MORPHOLOGY OF THE ARTERIAL SYSTEM. 



507 



much as the course of the posterior tibial nerve in the leg agrees closely with that of the 

 median nerve in the forearm, it would seem probable that the posterior tibial and median 

 arteries are homologous channels, and that the ulnar artery proper is not represented in the 

 leg. When the posterior tibial nerve divides into the internal and external plantar, corre- 

 sponding respectively to the median and ulnar nerves in the hand, the arterial channel also 

 becomes double ; the internal plantar artery then represents the occasional continuation of 

 the median in the hand, while the external plantar artery with its arch reproduces the deep 

 part of the ulnar artery and the deep palmar arch. The superficial palmar arch has no 

 representative in the foot, and the digital arteries of the toes are furnished by branches 

 homologous to the palmar interosseous arteries of the hand. 



The peroneal artery is the obvious homologue of the anterior interosseous-artery of the 

 forearm, and the anterior tibial of the posterior interosseous. The continuation of the anterior 

 tibial artery into the dorsal artery of the foot, the termination of which like that of the radial 

 in the hand is an enlarged perforating artery of the first space, may be compared to a channel 

 formed through the anastomotic network on the back of the wrist from the end of the 

 posterior interosseous to the radial artery. The termination of the anterior interosseous artery 

 and the anterior peroneal agree in entering the dorsal anastomosis ; and the formation of two 



Fig. 398. THE ARTERIES OP THE STOMACH AND 



INTESTINE IN THE EMBRYO OP SIX WEEKS. 



(Toldt.) 



ao, aorta ; cl, ccelic axis, sending its branches 

 forwards through mg, the mesogastrium to st, 

 the stomach, where the splenic and hepatic 

 arteries communicate along gc, the great curva- 

 ture, and supply spl, the spleen, and p, the 

 pancreas, while the coronary artery crosses to the 

 right of the lower end of the oesophagus and 

 then descends along Ic, the small curvature ; 

 s. mcs. a, superior mesenteric artery passing 

 through me, the mesentery to the primary intes- 

 tinal loop ; i. Tines, a, inferior mesenteric artery. 



arches (tarsal and metatarsal) in the dorsal 



network of the foot in comparison with the 



single carpal arch at the wrist may be explained as resulting from the greater development 



of the tarsal region of the foot. 



2. Splanchnic arteries. It may be assumed that these were originally double in accord- 

 ance with the primitive disposition of the aortic trunk from which they arise, and that the 

 azygos stems have been formed either by the fusion of paired vessels, or more probably by the 

 suppression of the arteries on one side. They comprise the small cesophageal branches of the aorta 

 in the thorax, and the three large arteries, coeliac, superior mesenteric and inferior mesenteric, 

 in the abdomen. In some of the lower vertebrates the arteries to the alimentary canal are 

 more numerous, and the cceliac and mesenteric arteries may be regarded as trunks resulting 

 from the union of several such branches. The primary offsets of these trunks, passing in the 

 primitive mesentery towards the dorsal border of the alimentary canal, divide each into an 

 ascending and a descending branch, which join the neighbouring branches in arches, thus 

 giving rise to the dorsal splanchnic anastomosis. From the arches, of which there may be 

 more than one series, offsets proceed to the wall of the canal on each side. 



The coeliac axis is the artery of the abdominal portion of the foregut, and supplies also 

 the glandular structures developed in connection therewith, viz., the liver, pancreas and 

 spleen. Its three divisions probably represent as many primitive splanchnic arteries, all of 

 which pass through the mesogastrium to the stomach, although their original disposition in 

 relation to the peritoneum is much obscured by developmental modifications (see Vol. I, 

 " Embryology, Development of the Alimentary Canal "). Of the three divisions, the highest and 

 lowest, viz., the coronary and hepatic, agree closely in their type : they run to the extremities 

 of the stomach, where they give off their ascending and descending branches, and are peculiar 

 in then being prolonged over the original right side of the tube and through the small 

 omentum (ventral mesogastrium) to the liver, forming at the same time along the small 

 curvature of the stomach a rcntral splanchnic anastomosis, which is not developed elsewhere 

 on the canal. The ascending and descending portions of the coronary artery are respectively 

 represented by the branches to the oesophagus and the fundus of the stomach ; the hepatic 

 branch is distinct in the fostus, but is only occasionally well developed in the adult (p. 460) ; 

 and the offset to the ventral anastomosis is so much enlarged as to form the continuation of 

 the trunk. The primitive stem of the hepatic artery is the gastro-duodenal, which divides 

 into its ascending, right gastro-epiploic, and descending, superior pancreatico-duodenal, 



