MORPHOLOGY OF THE ARTERIES. 891 
branches of the seventh and the 
following somatic segmental arteries. 
The continuation of the subclavian 
artery, beyond the inner margin of 
the first rib, is the persistent and 
enlarged lateral offset of the ventral 
branch of the seventh somatic 
segmental artery, which is continued 
outwardsintotheupper limb behind, 
or postaxial to the shoulder girdle. 
The thyroid axis and the superior 
intercostal artery, both branches of 
the subclavian artery, are persistent 
pre-costal anastomoses; and the 
ascending eervical artery belongs to 
the same series of vessels. The 
vertebral artery, which appears as 
a branch of the subclavian in the 
adult, is morphologically somewhat 
complex. The first part represents 
the dorsal branch of the seventh 
somaticsegmental artery ; the second 
part, that passing through the 
cervical transverse processes, consists 
of the persistent post-costal anasto- 
moses between the first seven seg- 
mental arteries; a third part, that 
lying on the arch of the atlas, is the 
‘spinal branch of the first somatic Fic, 610.—DriacRaM OF THE SEGMENTAL AND INTERSEGMENTAL 
segmental artery and its neural con- ARTERIES AT A LATER PERIOD OF DEVELOPMENT THAN IN 
tinuation ; whilst finally the upper Fic. 609. 
part of the vertebral artery, that in © CGelom ; D.A, Dorsal aorta ; D.Sp, Dorsal splanchnic anastomosis ; 
the cranial cavity, appears to repre- In, Intestine ; V.E.D, Branch to ventral enteric diverticulum ; 
sent a prolongation of the pre- V.Sp, Ventral splanchnic anastomosis. 
neural anastomoses, which - still 
farther upwards are probably represented by the basilar artery. As already stated, the post- 
costal anastomoses below the seventh segmental artery occasionally persist, and im such cases the 
vertebral may lose its connexion with the subclavian, and spring from one or other of the 
dorsal branches of the upper intercostal 
arteries. 
The profunda cervicis artery is to 
be regarded as a remnant of the post- 
transverse longitudinal anastomoses. 
The origin of the seventh somatic 
segmental artery from the dorsal longi- 
tudinal trunk is at first some distance 
behind the fifth aortic arch, but, simul- 
taneously with the elongation of the 
neck and the retraction of the heart 
into the thoracic region, it is shifted 
forward until it is opposite the dorsal 
end of the fourth aortic arch. 
The middle sacral artery is formed 
by the fusion of two vessels, each of 
which springs from the primitive aorta 
in exactly the same manner as a somatic 
segmental artery ; it may therefore be 
looked upon as consisting of fused 
somatic segmental vessels which have 
been prolonged backward for the supply 
5 of the caudal appendage. It is, how- 
Fic. 611.—DIAGRAM SHOWING THE ARRANGEMENT AND ComM- ever, commonly regarded as the direct 
5 ah lear a a la THE continuation of the descending aorta, 
Eon ease e ge ae eae ty bev and consequently as being mainly inter- 
).A.A, Cephalic aortic arch ; Iv, Intestine ; L.B, ateral branch goomental. Its mode of origin and 
of a somatic segmental artery ; L.E.D, Branch to a lateral | © 7 
enteric diverticulum ; P.C, Post-central anastomosis ; P.D.A, 
Primitive dorsal aorta; Po.C, Post-costal anastomosis; Po.N, ; F : r : 
Post-neural anastomosis ; Po.T, Post-transverse anastomosis ; The intermediate visceral arteries 
Pr.C, Pre-costal anastomosis ; Pr.L, Pre-laminar anastomosis ; supply the organs derived from the 
Pr.N, Pre-neural anastomosis; P.V.A, Primitive ventral aorta; intermediate cell mass. They form a 
So.S.A, Somatic segmental artery; V.E.D, Branch to a ventral somewhat irregular series of vessels in 
enteric diverticulum ; V.So, Ventral somatic anastomosis. the adult, but presumably in the 
general nature do not lend much 
support to the latter view. 
