Chapter V: Muscles of the Leg and Foot (11)

The =pericardium= is the fibro-serous sac which surrounds the heart, and also the great vessels in connection with it to a greater or less extent. Its form is in general similar to that of the heart. The =fibrous layer= is relatively thin, but strong and inelastic. It is attached dorsally to the large vessels at the base of the heart, and is continued in part up to the longus colli muscle. Ventrally it is firmly attached to the middle of the posterior half of the thoracic surface of the sternum. The =serous layer= is a closed sac, surrounded by the fibrous pericardium, and invaginated by the heart. It is smooth and glistening, and contains a small amount of clear serous fluid, the =liquor pericardii=. Like other serous membranes, it may be regarded as consisting of two parts, parietal and visceral. The =parietal part= lines the fibrous layer, to which it is closely attached. The =visceral layer= covers the heart and parts of the great vessels, and is therefore also termed the =epicardium=. The serous pericardium is composed of a connective-tissue membrane, rich in elastic fibers, and covered on its free surface by a layer of flat endothelial cells.

The pericardium is covered by the mediastinal pleura (Pleura pericardica) and is crossed laterally by the phrenic nerves. Its lateral surfaces are related chiefly to the lungs, but the lower part is in partial contact with the chest-wall. On the left side the area of contact is from the third to the fifth intercostal space inclusive. On the right side the contact is smaller and is at the third and fourth ribs. The anterior border of the base is opposite to the second intercostal space, and the posterior border is opposite to the sixth rib or space. The base is related to the great vessels, the trachea and its bifurcation, the bronchial lymph glands, the œsophagus, and the vagus, left recurrent, and cardiac nerves.

The two parts of the serous pericardium are, of course, continuous
with each other at the line of reflection on to the great vessels. The
latter are covered in varying degree by the visceral layer. The aorta
and pulmonary artery are inclosed in a complete common sheath as far
as the bifurcation of the latter. A pouch passes inward between the
pulmonary artery and the left auricle, and is connected with another
pouch which passes backward between the right auricle and the aorta,
thus forming the great transverse sinus of the pericardium. The
posterior vena cava is covered on the right and below for a distance
of an inch or a little more (ca. 3 cm.). The pulmonary veins receive
practically no serous covering. The epicardium is closely adherent to
the muscular tissue of the heart, but is attached to the vessels by
loose tissue and fat, and hence is easily dissected off them.

THE HEART

The =heart= (Cor) occupies the greater part of the middle mediastinal space. Its shape is that of an irregular and somewhat flattened cone. It is attached at its base by the great vessels, but is otherwise entirely free in the pericardium. It presents two surfaces, two borders, a base, and an apex for description.

FIG. 419.—HEART OF HORSE, LEFT VIEW. HARDENED _in situ_.

The dotted line indicates the line of reflection of the serous
pericardium. The epicardium and subepicardial fat have not been
removed from the heart.
]

In =position= the heart is decidedly asymmetrical, about two-fifths being to the right of the median plane and three-fifths to the left. The long axis (from the middle of the base to the apex) is directed downward, backward, and somewhat to the left. The =base= (Basis cordis) is directed dorsally and lies a little above the middle of the dorso-ventral diameter of the thorax. It is opposite to the ribs from the third to the sixth inclusive. The =apex= (Apex cordis) lies above the last segment of the sternum, and is separated by a very short interval from the sternal part of the diaphragm. The =anterior border= (Margo cranialis) is strongly convex and is directed obliquely downward and backward; its lower part is parallel with the sternum. The =posterior border= (Margo caudalis) is much shorter, nearly vertical, and is opposite to the sixth rib or intercostal space. The =surfaces=, =right= and =left= (Facies dextra, sinistra), are convex and are marked by =grooves= which indicate the division of the heart into four chambers, the two atria above and two ventricles below. The left surface (covered by the pericardium) is in contact with the lower third of the chest-wall from the third to the fifth intercostal space. On the right side the cardiac notch of the lung is smaller and further forward, so that the area of contact is at the third and fourth ribs.

FIG. 420.—HEART OF HORSE, RIGHT VIEW. HARDENED _in situ_.

Line of reflection of serous pericardium dotted. Epicardium and
subepicardial fat have not been removed from heart. Left ventricle
is considerably contracted.
]

The =coronary= or =atrio-ventricular groove= (Sulcus coronarius) indicates the division between the atria and the ventricles. It almost completely encircles the heart, but is interrupted by the origin of the pulmonary artery. The =longitudinal= or =interventricular grooves=, right and left (Sulci longitudinales sinister et dexter), correspond to the septum between the ventricles. The left groove is left-anterior in position. It begins at the coronary groove behind the origin of the pulmonary artery, and passes downward almost parallel to the posterior border, but does not reach the apex. The right groove is right posterior in position. It begins at the coronary groove below the termination of the posterior vena cava and passes downward and a little backward, ending about an inch and a half (ca. 3 to 4 cm.) above the apex. The grooves are occupied by the coronary vessel and a variable quantity of fat.

FIG. 421.—BASE OF HEART OF HORSE WITH LARGE VESSELS, DORSAL VIEW.
SPECIMEN HARDENED _in situ_.
]

=Size and Weight.=—In horses of medium size the heart usually weighs about seven or eight pounds (ca. 3.5 kg.), or about 0.7 per cent. of the body-weight. There is, however, great range of variation in apparently normal specimens.

The following measurements were obtained in medium-sized hearts in
diastole:

Sagittal diameter of base 25 cm. Greatest width of base 18 to 20 cm. Circumference at coronary groove 65 to 70 cm. Distance between origin of pulmonary artery and apex 25 cm. Distance between termination of posterior vena cava and apex 18 to 20 cm.

THE RIGHT ATRIUM

The =right atrium= (Atrium dextrum) or auricle forms the right-anterior part of the base of the heart, and lies above the right ventricle. It consists of a =sinus venosus=, into which the veins open, and an =auricle= or auricular appendix. The latter is a conical diverticulum which curves around the right and anterior surfaces of the aorta, its blind end appearing on the left side in front of the origin of the pulmonary artery.

FIG. 422.—RIGHT SIDE OF HEART OF HORSE OPENED UP BY REMOVAL OF GREATER
PART OF RIGHT WALL. ORGAN HARDENED _in situ_.

The right ventricle was in diastole. _r_, Intervenous crest; _v_, _v_,
tricuspid valve; _p_, _p_, papillary muscles; _m_, _m_, moderator
bands. Arrow points into origin of pulmonary artery.
]

There are five chief openings in the right atrium. The =opening of the anterior vena cava= (Ostium venæ cavæ cranialis) is in the upper and fore part. The =opening of the posterior vena cava= (Ostium venæ cavæ caudalis) is at the lower posterior part. Between the two the wall pouches upward somewhat, forming what is sometimes termed the sinus venosus or sac of Lower. In the anterior part of this the =vena azygos= opens. The =coronary sinus= opens immediately below the posterior vena cava; the orifice is provided with a small semilunar valve or valve of Thebesius (Valvula sinus coronarii). The middle coronary vein has a separate opening in some cases close to that of the coronary sinus. The =atrio-ventricular opening= is in the lower part, and leads into the right ventricle. In addition to the foregoing there are several small orifices of the venæ cordis parvæ; these are concealed in the depressions between the musculi pectinati.

The atrium is lined with a glistening membrane, the =endocardium=. Its walls are smooth except on the right and in the auricle (or appendix), where it is crossed in various directions by muscular ridges, the =musculi pectinati=. Small bands extend across some of the spaces inclosed by the musculi pectinati. The latter terminate above on a curved crest, the =crista terminales=, which indicates the junction of the primitive sinus venosus of the embryo with the atrium proper, and corresponds with the sulcus terminalis externally. The openings of the venæ cavæ are without valves. A ridge, the intervenous crest,[156] projects downward and forward from the roof just in front of the opening of the posterior vena cava; it tends to direct the flow of blood from the anterior vena cava to the atrio-ventricular opening. The =fossa ovalis= is an oval depression in the septal wall at the point of entrance of the posterior vena cava, bounded internally by a thick margin (Limbus fossæ ovalis). The fossa is the remains of an opening, the =foramen ovale=, through which the two atria communicate in the fœtus.

THE LEFT ATRIUM

The =left atrium= (Atrium sinistrum) or auricle forms the posterior part of the base of the heart. It lies behind the pulmonary artery and the aorta and above the left ventricle. The auricle (or appendix) extends outward and forward on the left side, and its blind end is behind the origin of the pulmonary artery. The =pulmonary veins=, usually seven or eight in number, open into the atrium behind and on the right side. The cavity of the atrium is smooth, with the exception of the auricle (or appendix), in which the musculi pectinati are present. In some cases there is a depression on the septal wall opposite the fossa ovalis, bounded above by a fold which is the remnant of the valve of the foramen ovale of the fœtus. The =atrio-ventricular opening= is situated below and in front; it usually appears smaller than the right one on account of the contraction of the left ventricle in the dead subject. The apertures of small veins of the heart are found in the spaces inclosed by the musculi pectinati.

The number and the arrangement of the pulmonary veins are variable.
They may be five to nine in number. The largest orifice is posterior.
Usually three veins of considerable size enter close together on the
right above the posterior vena cava, and three or four open close to
the ridge which projects from the roof at the base of the auricle
(appendix).

THE RIGHT VENTRICLE

The =right ventricle= (Ventriculus dexter) constitutes the right-anterior part of the ventricular mass. It forms almost all of the anterior border of the heart, but does not reach the apex, which is formed entirely by the left ventricle. It extends from the third rib to the fourth intercostal space on the left side, to the fifth rib and space on the right side. It is somewhat triangular in outline, and is semilunar in cross-section. Its base faces upward and a little to the right and is connected largely with the right atrium, with which it communicates through the atrio-ventricular orifice; but its left part projects higher and forms the =conus arteriosus=, from which the pulmonary artery arises. Its apex is two inches or more (ca. 5 to 6 cm.) above the apex of the heart. On opening the cavity it is seen that the two openings are separated by a thick rounded ridge (Crista supraventricularis). The axis of the cavity, taken from this ridge to the apex forms a spiral curve downward and to the right. The septal wall is convex and faces obliquely forward and to the right.

FIG. 423.—SECTION OF HEART OF HORSE.

Specimen hardened _in situ_ and cut nearly at right angles to the
ventricular septum. The left ventricle is contracted, but not _ad
maximum_. _V. a._, Segment of aortic valve.
]

The =right atrio-ventricular orifice= (Ostium venosum dextrum) is oval and is situated opposite to the lower parts of the third and fourth ribs. It is guarded by a =tricuspid valve= (Valvula tricuspidalis); of the three large cusps of this valve, one is between the atrio-ventricular opening and the conus arteriosus, one is septal, and the third is on the right margin. Small intermediate cusps intervene between the large ones. The peripheral edges of the cusps are attached to the fibrous ring at the atrio-ventricular opening. The central edges are irregular and hang down into the ventricle; they give attachment to chordæ tendineæ. The auricular surfaces are smooth. The ventricular surfaces are rough and furnish attachment to interlacing branches of the chordæ tendineæ. The valves are folds of the endocardium, strengthened by fibrous tissue and at the periphery by muscular fibers also. The =chordæ tendineæ= are attached below to the three =musculi papillares=, which project from the ventricular wall; superiorly they divide into branches which are inserted into the ventricular surfaces and the free edges of the valves. Each segment of the valve receives chordæ tendineæ from two papillary muscles. Of the latter, two are on the septum and the third and largest springs from the anterior wall.

The =pulmonary orifice= (Ostium arteriæ pulmonalis) is circular and is situated at the summit of the conus arteriosus, opposite to the lower part of the third intercostal space. It is guarded by the =pulmonary valve=, composed of three semilunar cusps (Valvulæ semilunares arteriæ pulmonalis); of these, one is internal, one external, and the third posterior. The convex peripheral border of each cusp is attached to the fibrous ring at the junction of the pulmonary artery and the conus arteriosus. The central border is free and slightly concave. Each cusp consists of a layer of endocardium on its ventricular surface, a continuation of the inner coat of the artery on its arterial surface, and an intermediate layer of fibrous tissue. The upper edge of the conus arteriosus forms three arches with intermediate projecting angles or horns, to all of which the cusps are attached; and the artery forms opposite each cusp a pouch, the sinus of the pulmonary artery (or of Valsalva).

FIG. 424.—SECTION OF HEART OF HORSE. SPECIMEN HARDENED _in situ_.

The section is cut nearly at right angles to the ventricular septum,
and is viewed from the right and posteriorly.
]

The walls of the ventricle (except in the conus arteriosus) bear muscular ridges and bands, termed =trabeculæ carneæ=. These are of three kinds, viz., (1) ridges or columns in relief; (2) musculi papillares, somewhat conical flattened projections, continuous at the base with the wall and giving off the chordæ tendineæ to the tricuspid valve; (3) moderator bands (Musculi transversi cordis) which extend from the septum to the opposite wall. The latter are partly muscular, partly tendinous, and vary in different subjects. The strongest one is usually about midway between the base and apex and extends from the septum to the base of the anterior musculus papillaris. It is considered that they tend to prevent overdistention.

THE LEFT VENTRICLE

The =left ventricle= (Ventriculus sinister) forms the left posterior part of the ventricular mass. It is more regularly conical than the right ventricle and its wall is much thicker except at the apex. It forms all of the posterior border of the ventricular part and the apex of the heart. Its base is largely continuous with the left atrium, with which it communicates through the left atrio-ventricular opening, but in front it opens into the aorta. The cavity usually appears smaller than that of the right ventricle in the dead subject, on account of the greater contraction of its wall. It is almost circular in cross-section.

FIG. 425.—THE BASES OF THE VENTRICLES OF THE HEART OF THE HORSE.

The atria have been removed and the aorta and pulmonary artery cut off
short. The left ventricle is contracted. _p.v._, Pulmonary valve;
_a.v._, aortic valve.
]

The =left atrio-ventricular orifice= (Ostium venosum sinistrum) is opposite to the fifth rib and intercostal space. It is almost circular and is guarded by the =bicuspid= or =mitral valve= (Valvula bicuspidalis). The two segments of this valve are larger and thicker than those of the right side of the heart. One is placed in front and separates the atrio-ventricular and aortic openings. The other is placed behind and laterally and is usually divided into two or three flaps.

The =aortic orifice= (Ostium aorticum) is directed upward and slightly forward. It is situated opposite to the fourth rib. It is guarded by the =aortic valve=, composed of three semilunar cusps (Valvulæ semilunares aortæ); one cusp is anterior, the others right and left posterior. They are similar to those of the pulmonary valve, but are much stronger. The free edge of each contains a central nodule of fibrous tissue, the corpus Arantii (Nodulus valvulæ semilunaris).

The =chordæ tendineæ= are fewer but larger than those of the right ventricle.

There are two large =musculi papillares=, one on each side; they are usually compound. The moderator bands are variable. Commonly two or three larger ones (which are often branched) extend from the musculi papillares to the septum. Smaller ones may be found in various places, especially at the apex. The other trabeculæ are fewer and less prominent than in the right ventricle.

The =interventricular septum= (Septum ventriculorum) is the musculo-membranous partition which separates the cavities of the two ventricles. It is placed obliquely. One surface is convex, faces forward and to the right, and bulges into the right ventricle. The other surface, which faces into the left ventricle, is concave and looks backward and to the left. The greater part of the septum is thick and muscular (Septum musculare), but the upper part is thin and membranous (Septum membranaceum). The latter intervenes between the left ventricle, on the one hand, and the right ventricle and atrium on the other.

STRUCTURE OF THE HEART

The heart-wall consists mainly of peculiar striped muscle, the myocardium, which is covered externally by the visceral part of the serous pericardium or epicardium, and is lined by the endocardium.

The =epicardium= is in general closely attached to the muscular wall, but is loosely attached over the coronary vessels and the associated subepicardial fat. It consists of a layer of flat polygonal cells, resting on a membrane of white and elastic fibers.

The =myocardium= consists of planes of fibers arranged in a somewhat complicated manner. The muscular tissue of the atria is almost completely separated from that of the ventricles by the fibrous rings around the atrio-ventricular orifices.

In the atria the muscle bands fall naturally into two groups—superficial and deep. The former are common to both atria, the latter special to each. The =superficial= or =common fibers= for the most part begin and end at the atrio-ventricular rings, but some enter the interatrial septum. The =deep= or =special bundles= also form two sets. Looped fibers pass over the atria from ring to ring, while annular or spiral fibers surround the ends of the veins which open into the atria, the auricles, and the fossa ovalis.

The muscular wall of the ventricles is much stronger than that of the atria. That of the left ventricle is in general about three times as thick as that of the right one, but is thin at the apex. The =superficial fibers= are attached above to the atrio-ventricular fibrous rings and pass in a spiral toward the apex. Here they bend upon themselves and pass deeply upward to terminate in a papillary muscle of the ventricle opposite to that in which they arose. The loops so formed at the apex constitute a whorl, the =vortex cordis=. The =deep fibers=, although they appear to be proper to each ventricle, have been shown by MacCallum to be in reality almost all common to both. Their arrangement is scroll-like. They begin on one side, curve around in the wall of that ventricle, then pass in the septum to the opposite side, and curve around the other ventricle. There is a layer of deep fibers which is confined to the basal part of the left ventricle; it is attached to the left atrio-ventricular ring.

Four =fibrous rings= (Annuli fibrosi) surround the orifices at the bases of the ventricles. The atrio-ventricular rings separate the musculature of the atria from that of the ventricles. Those which surround the origins of the pulmonary artery and aorta are festooned in conformity with the attached borders of the valves. The aortic ring contains on the right side a plate of cartilage (Cartilago cordis), which frequently becomes more or less calcified in old animals. Sometimes a smaller plate is present on the left side.

The =endocardium= lines the cavities of the heart and is continuous with the internal coat of the vessels which enter and leave the organ. Its free surface is smooth and glistening and is formed by a layer of endothelial cells. The latter rest on a thin layer of connective tissue, which is connected with the myocardium by a subendocardial elastic tissue containing vessels and nerves.

=Vessels and Nerves.=—The heart receives its blood-supply through the two =coronary arteries= which arise from the aorta opposite to the anterior and left cusps of the aortic valve. Most of the blood is returned by the =coronary veins=, which open into the right atrium by the coronary sinus.[157] A few small veins open directly into the right atrium, and others are said to open into the left atrium and the ventricles. The =lymph vessels= form a subepicardial network which communicates through stomata with the cavity of the pericardium. There is a less distinct subendocardial network. The vessels converge usually to two trunks which accompany the blood-vessels in the atrio-ventricular grooves and enter the glands at the bifurcation of the trachea. The =nerves= are derived from the =vagus= and =sympathetic= through the cardiac plexus.

THE ARTERIES

THE PULMONARY ARTERY

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.

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.

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.

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.

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.

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.).

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.

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

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

_a_, _a_, Ends of sterno-cephalicus; _b_, anterior part of
omo-hyoideus; _c_, sterno-thyro-hyoideus; _d_, trachea; _e_,
œsophagus; _f_, cariniform cartilage; _g_, rectus cap. ant. major;
_h_, stump of trachelo-mastoideus; _i_, intertransversales; _k_,
multifidus; _l_, _m_, scalenus; _n_, serratus cervicis; _o_, _o_,
stumps of splenius, _p_, longissimus, _q_, complexus (most of which
is removed); _r_, rhomboideus; _s_, trapezius; _t_, spinalis et
semispinalis; _u_, lamellar part of lig. nuchæ; _v_,
mastoido-humeralis; _w_, anterior deep pectoral; _x_, supraspinatus;
_y_, anterior superficial pectoral; _z_, scapular tuberosity; _1_,
_1_, articular processes of cervical vertebræ; _2_, _2_, transverse
processes of same; _3_, atlas; _3′_, axis; _4_, _4_, jugular vein
(remainder removed); _5_, common carotid artery, from which a piece
is removed to show the accompanying nerves; _6_, vago-sympathetic
trunk; _7_, _7_, tracheal and muscular branches of carotid artery;
_8_, recurrent nerve; _10_, left tracheal lymph duct; _11_, _12_,
ascending and descending branches of inferior cervical artery
(_13_); _14_, cephalic vein; _15–20_, ventral branches of second to
seventh cervical nerves; _21_, roots of phrenic nerve; _22_,
thoracic or pectoral nerves; _22′_, nerve to serratus magnus; _23_,
musculo-cutaneous nerve; _24_, median nerve; _25_, ulnar nerve;
_26_, radial nerve; _27_, axillary nerve; _28_, dorsal branches of
cervical nerves; _28′_, accessory nerve (cut); _29_, superior or
deep cervical artery; _30_, muscular branch of vertebral artery;
_31_, posterior (anastomotic) branch of occipital artery; _32_,
vertebral artery; _33_, muscular branches of occipital artery; _34_,
obliquus capitis post.; _35_, obi. cap. ant.; _36_, twig from dorsal
branch of third cervical nerve. (After Ellenberger-Baum, Top. Anat.
des Pferdes.)
]

The =occipital artery= (A. occipitalis) is the second in size of the terminals of the carotid. It arises usually just in front of the internal carotid, but in some cases with that artery by a common trunk of variable length. It pursues a somewhat flexuous course to the fossa atlantis, where it divides into anterior and posterior branches. It is related superficially to the submaxillary gland and the mastoido-humeralis, and deeply to the guttural pouch and the rectus capitis anterior major.[158] The internal carotid artery, the inferior cerebral vein, and the accessory, vagus, and sympathetic nerves cross its deep face. It gives twigs to the submaxillary gland, the anterior straight muscles, the guttural pouch and the adjacent lymph glands, and two named collateral branches. The =condyloid= or =prevertebral artery= (A. condyloidea) is a small vessel which passes upward and forward on the guttural pouch, and divides into muscular and meningeal branches. The latter enter the cranium through the foramen lacerum and hypoglossal foramen and are distributed to the dura mater. This artery is very variable in its origin. The =posterior meningeal= or =mastoid artery= (A. meningea caudalis) is a much larger vessel which runs upward and forward between the small oblique muscle and the paramastoid process, passes through the mastoid foramen into the parieto-temporal canal, enters the cranial cavity, and is distributed to the dura mater.

FIG. 434.—VESSELS AND NERVES OF BASE OF BRAIN OF HORSE.

_13_, Cerebro-spinal artery; _14_, middle spinal artery; _15_, basilar
artery; _16_, posterior cerebellar artery; _17_, anterior cerebellar
artery; _18_, internal auditory artery; _19_, posterior cerebral
artery; _20_, deep cerebral artery; _21_, stump of internal carotid
artery; _22_, anterior choroid artery; _23_, anterior meningeal
artery; _24_, middle cerebral artery; _25_, artery of corpus
callosum; _26_, anterior communicating artery; _1_, _1′_, _1″_,
olfactory tracts; _1‴_, olfactory peduncle; _2–12_, cranial nerves;
_a_, olfactory bulb; _b_, trigonum olfactorium; _c_, lamina
perforata anterior; _d_, fossa transversa; _e_, pyriform lobe; _f_,
cerebral peduncle; _g_, tractus transversus; _h_, corpus mammillare;
_i_, tuber cinereum; _k_, lateral fissure (of Sylvius); _l_,
presylvian fissure; _m_, pons; _o_, pyramid; _p_, facial eminence;
_q_, corpus restiforme; _r_, cerebellum; _s_, middle peduncle of
cerebellum. (After Ellenberger Baum, Top. Anat. d. Pferdes.)
]

The =posterior= or =retrograde branch= (Ramus descendens) of the occipital passes up through the foramen transversarium of the atlas and joins the vertebral artery. It gives branches to the great oblique muscle of the head, which covers it.

The =anterior= or =occipital branch= (Ramus occipitalis) passes through the alar (antero-external) foramen of the atlas and supplies the muscles and skin of the poll, anastomosing with the deep cervical artery and its fellow of the opposite side. In the alar furrow it gives off the =cerebrospinal artery=, which enters the spinal canal through the intervertebral (antero-internal) foramen of the atlas, perforates the dura mater, and divides into cerebral and spinal branches. The =cerebral branch= unites with that of the opposite side to form the =basilar artery=, and the =spinal branch= similarly forms by union with its fellow the =middle spinal artery=.

The =basilar artery= (A. basilaris cerebri) passes forward in the median groove on the ventral surface of the medulla and pons and divides into the two posterior cerebral arteries. The collateral branches of the basilar are:

1. Medullary branches (Rami medullares), ten or twelve in number, distributed to the medulla oblongata.

2. Posterior cerebellar arteries (Aa. cerebelli caudales) which pass outward around the medulla behind the pons to the cerebellum, to which they are distributed after giving twigs to the medulla and pons.

3. The small auditory artery (A. auditiva interna) accompanies the eighth nerve to the internal ear. It often arises from the posterior cerebellar.

4. Anterior cerebellar arteries (Aa. cerebelli nasales). These are very variable in number and origin. There are often two or three on either side and they frequently arise from the posterior cerebrals. They pass outward in front of the pons and supply the anterior part of the cerebellum.

The =posterior cerebral arteries= (Aa. communicantes caudales) diverge at an acute angle and join the posterior communicating branches of the internal carotid arteries on the inferior surface of the cerebral peduncles. They are connected by a transverse branch and by a network of fine twigs which form often a rete mirabile.

THE INTERNAL CAROTID ARTERY (Figs. 434, 436, 437)

This artery (A. carotis interna) is somewhat smaller than the occipital. It usually arises just behind that artery, crosses its deep face, and runs upward and forward on the guttural pouch to the foramen lacerum. It is closely related to the vagus nerve and the superior cervical ganglion of the sympathetic nerve, fibers from which accompany it. It is crossed externally by the ninth and twelfth cranial nerves and the pharyngeal branch of the vagus. It passes through the inferior petrosal sinus and enters the cavernous sinus, within which it forms an =S=-shaped curve. It is connected with the opposite artery by a transverse branch (A. intercarotica), which lies behind the pituitary body in the intercavernous sinus. A branch (A. caroticobasilaris) sometimes connects it with the basilar artery. It then perforates the dura mater, gives off the posterior communicating branch, and passes forward and divides at the side of the optic chiasma into anterior and middle cerebral arteries.

The =posterior communicating artery= (A. communicans posterior) turns backward and joins the posterior cerebral. It gives off the =deep cerebral artery= (A. cerebri profunda) which winds around the cerebral peduncle and is distributed chiefly to the mid-brain. A smaller collateral branch is the anterior choroid artery (A. chorioidea nasalis) which passes along the optic tract and is distributed in the choroid plexus of the lateral ventricle.

The =anterior cerebral artery= (A. cerebri anterior) unites with the corresponding branch of the opposite artery above the optic chiasma. From this junction proceeds the =artery of the corpus callosum= (A. corporis callosi), which turns around the genu of the corpus callosum, enters the great longitudinal fissure, divides into two branches, and is distributed to the inner aspect of the cerebral hemispheres. A small =anterior meningeal= branch (A. meningea nasalis) of the anterior cerebral is distributed to the anterior part of the dura, and assists in forming a network in the ethmoidal fossa (Rete ethmoidale), anastomosing with the ethmoidal branch of the ophthalmic artery.

The =middle cerebral artery= (A. cerebri media) passes outward in the lateral fissure (of Sylvius) and divides into branches on the outer surface of the hemisphere.

The =arterial circle of Willis= (Circulus arteriosus) (Fig. 434) is formed at the interpeduncular space of the base of the brain by the union of the anterior cerebral arteries in front, by the diverging posterior cerebral arteries behind, and is completed laterally by the junction of the latter with the posterior communicating arteries and by the internal carotid. It is irregularly polygonal in outline.