THE INFERIOR CAVAL SYSTEM. 907 



tributaries are venae comites of the branches of the artery, and its usually single 

 main stem passes through the greater sacro-sciatic foramen below the pyriformis to 

 empty into the internal iliac. 



Anastomoses of comparati\ely large calibre occur between the extrapelvic 

 portions of the sciatic vein and the internal circumflex and hrst perforating tribu- 

 taries of the deep femoral vein, thus establishing a collateral venous path between 

 the tributaries of the internal and external iliacs. 



5. The Internal Pudic Vein. — The internal pudic vein (v. pudenda interna) is 

 associated throughout the greater part of its course with the artery of the same name. 

 It differs, however, somewhat in its origin, since it is not the direct continuation of the 

 dorsal vein of the penis (or clitoris), although it communicates with that vessel by a 

 small branch immediately below the symphysis pubis, but is rather the continuation 

 of the veins of the corpus cavernosum which accompany the artery to that structure. 

 It is throughout the most of its length double, anastomoses between the two stems 

 surrounding the internal pudic artery. It has its origin between the two layers of the 

 triangular ligament of the perineum and passes backward into the ischio-rectal fossa, 

 lying with the artery at the side of that cavity in a canal {Alcock' s canal) formed by a 

 splitting of the lower edge of the obturator fascia. It leaves the ischio-rectal fossa by 

 the lesser sacro-sciatic foramen and, curving around the spine of the ischium, enters 

 the pelvis through the lower part of the greater sacro-sciatic foramen and empties 

 into the internal iliac. 



In addition to the communication with the dorsal vein of the penis (or clitoris) 

 already mentioned, the internal pudic vein makes near its origin a connection with the 

 pudendal plexus and, as it curves over the spine of the ischium, with the sciatic vein. 



It possesses several valves arranged in a rather characteristic manner. Through- 

 out its course through the perineum it is valveless, but both its terminal portion and 

 its communication with the pudendal plexus possess valves whose concavities look in 

 the one case towards the internal iliac and in the other towards the plexus. Blood 

 contained in the perineal portion of the vein may flow, therefore, either towards the 

 internal iliac directly or to the pudendal plexus (Fenwick), and the communication 

 with the latter cannot well be regarded as the origin of the vein, as is sometimes 

 done. 



Tributaries. — In addition to [a) the vein of the corpus cavernosum (v. profunda penis vel 

 clitoridis) already mentioned, the internal pudic \ein receives numerous tributaries which cor- 

 respond with the branches of the artery. Among these may be mentioned : {b) the veins of the 

 bulb (vv. bulbi urethrae), which are quite numerous and issue from the bulb of the urethra or from 

 the bulbus vestibuH in the female, these latter vessels being quite large ; (r) the superficial peri- 

 neal veins (vv. scrotales posteriores), which return the blood from the integument and superficial 

 muscles of the perineum and from the posterior surface of the scrotum and the posterior portion 

 of the labia majora, anastomosing in these structures with the tributaries of the external pudic 

 veins; {d) the inferior hemorrhoidal veins (vv. haemorrhoidales inferiores), which traverse the 

 ischio-rectal space from the neighborhood of the anus, where they make communications with 

 the hemorrhoidal plexus of the rectum. 



6. The Obturator Vein. — The obturator vein (y. obturate ria) accompanies the 

 obturator artery and shares in the variations which that vessel presents (page 814). 

 It takes its origin in the adductor muscles of the thigh, its tributaries uniting to form 

 an internal and an external branch, which curve around the margins of the obturator 

 foramen. The vein formed by the union of these two branches passes through the 

 opening in the upper part of the obturator membrane and passes across the lateral 

 pelvic wall, lying immediately below the artery. It opens, as a rule, into the internal 

 iliac vein. 



Its communications are somewhat extensive and important. Its external tribu- 

 tary branch receives branches from the scrotum or labia majora and through these 

 communicates v/ith the external pudic veins. At its passage through the opening 

 in the obturator membrane it receives branches from the obturator plexus, which 

 cover both surfaces of the membrane and drain the obturator muscles, and also a 

 branch which passes downward and inward upon the inner surface of the os pubis, 

 frequently communicating above with the pubic tributary of the deep epigastric vein. 



