1046 



THE VASCULAR SYSTEM. 



branches of the somatic intersegmental arteries persist as the internal mammary and superior 



and inferior epigastric arteries. 



The lateral offsets of the ventral branches are represented by the cutaneous arteries which 



accompany the lateral cutaneous branches of the spinal nerves, and the lateral branch of the seventh 



somatic intersegmental artery forms 

 the greater part of the arterial 

 stem of the upper limb. 



The post-costal and post-trans- 

 verse anastomoses usually disappear 

 in the thoracic and lumbar regions, 

 but the post - costal anastomoses 

 occasionally persist in the upper 

 thoracic region, and take part in the 

 formation of the vertebral artery, 

 which in such cases arises from the 

 first or second intercostal artery. 

 In some carnivora the post-costal 

 longitudinal vessels persist in the 

 upper thoracic region, and form, on 

 each side, a trunk which is connected 

 with the first aortic intercostal, and 

 which supplies the first five inter- 

 costal spaces. 



The pre -laminar, the post- 

 central, and the pre- and post- 

 neural anastomoses persist, the 

 latter two aiding in the formation 

 of the thoracic and lumbar portions 

 of the anterior and posterior spinal 

 arteries respectively. 



It is in the cervical region, 

 however, that the most interesting 

 changes occur. The first six pairs 



FIG. 843. DIAGRAM OF THE SEGMENTAL AND INTERSEGMENTAL of somatic intersegmental arteries 

 ARTERIES AT A LATER PERIOD OF DEVELOPMENT THAN IN lose their connexions with the 

 FIG. 842. dorsal roots of the aortic arches, i.e., 



C, Coelom ; D.A, Dorsal aorta ; D.Sp, Dorsal splanchnic anastomosis ; in other words, with the longi- 



IN, Intestine ; V.E.D, Branch to ventral enteric diverticulum ; tudinal anastomosing channels in 



. V.Sp, Ventral splanchnic anastomosis. that region. The seventh pair, 



however, persist in their entirety ; 



and from them are formed, on the right side, a portion of the subclavian trunk, and on the 



left side the whole of the subclavian stem from its commencement up to the origin of the 



vertebral artery. On each side the ventral branch of the seventh intersegmental artery forms 



that portion of the 



subclavian artery 



which lies between 



the origins of the 



vertebral and internal 



mammary arteries, 



and also the trunk of 



the internal 



Somatic intersegmental 

 artery" 



Post-neural anastomosis 

 Post-central anastomosis 

 Post-transverse anastomosis 

 Post-costal anastomosis 



Pre-costal anastomosis 



Pre-laminar anastomosis 

 | Prc-iieural anastomosis 



mam- 

 mary artery as far as 

 the upper border of 

 the first costal cartil- 

 age. The remainder 

 of the internal mam- 

 mary artery repre- 

 sents the ventral 

 longitudinal anasto- 

 moses between the 

 ventral branches of 

 the seventh and the 

 following somatic in- 

 tersegmental arteries. 

 The continuation of 

 the subclavian artery, 

 beyond the inner 

 margin of the first 

 rib, is the persistent 



Primitive dorsal aorta 



Lateral branch of a somatic 

 intersegmental artery 



Cephalic aortic arch 



Branch to a lateral enteric 

 diverticulum 



Primitive ventral aorta 



Ventral somatic anastomosis 



Branch to a ventral enteric diverticulun 



FIG. 844. DIAGRAM SHOWING THE ARRANGEMENT AND COMMUNICATIONS OF THE 

 SEGMENTAL ARTERIES IN THE KEGION OF THE CEPHALIC AORTIC ARCHES. 



IN, Intestine. 



and enlarged lateral offset of the ventral branch of the seventh somatic intersegmental artery, 

 which is continued into the upper limb, caudal or postaxial to the shoulder girdle. The thyre 

 cervical trunk and the superior intercostal artery, both branches of the subclavian artery, ai 



