Part 174

3. The =anterior branch= divides after a very short course into the dorsal and middle colic arteries.

(1) The =dorsal=, =left=, or =retrograde colic artery= (A. colica dorsalis) is a large vessel which passes along the dorsal (fourth and third) parts of the great colon to the pelvic flexure, where it joins the ventral colic artery.

(2) The =middle colic artery= or =first artery of the small colon= (A. colica media) is a much smaller vessel which passes to the origin of the small colon, enters the colic mesentery, and forms an arch by joining the first branch of the posterior mesenteric artery close to the lesser curvature of the bowel. It sends an anastomotic branch to the dorsal colic artery.

III. The =renal arteries= (Aa. renales), right and left, are relatively large vessels which arise from the aorta near the anterior mesenteric. The =right artery= is the longer of the two. It crosses over the dorsal surface of the vena cava to the right and somewhat forward. At the hilus it divides into several (five to eight) branches; some of these enter the gland at the hilus, while others pass to the ventral surface and enter there. The =left artery= is short and usually arises a little further back; it passes directly outward to the kidney and is then disposed like the right one. Small collateral branches are supplied to the ureters and the adrenals. The latter also receive small vessels directly from the aorta. The distribution within the kidney has been described.

Variations in the renal arteries are frequent. Two or more arteries may occur on one or both sides. Accessory arteries are more common on the left side and usually enter the posterior pole of the gland. They may arise from the aorta, the external iliac, or the circumflex iliac artery.

IV. The =posterior= or =small mesenteric artery= (A. mesenterica caudalis) is an unpaired vessel which arises from the ventral face of the aorta at the fourth lumbar vertebra, i. e., about five or six inches (ca. 12 to 15 cm.) behind the origin of the anterior mesenteric artery. It is much smaller than the latter and supplies the greater part of the small colon and rectum. It descends in the colic mesentery and divides into two branches. The anterior branch (A. colica sinistra) gives off three or four arteries which divide and form anastomotic arches close to the bowel. The first arch is formed by union with the middle colic branch of the anterior mesenteric. The posterior branch or anterior hæmorrhoidal artery (A. hæmorrhoidalis cranialis) passes backward in the upper part of the mesentery and the mesorectum and terminates near the anus by anastomosing with the internal pudic artery. Three or four of its anterior collateral branches form arches.

V. The =spermatic arteries= (Aa. spermaticæ internæ), right and left, are long slender arteries which arise from the aorta near the posterior mesenteric and supply the testicle and epididymis. Each passes backward in a narrow fold of peritoneum (Plica vasculosa) to the internal inguinal ring and descends through the inguinal canal to the scrotum. In the canal and scrotum it pursues a very flexuous course in the anterior border of the spermatic cord, surrounded by the pampiniform plexus of veins, and associated closely with the spermatic nerves and lymphatics and unstriped muscle-fibers. It passes between the epididymis and testicle, runs in flexuous fashion along the attached border of the latter, turns around the posterior pole, and runs forward on the free border to the anterior pole. The largest branches arise from its ventral part, pass tortuously up either side of the gland embedded in the tunica albuginea, and give off fine branches to the gland substance. Small collateral branches are detached to the ureter and the spermatic cord.

The =utero-ovarian arteries= in the female correspond to the preceding vessels, but are much shorter. Each is placed in the anterior part of the broad ligament of the uterus and divides into ovarian and uterine branches. The =ovarian branch= (A. ovarica) pursues a flexuous course to the ovary, which it supplies. The =uterine branch= (A. uterina cranialis) passes to the concave border of the cornu of the uterus, which it supplies, anastomosing with the uterine artery.

VI. The =lumbar arteries= (Aa. lumbales) are in series with the intercostal arteries and have a similar origin and distribution. There are usually six pairs of lumbar arteries, of which five arise from the aorta and the sixth from the internal iliac or the lateral sacral at the junction of the last lumbar vertebra and the sacrum. Each passes across the body of a lumbar vertebra to the intertransverse space, gives branches to the sublumbar muscles, and divides into dorsal and ventral branches. The =dorsal branch= (Ramus dorsalis), the larger of the two, passes upward to ramify in the extensor muscles of the spine and the skin of the loins; it gives off a =spinal branch= (Ramus spinalis) which comports itself like the corresponding branch of an aortic intercostal artery. The =ventral branch= (Ramus ventralis) runs outward in the intertransverse space, passes between the transversus and obliquus internus abdominis, gives branches to these muscles, and ends in the obliquus externus, the panniculus, and the skin of the flank.

FIG. 451.—DISSECTION OF PELVIS, THIGH, AND UPPER PART OF LEG OF HORSE, INTERNAL VIEW.

L, Lumbar vessels; V.c., posterior vena cava; A, aorta (termination); C, sympathetic trunk; 1, internal iliac artery, 2, lateral sacral artery; 3, middle coccygeal artery; 4, lateral coccygeal artery; 5, umbilical artery (cut off); 6, internal pudic artery; 6′, vesico-prostatic artery; 7, obturator artery; 8, a. profunda penis (from left obturator); 9, posterior gluteal (or ischiatic) vessels; 10, external iliac artery; 11, 11′, deep femoral artery; 12, prepubic artery; 13, branch of deep femoral artery; 14, great sciatic nerve; 15, posterior gluteal nerve; 16 (above), internal pudic nerve; 16 (below near pelvic outlet), suburethral venous plexus; 17, sacro-coccygeus inferior; 18, coccygeus; 19, recto-coccygeus; 20, retractor ani; 21, 21′, two heads of obturator internus; 22, symphysis pelvis; 23, prepubic tendon; 24, suspensory ligament of penis; 25, retractor penis muscle; 26, bulbo-cavernosus muscle; 27, urethra (cross-section); 28, crus penis (cross-section); 29, suspensory ligaments of anus (= upper part of retractor penis muscle); 30, anal lymph glands; 31, tibial nerve. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

THE INTERNAL ILIAC ARTERY

The =internal iliac= or =hypogastric arteries= (Aa. hypogastricæ) result from the bifurcation of the aorta under the fifth or sixth lumbar vertebra. They diverge at an angle of about 60 degrees, and each passes backward under the wing of the sacrum, then inclines downward on the pelvic surface of the shaft of the ilium, and divides a little above the psoas tubercle into iliaco-femoral and obturator arteries. The chief branches are as follows:

FIG. 452.—DISSECTION OF PERINEUM OF HORSE (MALE).

1, Bulbo-urethral (Cowper’s) gland; 2, transversus perinei muscle (?); 3, ischio-urethral muscle; 4, retractor penis muscle; 5, bulbo-cavernosus muscle. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

1. The last pair of =lumbar arteries= pass up through the foramina at the junction of the last lumbar vertebra and the sacrum.

2. The =internal pudic artery= (A. pudenda interna) arises near the origin of the internal iliac. It passes backward and somewhat downward, at first along the iliac head of the obturator internus, then above the superior ischiatic spine on the inner surface of the sacro-sciatic ligament, perforates the latter and runs for a variable distance in its substance or on its external face. It then re-enters the pelvic cavity, passes backward on the retractor ani to the ischial arch, and divides into the perineal and the artery of the bulb in the male, perineal and artery of the clitoris in the female. It is accompanied by the internal pudic nerve. Its chief branches are as follows:

(1) The =umbilical artery= is given off from the internal pudic about an inch (ca. 2 to 3 cm.) from the origin of that vessel. It is a very large artery in the fœtus, in which it curves downward and forward at the side of the bladder in the edge of the umbilical fold of peritoneum, passes through the umbilical opening, becomes a component of the umbilical cord and ramifies in the fœtal placenta. After birth it extends only to the vertex of the bladder and is much reduced. Its lumen is almost obliterated and its wall is very thick, giving the vessel a cord-like character, hence it is commonly termed the round ligament of the bladder. It gives off small =vesical branches= (Aa. vesicates craniales) to the bladder, and twigs to the prostate and vasa deferens in the male. In the mare a small branch runs forward along the ureter into the broad ligament of the uterus.

FIG. 453.—DISSECTION OF PERINEUM OF MARE.

1, Sacro-coccygeus superior; 2, sacro-coccygeus lateralis; 3, intertransversales; 4, sacro-coccygeus inferior; 5, recto-coceygeus; 6, sphincter ani externus; 6′, branch connecting sphincter ani with constrictor vulvæ; 7, vestibular bulb; 8, tuber ischii; 9, sacro-sciatic ligament. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

(2) The =middle hæmorrhoidal= or =vesico-prostatic artery= (A. hæmorrhoidalis media) in the male arises usually near the prostate and runs backward lateral to the rectum. It supplies branches to the rectum, bladder, urethra, and accessory genital glands. In the female the homologous vessel is much larger and gives off the =posterior uterine artery= (A. uterina caudalis). This runs forward on the side of the vagina, to which it gives branches, and ramifies on the body of the uterus, anastomosing with the anterior and middle uterine arteries. (In some cases this artery arises from the internal iliac or the umbilical.)

(3) The =perineal artery= (A. perinei) is relatively small in the male. It passes upward at the side of the anus, which it supplies, and gives twigs to the bulbo-cavernosus muscle and the skin of the perineum. In the female it is large and is distributed to the anus and vulva, and gives a large branch to the vestibular bulb.

(4) The =artery of the bulb= (A. bulbi urethræ) may be regarded as the direct continuation of the internal pudic in the male. It lies at the side of the urethra above the ischial arch, dips under the bulbo-cavernosus muscle, and ramifies in the corpus spongiosum. Before doing so it gives off a small branch which turns around the ischial arch to reach the dorsum penis, and anastomoses with the deep branch of the obturator.

(4a) The =artery of the clitoris= (A. clitoridis) is the homologue in the female of the preceding vessel, but is much smaller. It passes to the ventral surface of the vulva with the internal pudic nerve, supplies the clitoris, and gives twigs to the vulva.

FIG. 454.-CROSS-SECTION OF TAIL OF HORSE.

1, Dorso-lateral coccygeal vessels and nerve; 2, ventro-lateral coccygeal artery and nerve; 3, middle coccygeal artery; 4, sacro-coccygeus superior; 4′, sacro-coccygeus lateralis; 5, 5′, intertransversales; 6, sacro-coccygeus inferior; 7, recto-coccygeus; 8, coccygeal fascia; 9, fibro-cartilage between fourth and fifth coccygeal vertebra. The veins are black. ]

3. The =lateral sacral artery= (A. sacralis lateralis) arises at the lumbo-sacral articulation (Fig. 451). It passes backward under the wing of the sacrum, then along the pelvic surface of the bone below the inferior sacral foramina and the nerves emerging from them, and is continued by the lateral coccygeal artery. The branches are as follows:

(1) =Spinal branches= (Rami spinales) enter the vertebral canal through the foramen between the last lumbar vertebra and the sacrum and through the ventral sacral foramina. They give off branches to the spinal cord and its membranes which reinforce the ventral spinal artery, and others which emerge through the dorsal sacral foramina and supply the muscles and skin of the croup.