VARIATIONS OF THE ARTERIES 639 



The branches of the hypogastric artenj show great variation in their origin, and there is 

 frequently no separation of the hypogastric into anterior and posterior divisions. Rarely 

 the branches all take origin from the external iliac, in which case the hypogastric (as such) 

 is absent. The obturator artery may arise from the inferior epigastric, or vice versa (p. 615). 

 The arteria comitans n. ischiadici may be larger than usual and form a very pronounced anas- 

 tomosis with the popliteal. In rare cases the main blood-supply of the lower limb is thus 

 derived from the inferior gluteal which is the primitive embryonic condition (p. 640). The 

 vesical and vaginal arteries are liable to variation in their relative areas of distribution. The 

 internal pudendal is sometimes small and may terminate as the perineal artery, in these cases 

 the urogenital region is supplied largely by the accessory pudendal (p. 610). 



B. ARTERIES OF THE EXTREMITIES 

 1. MORPHOGENESIS 



The arteries of the adult extremities represent surviving channels resulting from the 

 selection of a chosen path traversing the perineural arterial plexuses of the early embryonic 

 limb. 



At present there is little unanimity of opinion as to whether the pattern of the developing 

 nerve trunks is specifically reproduced by the primitive arterial plexuses or whether the un- 

 doubted similarity between the two is of a more general nature. There occurs in either ex- 

 tremity one case in which an artery of fundamental importance follows a course practically 

 independent of nerve distribution. The volar interosseous, in the forearm, and the peroneal, 

 in the leg, are accompanied by insignificant nerves (n. to pronator quadratus, and n. to flexor 

 hallucis longus respectively) which, moreover, do not extend the full length of the arteries in 

 question. 



The blood of a developing limb, having traversed the proximal segment by means of the 

 arterial plexus around a single nerve, has the choice of several possible paths by which to 

 reach the digits. The selected channel becomes, for the time, the principal artery of the distal 

 segment. This presently gives way to a second favoured route, which may persist or again 

 give way to a third. Thus, finally, the adult arrangement is established. This process of 

 alternation is the cause of many of the commoner variations for, if it does not proceed to its 

 usual termination, a small vessel, commonly rated as a branch, may testify to its earUer im- 

 portance by appearing as one of the chief vessels of the part. 



In the upper extremity the blood first traverses the peri-median plexus (which becomes 

 later the axillo-brachial trunk) and flows to the digits mainly by the volar interosseous route. 

 Next the volar interosseous dwindles in favour of the median. The median afterward relin- 

 quishes its function to the radial and ulnar. 



In the lower extremity the main blood-flow at first foUows the peri-sciatic plexus from 

 which it is delivered to the digits chiefly by the peroneal artery. The peroneal artery passes 

 from the sole to the dorsum of the foot through the sinus pedis, and from here supphes the digits. 

 The anterior and posterior tibial are at first small, the latter supplying the plantar digital 

 arteries. At a stage of 10 millimetres the femoral artery is represented by a peri-saphenous 

 plexus which anastomoses with the peri-sciatic plexus near the knee. The peri-femoral plexus 

 rapidly consolidates into the femoral and genu suprema arteries. The femoral later takes 

 over the pophleal as its direct continuation, and the origin of the genu suprema marks the 

 boundary between the femoral and ischiadic zones of the main trunk. Finally the peroneal 

 gives place to the anterior and posterior tibial arteries. The portion of the peroneal perforating 

 the tarsus disappears. In so doing it leaves the original termination of the peroneal artery 

 connected with the dorsalis pedis to become the arcuate branch of the latter. 



2. VARIATIONS 



The variations of the arteries of the upper extremity may be divided into two categories, 

 A certain number of them, particularly those occurring in the forearm and hand, are directly 

 traceable to the unusual persistence of one or more of the embryonic channels; or, when varia- 

 tion involves magnitude only, to reciprocal variations in the size of the normal vessels. The 

 commoner and more important variations of the arterial distribution, however, arise in a manner 

 much less susceptible to ready explanation. They depend, in fact, upon variations in the course 

 taken by the single or double route which, surviving from the intricacies of the peri-median 

 plexus, persists to maturity. These wiU be referred to later. 



The volar interosseous artery may be unusually large. It may reinforce a deficient radia 

 or ulnar through the volar carpal artery, or its dorsal carpal branch may join the radial at the 

 back of the wrist. In very rare cases the volar interosseous, together with a large ulnar artery, 

 replaces the radial altogether. 



A large median artery may participate in the palmar supply of the fingers, either by joining 

 the superficial volar arch or (the arch being absent) by breaking directly into digital 

 branches. The median, when large, occasionally replaces the ulnar, very rarely the radial, 

 and frequently the superficial volar. 



The superficial volar arch may be small, with compensation by the deep, or absent. In 

 the latter case the digital arteries may come directly from the ulnar and radial, ulnar and median 

 or median and radial. In the absence of the superficial volar, which is very frequent, the super- 

 ficial arch is completed by the princeps poUicis or the volaris radiahs indicis. 



