INTEESEGMENTAL AKTEEIES AND THEIE ANASTOMOSES. 1045 



original characters are retained. The paired vessels pass dorsally, by the sides of the vertebrae, 

 and divide into dorsal and ventral branches which accompany the corresponding anterior and 

 posterior divisions of the spinal nerves. 



The ventral branches run ventro-laterally, between the ribs, in the thoracic region, and in 

 corresponding positions in the lumbar region, and together with the stems they form the main 

 parts or trunks of the vessels in the thoracic and lumbar regions. They are connected together, 

 near their commencements, by a series of pre-costal anastomoses which pass in front of the necks 

 of the ribs, and they are also connected together, near their terminations, by ventral anastomos- 

 ing channels which run, in the thoracic region, behind the costal cartilages, and in the lumbar 

 region behind or in the substance of the rectus abdominis muscle. Each ventral branch gives off 

 a lateral offset which is distributed like the lateral cutaneous branch of a spinal nerve, and 

 the ventral branch together with the stem of the intersegmental artery forms the trunk of 

 an intercostal or lumbar artery in the adult. 



The dorsal branches, which are present before the ventral branches, run dorsally between the 

 transverse processes of the vertebrae, and form the posterior branches of the intercostal arteries and 

 the dorsal branches of the lumbar arteries of the adult ; they are connected, behind the necks of 

 the ribs, by post-costal anastomoses, and again, behind the transverse processes of the vertebrae, by 



Pre-laminar anastomosis 

 Post-neural anastomosis \ 



Pre-neural anastomosis 





Post-transverse anastomosis 

 Post-central anastomosi 



Post-costal anastomosis 

 Pje-costal anastomosis 



c intersegmental artery 



rmediate visceral artery- 

 Primitive dorsal aorta 



Splanchnic artery 

 Lateral branch of the ventral 

 division of a somatic 

 intersegmental artery 



rsal splanchnic anastomosi 



Ventral splanchnic anastomosis 

 Branch to a ventral enteric diverticulum 



Ventral somatic anastomosis 



842. DIAGRAM SHOWING THE ARRANGEMENT AND COMMUNICATIONS OF THE SEGMENTAL AND 

 INTERSEGMENTAL ARTERIES AT AN EARLY STAGE OF DEVELOPMENT. 



C, Coelom ; IN, Intestine. 



-transverse anastomosing channels. Moreover, each dorsal branch, as it passes by the cor- 

 esponding intervertebral foramen, gives off a spinal offset which enters the spinal canal, along 

 he corresponding nerve-root, and divides into a dorsal, a ventral, and a neural branch. The 

 lorsal branches of these spinal arteries are connected together along the ventral surfaces of the 

 aminae by pre-laminar anastomoses, and the ventral branches are united on the dorsal surfaces 

 f the vertebral bodies (or centra) with their fellows above and below by post-central anastomoses ; 

 hey are also united with their fellows of the opposite side by transverse communicating channels. 

 The neural branches of the spinal arteries divide similarly into dorsal and ventral branches ; 

 he dorsal branches of each side are connected together by post-neural anastomoses, which form 

 he posterior spinal arteries ; and the ventral branches unite, in the median line, both with their 

 ellows above and below and with those of the opposite side, forming a single longitudinal 

 >re-neural trunk, the anterior spinal artery. 



In the thoracic and lumbar regions of the body the somatic intersegmental arteries persist, and 

 3rm the intercostal and lumbar arteries. These vessels spring from the dorsal aspect of the 

 ascending aorta, usually in pairs. The corresponding vessels of opposite sides, however, occasion - 

 lly fuse together at their origins, simultaneously with the fusion of the dorsal longitudinal 

 runks to form the descending aorta, and then the arteries of opposite sides arise by common 

 terns. 



The pre-costal anastomoses between the ventral branches of the somatic intersegmental arteries 



only represented in the thoracic region by the superior intercostal arteries ; in the lumbar 



igion they disappear entirely. The anastomoses between the anterior ends of the ventral 



