506 MORPHOLOGY OF THE ARTERIAL SYSTEM. 



is the dorsal branch of the seventh segmental artery, is thence to the transverse process of the 

 atlas the series of postcostal anastomoses ; the part of the last artery lying in the vertebral 

 groove of the atlas is the spinal offset of the first segmental vessel, which enters the cranium, 

 having first contributed its posterior spinal branch to the dorsal neural anastomosis, and joins 

 its fellow in the basilar artery a part of the ventral neural anastomosis. The ventral division 

 of the eighth segmental artery is wanting, while its dorsal branch, continued by the postverte- 

 bral anastomoses, forms the deep cervical artery ; and this together with the intercostal arteries 

 of the first and second spaces arise from the subclavian (the seventh segmental artery) by 

 means of the superior intercostal trunk, which is a precostal anastomosis. 



The origin and morphological relations of the terminal branches of the aorta and of the 

 arteries of the lower limbs are by no means clearly understood. The middle sacral artery 

 (caudal aorta of the lower animals) being generally regarded as the continuation of the aortic 

 trunk, the common and external iliac arteries would appear to be the ventral division of the 

 artery of the fourth lumbar segment, of which the last lumbar artery is the dorsal branch 

 only (Hochstetter). The femoral artery is the lateral perforating offset, and the continuation 

 of the segmental vessel forms the first portion of the epigastric artery, which communicates 

 through ventral anastomoses with the internal mammary and subclavian above, and the 

 obturator below. The normal obturator artery may be looked upon as the ventral division of 

 another segmental artery, and the gluteal and sciatic as lateral perforating branches, while 

 the internal iliac is a precostal anastomosis by which they have acquired a secondary origin 

 from the common iliac, and which is continued by the lateral sacral arteries. An obturator 

 artery arising from the epigastric is a development of the ventral anastomosis. 



An entirely different explanation is given by A. H. Young, who looks upon the common 

 iliac and hypogastric arteries as the direct prolongations of the primitive aortse, while the 

 middle sacral is a new formation. From the primitive aorta the external iliac artery is given 

 off as a segmental parietal vessel, while the internal iliac is a common stem for parietal and 

 visceral arteries. 



Arteries of the limbs. It has already been seen that the segmental arteries of the body- 

 wall follow closely in their course and branching the corresponding nerves, and the same is 

 the case with the arteries of the limbs. As a rule the chief nerve-trunks are accompanied by 

 an arterial canal, which may attain considerable development, so as to form a principal vessel, 

 or may exist only as an anastomotic chain. Where there are differences in the nerves of the 

 two limbs, therefore, correlated differences in the arteries may be expected. The large 

 arteries may also be developed from different channels in the two cases, so that a main trunk 

 in the one limb is only represented by a small vessel or an anastomotic connection in the 

 other. In the neighbourhood of the large joints numerous anastomoses are formed between 

 the different channels, and by the development of these it may happen that the chief arteries 

 accompany different nerves in the several limb-segments. From these considerations it will 

 be evident that the homologies of the arteries are mainly dependent upon the homologies that 

 are recognized between the nerves of the limbs. 



In comparing together the main arterial trunks of the two limbs, it will be observed that 

 they show an important difference in their relation to the limb-girdle ; the axillary artery 

 enters the limb with the nervous trunks on the postaxial or caudal side of the shoulder-girdle, 

 while the femoral artery runs on the preaxial or cephalic side of the hip-girdle in company 

 with only a small part of the limb-nerves. From this it may be inferred that the two vessels 

 are not strictly homologous. In the Sauropsida generally the main artery enters the hind 

 limb on the caudal side of the pelvic girdle, in company with the sciatic nerve, and therefore 

 corresponds more closely to the artery of the fore limb ; and in the early mammalian embryo 

 a similar condition exists, the sciatic artery being continued into the limb, while the femoral 

 artery is small and does not extend beyond the thigh. Subsequently the latter artery grows 

 more rapidly, and forming a connection with the primitive vessel near the knee becomes the 

 main trunk, while the sciatic between the pelvis and the ham in great part disappears 

 (Hochstetter). The rare cases in which the main artery of the limb is continued from the 

 sciatic (p. 491) are instances of persistence of the primitive condition. 



While the lower part of the brachial artery may therefore be regarded as represented in 

 the popliteal, the place of division of the former corresponds to the point of origin of the lower 

 internal articular branch of the latter, that branch being homologous to the radial recurrent 

 artery, while the radial trunk itself is not farther represented in the leg. The remainder of 

 the popliteal artery may accordingly be compared to the ulnar-interosseous trunk of the fore- 

 arm, although from the difference in their relation to the pronator teres and popliteus muscles 

 respectively it would appear that the correspondence of the two vessels is not complete. 



In the forearm the nerves of the ventral side of the limb form two trunks, the ulnar and 

 median, each of which is accompanied by an artery the ulnar proper and the comes nervi 

 mediani, the latter being usually small, although in some cases it is enlarged and continued 

 beneath the annular ligament into the palm. These two nerves are represented in the lower 

 limb by one trunk the posterior tibial, with which runs the artery of the same name. Inas- 



