848 



HUMAN ANATOMY, 



Fig 746. 



then lose their original connections with the aorta, so that they seem in the foetus to arise 

 from the iliac vessels, and these latter, although primarily somatic in their distribution, give off 

 a number of visceral branches. 



Of the unpaired visceral branches representatives are to be found in the thoracic region in 

 the oesophageal and mediastinal vessels and m the abdomen m the coeliac axis and the superior 

 and inferior mesenteric arteries, the superior mesenteric representing the omphalo-mesenteric 

 or vitelline arteries of the embryo which primarily arise by several roots, only the lowest of which 

 persists to form the adult vessel. 



According to the general plan of the embryonic arterial system thus outlined, the only ves- 

 sels which have primarily a longitudinal course are the dorsal and ventral longitudinal stems, 

 the dorsal aorta, and its prolongation, the a. sacra media. In the adult, however, several other 

 longitudinal vessels exist, such, for instance, as the vertebrals, the internal mammaries, and the 

 superficial and deep epigastrics. All these vessels are secondary formations due to the end-to- 

 end anastomoses of upwardly and downwardly 

 directed branches of the lateral segmental ves- 

 sels. The internal mammaries and the epigas- 

 trics (Fig. 746) are formed in this manner from 

 branches of the intercostal arteries, with which 

 they remain connected to p greater or less ex- 

 tent , the vertebrals are formed from branches 

 of the lateral cervical vessels, and become inde- 

 pendent stems by the separation of these vessels 

 from the dorsal longitudinal stems, as already 

 described. 



The arteries of the limbs are formed, as 

 already stated, by the lateral somatic branches 

 of the seventh cervical and fifth lumbar segments 

 respectively, but in both limbs a series of changes 

 is necessary before the adult arrangement is 

 acquired. In the arm the subclavian artery at 

 first extends as a single main stem as far as the 

 carpus, where it terminates by dividing into 

 digital branches for the fingers (Fig. 747, A). 

 Throughout its course in the forearm it lies 

 between the two bones, resting on the interos- 

 seous membrane, in the position occupied by 

 the adult anterior interosseous artery ; from 

 the upper part of this portion of its course a 

 branch is given ofif which takes a more super- 

 ficial course, accompanying the median nerve. 

 This median artery gradually becomes larger, 

 while the anterior interosseous undergoes a cor- 

 responding retrogression, and eventually the 

 median, by fusing with the lower portion of the 

 interosseous, forms the main channel for the digi- 

 tal branches and becomes the principal artery of 

 the forearm (Fig. 747, B). A further stage is marked by the development of the ulnar artery as a 

 branch from the brachial, and this, extending down the ulnar side of the forearm, unites with the 

 median to form a carpal arch from which the digital branches arise ( C). Later there develops 

 high up upon the brachial a superficial brachial artery, which, after traversing the brachium, 

 passes down the radial side of the forearm and near the wrist passes to the posterior surface, 

 dividing over the carpus into branches for the dorsum of the thumb and index-finger. After the 

 appearance of the ulnar artery a retrogression of tlie median begins, whereby it becomes the 

 a. comes nervi mediani of the adult ; a branch, the superficial volar, arises from the lower part of 

 the superficial brachial and passes downward into the palm to unite with the palmar arch already 

 present (D) ; and, finally, a branch arising from the lower part of the brachial anastomoses with 

 the superficial brachial just below the bend of the elbow and together with the antilirachial part 

 of the superficial brachial, forms the radial artery. The upper part of the superficial brachial 

 then degenerates until it is normally represented in the adult by a small branch of the brachial 

 which passes to the biceps muscle ( ^E") . 



In the leg the changes are equally complicated. Primarily it is the sciatic artery which 

 forms the main stem, extending the entire length of the posterior surface of the limb into the 

 plantar surface of the foot, where it divides into the digital branches (Fig. 748, A). The ex- 

 ternal iliac at this stage is a relatively slender vessel which extends but a short distance down the 

 thigh and terminates in what is later the profunda femoris. In a later stage there arises from 



Trunk-arteries of embryo of six weeks, showing; 

 origin of internal mammary (im) and epigastric arteries 

 ( J?, superficial, rff, deep) ; a, aorta ; z/, vertebral ; ct, 

 common iliac, continuing as large hypogastric (//) ; 

 external iliac, giving off deep epigastric and femoral, 

 is still small. X 5- {Mall.) 



