Circulatory System of the Dog

The Pericardium and Heart

The pericardium is attached to the sternal part of the diaphragm by a fibrous band, and is connected with the sternum only by the mediastinal pleura.

The heart is almost globular in form, the apex being much blunter than in the other animals, and marked by a notch. Its long axis is almost parallel with the sternum and the apex (covered by the pericardium) is directed against the sternal part of the diaphragm. Its weight averages about 0.8 to 1 per cent. of the body-weight.

The base extends forward to a transverse plane through the lower ends of the third pair of ribs. The apex lies opposite the sixth costal cartilage in expiration. On the right side the pericardium is in direct contact with the chest-wall over a narrow triangular area (base anterior) from the fourth to the sixth intercostal space inclusive. On the left side the area of contact is smaller and is at the fourth, fifth, and sixth cartilages.

The two coronary arteries may arise by a common trunk. Each divides into circumflex and descending branches. The left artery is twice as large as the right one.

The great coronary vein ascends in the left longitudinal groove to the coronary groove, in which it winds around the posterior face of the heart to the right side and opens into the right atrium below the posterior vena cava. Near its termination it receives one or two veins which ascend along the right longitudinal furrow. Several small veins from the wall of the right ventricle open into the right atrium separately or by a common trunk.

The Arteries

There is no common brachiocephalic trunk (anterior aorta). Two large vessels arise from the convexity of the aortic arch; these are the brachiocephalic and left brachial arteries. The brachiocephalic gives off the two carotid arteries, no common trunk being present. It frequently also supplies the posterior thyroid artery. The intrathoracic branches of the brachial arteries are as follows:

A common trunk gives off the dorsal, subcostal, and superior cervical arteries. The dorsal artery passes out in front of the first rib, and the superior cervical through the first intercostal space. The latter is relatively small.

The vertebral artery may arise either in front of or behind the trunk above mentioned; in the latter case it crosses the inner face of the trunk. It passes along the neck in typical fashion to the third cervical vertebra and divides into three branches. The largest of these supplies the muscles in this region, compensating for the absence of branches of the superior cervical artery. The second passes between the second and third cervical vertebræ into the vertebral canal and unites with the opposite artery and a branch of the occipital artery to form the basilar. The third branch is the small continuation of the parent trunk; it passes to the wing of the atlas and anastomoses with the occipital.

The inferior cervical artery gives off the superficial cervical which ascends in front of the supraspinatus.

The internal thoracic artery is large and sends perforating branches to the pectoral mammary glands.

The external thoracic artery arises outside of the thorax and sends branches to the mammary glands.

The common carotid arteries, left and right, arise in that order from the brachiocephalic. In the neck the right one lies on the trachea, the left on the œsophagus. Collateral branches are the posterior thyroid (which may, however, arise from the right or left brachial or the inferior cervical), pharyngeal, thyro-laryngeal muscular, tracheal, and glandular (to the submaxillary gland). Each carotid divides under the wing of the atlas into occipital and internal and external carotid branches.

The occipital artery is small. It gives off condyloid and posterior meningeal branches, passes up through the notch on the anterior margin of the wing of the atlas, and is distributed to the muscles of this region. Its cerebrospinal branch passes through the intervertebral foramen, and unites in the vertebral canal with its fellow and a branch from the vertebral artery to form the basilar. A retrograde branch anastomoses with the vertebral as in the horse.

The internal carotid artery, also small, passes to the foramen lacerum posterius, enters the carotid canal and forms a bend before entering the cranium through the carotid foramen. It forms a plexus which is connected by branches with the middle meningeal and ophthalmic arteries. It then perforates the dura mater and enters into the formation of the arterial circle (of Willis).

The external carotid artery is the direct continuation of the common carotid. It passes along the lateral wall of the pharynx, emerges from beneath the digastricus, and divides into superficial temporal and internal maxillary arteries. It gives off the large flexuous lingual artery which has a course similar to that of the horse. The external maxillary, smaller than the lingual, passes along the upper border of the digastricus, gives off the sublingual, gains the anterior border of the masseter, and divides into superior and inferior labial and the angularis oris. The last-named vessel passes forward on the cheek between the labials to the angle of the mouth. The sublingual artery passes at first along the upper border of the digastricus and continues between the mylo-hyoideus and the ramus of the mandible. The posterior auricular artery arises at the anterior border of the digastricus, gives branches to the salivary glands and the adjacent muscles, and ascends on the convex face of the concha. It divides into two branches which return along the borders of the external ear.

The superficial temporal artery, after giving off the anterior auricular artery and a small transverse facial artery, turns forward under the temporal fascia toward the eye, and divides into upper and lower branches which supply the frontal region and the eyelids. It also supplies branches to the parotid gland and the masseter and temporalis muscles.

The internal maxillary artery pursues a course similar to that of the horse. The mental branches of the inferior alveolar (or dental) artery are of considerable size and are distributed in the lower lip and gums. Two or three deep temporal arteries are present. The anterior one may arise from the buccinator; it gives off the middle meningeal, which sends a branch to the carotid plexus. The ophthalmic artery gives off a branch which enters the cranium through the foramen lacerum orbitale and connects with the internal carotid; this is termed the internal ophthalmic and gives off the arteria centralis retinæ. The superficial branches of the infraorbital artery replace the dorsal and lateral nasal arteries, and compensate for the small size of the superior labial.

The brachial artery in its course in the arm presents no special features. At the elbow it passes between the biceps and the pronator teres, descends (as the median) under the flexor carpi radialis about a third of the way down the forearm, and divides into radial and ulnar arteries. Among its collateral branches are: (1) The subscapular artery, which passes up between the subscapularis and teres major, turns around the posterior angle of the scapula and terminates in the supraspinatus, deltoid, trapezius, and mastoido-humeralis. In about half the cases it gives off the anterior circumflex, which often arises with the posterior circumflex. Its other branches resemble those of the horse. (2) The anterior circumflex artery (in about half the cases). (3) The deep brachial arises about a third of the way down the arm. (4) The bicipital artery (for the biceps) is given off at the lower part of the arm. (5) The proximal collateral radial artery (not present in the horse) arises at the lower fourth of the arm, crosses over the terminal part of the biceps, descends on the extensor carpi radialis, and concurs with a branch of the volar interosseous in forming the rete carpi dorsale. It often supplies the bicipital artery. (6) The anterior radial artery (A. collateralis radialis distalis) is very small. (7) The common interosseous artery is given off a little below the elbow. It supplies branches to the flexor muscles and the dorsal interosseous artery, which passes through the interosseous space, gives branches to the extensor muscles, and by its terminal twigs concurs in the formation of the rete carpi dorsale. The direct continuation of the trunk is the volar interosseous artery, which descends under cover of the pronator quadratus, gives off a branch to the rete carpi dorsale and the fifth volar metacarpal artery, and terminates by joining the volar branch of the radial artery to form the deep volar arch. (8) The volar antibrachial artery (Ramus volaris antibrachii) arises below the interosseous and descends at first under the flexor carpi radialis, then between the heads of the deep flexor, and divides into ascending and descending branches.

The radial artery, the smaller terminal of the brachial, descends along the inner border of the radius, and divides near the carpus into dorsal and volar branches. The dorsal branch assists in forming the rete carpi dorsale. The larger volar branch descends behind the inner border of the carpus and joins the end of the volar interosseous in forming the deep volar arch. From this arch three deep volar metacarpal arteries descend in the second, third, and fourth intermetacarpal spaces and concur with the corresponding superficial volar and the dorsal arteries to form the common digitals.

The ulnar artery descends on the deep flexor of the digit, and sends a connecting branch to the radial above the carpus. Near the middle of the metacarpus it gives off the first volar metacarpal artery and divides into the second, third, and fourth superficial volar metacarpal arteries. The first volar artery unites with the corresponding dorsal artery to form the first common digital artery, which supplies proper digital arteries for the opposed surfaces of the first and second digits. The superficial volar arteries descend in the spaces between the other metacarpal bones, unite near the metacarpo-phalangeal joints with the corresponding deep volar and dorsal metacarpal arteries to form three common digital arteries. Each of the latter divides after a short course into two proper digital arteries, which run along the opposed surfaces of the second to the fifth digits. The external digital artery of the fifth digit comes from the superficial volar arch formed by the union of a branch of the volar interosseous with the fourth superficial volar metacarpal artery.

The rete carpi dorsale gives off four dorsal metacarpal arteries which descend in the intermetacarpal spaces and unite near the metacarpo-phalangeal joints with the volar arteries to form the common digitals.

The thoracic aorta supplies the last nine or ten pairs of intercostal arteries, but no anterior phrenic arteries. It gives off two or more œsophageal branches, in addition to the broncho-œsophageal, which arises close to or with the sixth intercostal and ramifies in the usual manner.

The abdominal aorta, after giving off the external iliac arteries, continues about half an inch to an inch (ca. 1 to 3 cm.) under the last lumbar vertebræ, gives off the internal iliacs, and is continued by the middle sacral artery. This small vessel runs backward under the sacrum and coccygeal vertebræ and gives off branches in segmental fashion.

The cœliac artery gives off the hepatic artery and forms a short gastro-splenic trunk. The hepatic artery gives off several hepatic branches, and the pyloric or right gastric artery, which passes along the lesser curvature of the stomach, anastomoses with the (left) gastric artery, and is continued by the gastro-duodenal. This divides near the pylorus into right gastro-epiploic and pancreatico-duodenal. The (left) gastric artery passes to the lesser curvature of the stomach and ramifies chiefly on the left part of the stomach, giving off a branch which anastomoses with the pyloric branch of the hepatic. The splenic artery gives off pancreatic branches, and reaches the lower part of the spleen. It gives off the left gastro-epiploic, and a branch which passes to the dorsal end of the spleen, which it supplies, besides giving off twigs to the left extremity of the stomach.

The anterior mesenteric artery arises close behind the cœliac. It gives off a common trunk for two colic arteries and the ileo-cæco-colic. The former supply the transverse and the anterior part of the left or descending colon, while the latter divides into branches for the ileum, cæcum, and first part of the colon. The trunk is continued as the artery of the small intestine, giving off fourteen to sixteen branches (Aa. jejunales), which form a series of anastomotic arches in the mesentery near the bowel.

Two phrenico-abdominal arteries (Aa. phrenicæ caudales) come off from the aorta behind the anterior mesenteric, and divide into phrenic and abdominal branches. Each phrenic artery diverges from its fellow in descending on the abdominal surface of the corresponding crus of the diaphragm to the sternal part. The inner branches anastomose with those of the opposite artery, the outer branches with intercostal arteries chiefly. The abdominal arteries give branches to the lumbar muscles, the renal fat and capsules, and ramify in the oblique abdominal muscles.

The renal and spermatic arteries offer no special features.

The utero-ovarian artery divides near the ovary into three or four branches which supply the ovary and uterus, anastomosing with the uterine artery.

The posterior mesenteric artery is small. It divides into two branches which supply the terminal part of the colon and the anterior part of the rectum.

The circumflex iliac artery usually arises from the terminal part of the aorta.

The external iliac artery usually gives off no collateral branches.

The femoral artery has the usual course. The deep femoral gives off the posterior abdominal and external pudic arteries by a very short common trunk or separately. In the female the external pudic divides into anterior and posterior branches; the former runs forward, supplies branches to the inguinal lymph glands, the mammary glands and skin, and anastomoses with the mammary branch of the internal thoracic artery; the posterior branch pursues a flexuous course between the thighs to the vulva, where its terminal branches anastomose with the internal pudic artery. The anterior femoral may arise by a short common trunk with the external circumflex. The former, accompanied by the anterior branch of the femoral nerve, dips in between the rectus femoris and vastus internus. The external circumflex artery passes forward between the sartorius internally and the rectus femoris and tensor fasciæ latæ externally and supplies branches to these muscles and the glutei. In addition to muscular branches of considerable size, an articular branch (A. genu suprema) arises a little below the middle of the thigh and runs downward and forward to the inner face of the stifle joint. The saphenous artery is large. It arises from the inner surface of the femoral a little below the middle of the femur, descends superficially to the upper part of the leg and divides into two branches. The smaller anterior branch (Ramus dorsalis) passes obliquely downward and forward across the inner surface of the tibia to the flexion surface of the hock and terminates in three superficial dorsal metatarsal arteries. The posterior branch (Ramus plantaris) passes down on the inner face of the gastrocnemius and the long digital flexor. It gives off a branch (A. tarsea lateralis) which descends obliquely to the external surface of the tarsus, and at the posterior face of the tarsus detaches the internal and external plantar arteries; these descend on either side of the deep flexor tendon and unite with the perforating metatarsal artery to form the proximal plantar arch. The artery continues down the middle of the plantar surface of the metatarsus and divides near the metacarpo-phalangeal joints into three superficial plantar metatarsal arteries. These vessels unite with three deep plantar metatarsal arteries which descend from the proximal plantar arch and with branches from the dorsal metatarsal arteries. From these anastomoses four digital arteries result; of these, the central two have a common digital trunk.

The popliteal and posterior femoral arteries present nothing of special interest.

The posterior tibial artery is small, being replaced largely by the saphenous. It supplies twigs to the flexor muscles at the upper part of the leg.

The anterior tibial artery descends on the anterior face of the tibia and tarsus and is continued as the perforating metatarsal artery, which passes through the upper part of the space between the second and third metatarsal bones and concurs with the plantar branches of the saphenous in the formation of the plantar arch. Besides muscular and articular branches, the anterior tibial supplies the external or fifth dorsal metatarsal artery for the outer side of the fifth digit. At the proximal part of the metatarsus it gives off three deep dorsal metatarsal arteries which descend in the intervals between the metatarsal bones and concur with the superficial dorsal and the plantar arteries in the formation of the digital arteries. The latter resemble in general arrangement the corresponding arteries of the thoracic limb.

The internal iliac artery (Fig. ) runs backward and a little outward across the ilio-psoas, and on reaching the ilium divides into parietal and visceral branches. The parietal branch is the larger. It runs backward on the lateral wall of the pelvis, passes out through the lesser sciatic notch, and breaks up into branches which supply the muscles in this region like the posterior gluteal (or ischiatic) artery of the horse. It gives off the following branches: (1) The ilio-lumbar artery passes outward between the ilio-psoas and the shaft of the ilium and ramifies in the gluteus medius, giving branches to the ilio-psoas and tensor fasciæ latæ; it may arise from the internal iliac. (2) The anterior gluteal artery passes out behind the posterior superior spine of the ilium and supplies branches to the glutei. (3) Muscular branches go to the obturator internus, coccygeus, and retractor ani. (4) The lateral coccygeal artery (A. caudalis lateralis superficialis) passes back at first on the outer face of the coccygeus and continues beneath the skin along the side of the tail. The visceral branch or internal pudic artery passes back below the parietal branch on the lateral face of the rectum, retractor ani, and coccygeus. Its chief collateral branches are: (1) The umbilical artery, which pursues a flexuous course and supplies twigs to the bladder, ureter, and vas deferens. In the bitch it gives off a large uterine artery which ramifies chiefly in the body and neck of the uterus and the vagina and anastomoses with the utero-ovarian. (2) The middle hæmorrhoidal artery arises near the ischial arch, passes upward and forward on the lateral surface of the rectum, gains its dorsal surface and anastomoses with the posterior mesenteric; it supplies the rectum and the anus, together with its muscles and glands. (3) A small perineal artery is detached to the perineum. In the male the trunk turns around the ischial arch as the artery of the penis (A. penis). This vessel, after giving off the deep artery of the penis (A. profunda penis), which supplies the artery of the bulb (A. bulbi urethræ) and enters the corpus cavernosum, is continued as the dorsal artery of the penis (A. dorsalis penis) along the dorsum penis. In the female the terminal branches of the trunk go to the vulva, vestibular bulb, and clitoris.

The Veins

The arrangement of the veins is, of course, correlated in general with the arterial system, but a few special features are worthy of mention.

The anterior vena cava is formed by the junction of short right and left brachiocephalic (or subclavian) veins, and each of the latter results from the confluence of jugular and brachial veins.

The vena azygos resembles that of the horse, and receives at the ninth or tenth thoracic vertebra a vena hemiazygos.

Two jugular veins are present on each side. The external jugular vein is formed by the union of external and internal maxillary veins at the posterior border of the submaxillary gland. The two external jugulars are commonly united by a transverse branch below the cricoid cartilage. Each passes along the neck on the sterno-cephalicus, covered only by the skin and panniculus, dips under the cleido-cervicalis, and joins the internal jugular.

The external maxillary vein arises on the lateral nasal region by the junction of the dorsal nasal vein with the angularis oculi. Near the infraorbital foramen it receives the lateral nasal vein, and a little lower the superior labial. In its course along the anterior border of the masseter it receives the vena reflexa, which arises in the pterygo-palatine fossa by the junction of infraorbital, sphenopalatine, and palatine radicles, together with a branch from the cavernous sinus. At the lower border of the mandible it is joined by the inferior labial vein, which receives the buccinator vein. The lingual vein is connected with its fellow by a superficial transverse branch at the insertion of the sterno-hyoidei. The sublingual and submental veins terminate in a variable manner, but often form a common trunk which joins the lingual.

The internal maxillary vein arises from the pterygoid plexus, formed chiefly by dorsal lingual, inferior alveolar, deep temporal, pterygoid, and meningeal tributaries. It receives the superior cerebral, auricular, superficial temporal, transverse facial, and masseteric veins, and often a trunk formed by the union of the inferior cerebral and occipital veins.

The internal jugular vein results usually from the junction of laryngeal and thyroid veins, but in some cases it is formed by the confluence of the inferior cerebral and occipital veins.

The brachial and radial veins are satellites of the arteries.

The ulnar vein is usually double. It unites below the carpus with a branch of the interosseous vein to form the superficial venous arch.

The cephalic vein accompanies the ulnar artery in the forearm and joins the superficial venous arch below.

The accessory cephalic vein arises from the union of three dorsal metatarsal veins. It joins the cephalic about the middle of the forearm.

There are three short volar metacarpal veins which open into the superficial venous arch. They are formed above the metacarpo-phalangeal joints by the junction of the volar digital veins, of which there are two for each of the chief digits. The volar vein of the first digit joins the superficial venous arch.

Each of the chief digits has two dorsal digital veins, while the first digit has one.

The posterior vena cava, its collateral affluents, and common iliac radicles present no special features of importance.

The internal iliac vein corresponds in regard to its tributaries with the branches of the artery, except that it is not divided into dorsal and ventral branches.

The external iliac, femoral, and popliteal veins with their collateral tributaries are satellites of the arteries.

The anterior tibial vein is usually double, and the posterior tibial vein is very small.

The internal saphenous vein is the upward continuation of the internal plantar metatarsal vein. It communicates by a large branch with the dorsal metatarsal vein, and ascends the leg as a satellite of the saphenous artery and its plantar (posterior) branch. The recurrent tarsal or external saphenous vein is larger. It is formed at the lower part of the leg by the union of dorsal and external plantar metatarsal veins, crosses the outer surface of the leg obliquely upward and backward, ascends behind the gastrocnemius and joins the posterior femoral vein.

The metatarsal and digital veins resemble in general the corresponding vessels of the fore limb.

The Lymph Vessels and Glands

The cisterna chyli is large and fusiform. The thoracic duct may be single throughout, but often divides anteriorly into two branches, which may then unite and form a dilatation which receives the left tracheal duct and the vessels from the fore limb. The primitive plexiform arrangement persists in varying degree. The duct terminates in the left brachiocephalic vein.

The tracheal ducts accompany the internal jugular veins. The right duct opens into the right brachiocephalic vein.

The submaxillary lymph glands (Fig. ) are commonly three in number on each side. They are situated in the angle between the masseter and the submaxillary salivary gland, in relation to the external maxillary vein and covered only by the skin and panniculus. The number may be reduced to two or one with a corresponding increase in size.

A small round parotid or auricular lymph gland is situated superficially between the upper part of the posterior border of the masseter and the parotid gland.

The pharyngeal (or retropharyngeal) lymph glands, one or two in number on each side, lie on the pharynx chiefly under cover of the submaxillary salivary gland.

The prescapular or superficial cervical lymph glands lie on the serratus magnus at the anterior border of the supraspinatus, embedded in a mass of fat. Usually two or three are present on each side, but there may be only one. They are oval and are about an inch long in a dog of medium size when two are present.

The axillary lymph gland lies in a mass of fat on the inner face of the lower part of the teres major. It is discoid.

The cubital lymph gland is inconstant. It is situated on the deep face of the long head of the triceps or on the latissimus dorsi.

The mediastinal lymph glands are small and quite variable in arrangement. Usually one or two glands are found on the course of the internal thoracic vessels at the second segment of the sternum.

The bronchial lymph glands are commonly four in number. The largest is situated in the angle of divergence of the chief bronchi, two lie on the origin of the right apical bronchus, and the fourth is between the aortic arch and the anterior face of the left bronchus. They are commonly pigmented.

The lumbar lymph glands are small.

The mesenteric lymph glands are represented chiefly by a long flattened mass (formerly known as the pancreas Aselli) which lies in the mesentery along the course of the anterior mesenteric artery and vein. It is succeeded by three or four glands which lie along the portal vein and appear to receive vessels from the stomach, spleen, liver, pancreas, and great omentum. A few very small nodes occur along the colon.

The iliac lymph glands are relatively large and are situated at the termination of the aorta and the origin of the iliac arteries. Commonly four are present. Of these, the two largest lie on the psoas minor on each side in relation to the aorta and vena cava respectively. The other two are in the angle of divergence of the internal iliac arteries.

The superficial inguinal lymph glands are situated in the subcutaneous fat behind the external inguinal ring. They are relatively large, especially when there is only one on each side; two smaller ones may be found instead.

The popliteal lymph gland—usually unique, but sometimes double—lies in a pad of fat on the upper part of the gastrocnemius between the biceps femoris and semitendinosus. It may project back so as to be superficial and palpable.