from the brachial outside of the thorax. In other cases it arises by a common trunk with the inferior cervical. It may be very small or even absent, in which case the perforating branches of the internal thoracic compensate.

6. The =inferior cervical artery= (Truncus omo-cervicalis) arises usually from the dorsal surface of the brachial opposite the first rib or where that vessel winds around the rib. It is directed downward and a little forward across the external surface of the jugular vein and the deep face of the scalenus among the lymph glands at the thoracic inlet, and divides into ascending and descending branches. The =ascending branch= (A. cervicalis ascendens) passes upward and forward along the external surface of the jugular vein, then turns sharply backward and runs upward along the anterior border of the anterior deep pectoral muscle, between the omo-hyoideus and mastoido-humeralis and in relation to the prescapular lymph glands; it gives branches to these muscles and the prepectoral and prescapular lymph glands. The =descending branch= (A. transversa scapulæ) passes downward and outward across the surface of the anterior deep pectoral and then runs in the groove between that muscle and the mastoido-humeralis in company with the cephalic vein. It supplies branches to these muscles and the skin of the breast.

THE COMMON CAROTID ARTERY

These two vessels (Arteriæ carotides communes) arise from the brachiocephalic artery by a common trunk. This stem, the =truncus bicaroticus= or =cephalic artery=, is detached from the inner face of the brachiocephalic opposite the first rib and passes forward in the median plane beneath the trachea. It is related ventrally to the prepectoral lymph glands, the terminal parts of the jugular veins, and the anterior vena cava, and laterally to the vagus and recurrent nerves. It is commonly two or three inches (ca. 5 to 7 cm.) in length, but it may vary between one and eight inches (ca. 2.5 to 20 cm.).

[Illustration:

FIG. 431.—VESSELS AND NERVES OF NECK OF HORSE.

_a_, Mastoido-humeralis; _b_, sterno-cephalicus; _c_, omo-hyoideus; _d_, sterno-thyro-hyoideus; _e_, trachea; _f_, position of cariniform cartilage; _g_, anterior superficial pectoral muscle; _h_, scalenus; _i_, intertransversales; _k_, insertion of serratus cervicis; _l_, origin of trapezius; _m_, rhomboideus; _n_, splenius; _o_, complexus; _p_, _q_, trachelo-mastoideus; _p′_, _q′_, tendons of same; _r_, longissimus; _s_, obliquus capitis posterior; _t_, wing of atlas; _u_, parotid gland; _v_, supraspinatus; _w_, anterior deep pectoral; _x_, spine of scapula; _y_, prescapular lymph glands; _1_, external maxillary vein; _2_, _3_, jugular vein; _4_, carotid artery, exposed by drawing jugular vein aside; _5_, _6_, ascending and descending branches of inferior cervical artery; _7_, cephalic vein; _8_, branches of superior or deep cervical artery; _9–14_, ventral branches of second to seventh cervical nerves; _15_, branches of dorsal divisions of cervical nerves. (Ellenberger-Baum, Top. Anat. d. Pferdes.) ]

The =right common carotid artery= passes obliquely from the ventral face of the trachea to its right side. In this position it runs upward and forward and divides at the crico-pharyngeus muscle and under the submaxillary gland into external carotid, internal carotid, and occipital arteries. It is inclosed in a fibrous sheath, and is accompanied dorsally by the vagus and sympathetic nerves, ventrally by the recurrent nerve. At the last two cervical vertebræ it is in contact superficially with the jugular vein, but further forward the omo-hyoideus muscle intervenes between the artery and vein. Near its termination the artery becomes more deeply placed and is related externally to the submaxillary and parotid glands, internally to the œsophagus. In some cases it is in contact ventrally with the thyroid gland, especially when the latter is larger than usual.

The =left common carotid artery= differs from the right one in that it is related deeply to the œsophagus, which separates it from the trachea in the greater part of its course.

The collateral branches of the common carotids are in the main small. They comprise:

1. =Muscular branches= (Rami musculares) of variable size, which go to the ventral muscles of the neck and the skin.

2. =Œsophageal= and =tracheal branches= (Rami œsophagei et tracheales). Small twigs go to the cervical lymph glands also.

3. The =parotid artery=. This comes off near the termination and enters the ventral part of the parotid gland. It also supplies the subparotid lymph glands, and sometimes sends a branch to the submaxillary gland. It is inconstant.

4. The =thyro-laryngeal artery= (A. thyreoidea cranialis). This, the largest collateral branch of the carotid, arises from the latter two or three inches before it divides. It curves over the anterior end of the thyroid gland, into which it sends several branches. It gives off a =laryngeal branch= (A. laryngea), which sends twigs to the external muscles of the larynx and the constrictors of the pharynx, passes between the cricoid and thyroid cartilages, and supplies the internal muscles and the mucous membrane of the larynx. A small =pharyngeal branch= (A. pharyngea ascendens) runs upward and forward to the crico-pharyngeus, and supplies twigs to the posterior part of the pharynx and the origin of the œsophagus. Small innominate twigs are given off to the trachea, the œsophagus, and the sterno-thyro-hyoideus and omo-hyoideus muscles.

In some cases the thyroid and laryngeal arteries arise from the carotid separately or by a short common stem. A laryngeal branch is often detached from the carotid in front of the thyro-laryngeal and enters the larynx with the superior laryngeal nerve. The pharyngeal branch frequently comes directly from the carotid.

[Illustration:

FIG. 432.—CROSS-SECTION OF NECK OF HORSE, PASSING THROUGH FIFTH CERVICAL VERTEBRA; ANTERIOR VIEW.

_a_, Branches of cervical nerves; _a′_, nuchal fat; _b_, intertransversalis muscle; _c_, longissimus muscle; _d_, vertebral artery; _e_, vertebral vein; _f_, vertebral nerve; _g_, spinal accessory nerve (upper division); _h_, recurrent nerve; _i_, vago-sympathetic trunk; _k_, tracheal lymph duct; _l_, body of fifth cervical vertebra; _l′_, transverse process of same; _m_, carotid artery; _n_, jugular vein; _o_, superior cervical artery; _o′_, satellite vein of _o_; _p_, spinal cord; _q_, dura mater; _r_, spinal vein; _s_, ligamentum nuchæ; _t_, rhomboideus muscle; _u_, splenius; _v_, complexus; _w_, multifidus; _x_, serratus cervicis; _y_, mastoido-humeralis; _z_, sterno-cephalicus; _1_, rectus capitis ant. major; _2_, omo-hyoideus; _3_, panniculus; _4_, sterno-thyro-hyoideus; _5_, longus colli; _6_, _7_, trachelo-mastoideus; _8_, trapezius; _9_, spinalis; _10_, œsophagus; _11_, trachea, with cartilaginous ring (_11′_), mucous membrane (_11″_), and muscular layer (_11‴_). (After Ellenberger, in Leisering’s Atlas.) ]

5. The =accessory thyroid artery= (A. thyreoidea caudalis) is an inconstant vessel which arises from the carotid at a variable distance behind the thyro-laryngeal—sometimes from the latter or from the parotid artery. It sends branches into the posterior part of the thyroid gland and detaches small tracheal and muscular twigs. In some cases it is distributed chiefly or entirely to the adjacent muscles.

THE OCCIPITAL ARTERY

[Illustration:

FIG. 433.—DEEP DISSECTION OF NECK OF HORSE.