890 THE VASCULAR SYSTEM. 
post-transverse anastomosing channels. Moreover, each dorsal branch, as it passes by the corre- 
sponding intervertebral foramen, gives a spinal offset which enters the spinal canal along the 
corresponding nerve-root, and divides into a dorsal, a ventral, and a neural branch. The dorsal 
branches of these spinal arteries are connected together along the ventral surfaces of the laminw 
by pre-laminar anastomoses, and the ventral branches are united on the dorsal surfaces of the 
vertebral centra with their fellows above and below by post-central anastomoses ; they are also 
united with their fellows of the opposite side by transverse communicating channels. The neural 
branches of the spinal arteries divide sunilarly into dorsal and ventral b ranches; the dorsal 
branches of each side are connected together by post-neural anastomoses, and the ventral branches 
unite in the middle line both with their fellows above and below and with those of the opposite 
side, forming a single longitudinal pre-neural trunk. 
In the dorsal and lumbar regions of the body the somatic segmental arteries persist and form 
the intercostal and Jumbar 
arteries. These vessels spring 
from the dorsal aspect of the 
descending aorta, usually in 
pairs. The corresponding 
vessels of opposite sides, how- 
ever, occasionally fuse together 
at their origins, simultaneously 
with the fusion of the dorsai 
longitudinal trunks to form 
the descending aorta, and then 
they arise by common stems. 
The precostal anastomoses 
between the ventral branches 
of the somatic segmental 
arteries are only represented 
in the dorsal region by the 
superior intercostal arteries ; 
in the lumbar region they 
disappear entirely. The 
anastomoses between the 
anterior ends of the ventral 
branches of the somatic seg- 
mental arteries persist as the 
internal mammary and deep 
epigastric arteries. 
The lateral offsets of the 
ventral branches are repre- 
sented by the cutaneous 
arteries which accompany the 
lateral cutaneous branches of 
Fic. 609.—D1aGRaM SHOWING THE ARRANGEMENT AND Communications the spinal nerves. 
OF THE SEGMENTAL AND INTERSEGMENTAL ARTERIES AT AN EARLY The post-costal and post- 
STAGE OF DEVELOPMENT. transverse anastomoses usually 
disappear in the dorsal and 
lumbar regions, but the post- 
C, Ceelom ; D.Sp, Dorsal splanchnic anastomosis ; IN, Intestine ; I.V, Inter- 
mediate visceral artery se lisBs Lateral branch of the ventral division of costal anastomoses occasionally 
a somatic segmental artery ; P.C, Post-central anastomosis ; P.D.A, mae f : aa 
Primitive dorsal aorta; Po.C, Post-costal anastomosis ; Po.N, Post- persist in_ the URRY dorsal 
neural anastomosis ; Po.T, Post-transverse anastomosis ; Pr.C, Pre- TVeS1on, and take part 1 the 
costal anastomosis ; Pr.L, Pre-laminar anastomosis; Pr.N, Pre-neural formation of the vertebral 
anastomosis ; So.8.A, Somatic segmental artery ; Sp.S.A, Splanchnic artery, which in such cases 
segmental artery ; V.E.D, Branch to a ventral enteric diverticulum ; arises from the first or second 
V.So, Ventral somatic anastomosis; V.Sp, Ventral splanchnic ana- intercostal artery. In some 
stomosis. carnivores the — post-costal 
longitudinal vessels persist in 
the upper dorsal region, and form, on each side, a trunk which is connected with the first 
aortic intercostal, and which supplies the five anterior intercostal spaces. “ 
The pre-laminar, the post-central, and the pre- and post-neural anastomoses persist, the two 
latter aiding in the formation of the dorsal and lumbar portions of the pre- and post-spinal 
arteries respectively. 
It is in the cervical region, however, that the most interesting changes occur. The first six 
pairs of somatic segmental arteries lose their connexions with the dorsal roots of the aortic arches, 
ze. in other words, with the longitudinal anastomosing channels in this 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 com- 
mencement up to the origin of the vertebral artery. On each side the ventral branch of the 
seventh segmental artery forms that portion of the subclavian artery which lies betweem the 
origins of the vertebral and internal mammary arteries, and also the trunk of the internal 
mammary artery as far as the upper border of the first costal cartilage. The remainder of the 
internal mammary artery represents the ventral longitudinal anastomoses between the ventral 
. 
