1044 



THE VASCULAK SYSTEM. 



RD.A. 



with one another, through the portions of the intersegmental vessels which they connect 

 together, and thus form longitudinal trunks. The longitudinal trunks are mainly, though 



not exclusively, seg- 

 mental. From them 

 the main stem vessels 

 of the individual are 

 formed, and from or 

 to these latter the in- 

 tersegmental vessels 

 appear to proceed as* 

 branches or tribu- 

 taries. 



In the course of 

 P' development the 

 longitudinal trunks 

 become the most im- 

 portant trunks in the 

 individual, and they 

 are formed before the 

 branches and tribu- 

 taries make their 

 appearance. 



So.SA. 1. 



CAA.I 



V.So. 



VV. 



FIG. 840. DIAGRAM OP THE CEPHALIC AORTIC ARCHES, AND OF THE SEGMENTAL 

 AND INTERSEGMENTAL ARTERIES IN THE KEGION IN FRONT OF THE UMBILICUS. 



C.A.A. I, II, III, IV, V. The cephalic 

 aortic arches. 



Co. Anastomosing vessel between the 

 primitive ventral aorta and the 

 ventral somatic anastomosis. 



D. D. Dorsal division of a somatic inter- 

 segmental artery. 



D.Sp. Dorsal splanchnic anastomosis. 



L.B. Lateral branch of ventral division 

 of somatic intersegmental 

 artery. 



L.E.D. Branch to lateral enteric diver - 

 ticulum. 



P.D.A. Primitive dorsal aorta. 



Po.C. Post-costal anastomosis. 

 Po.T. Post-transverse anastomosis. 

 Pr.C. Pre-costal anastomosis. 

 P.V.A. Primitive ventral aorta. 

 So.S.A. 1, 2, 3, 4, 5, 6, 7, 8. Somatic 



intersegmental arteries. 

 Sp.S.A. Splanchnic arteries. 

 V.D. Ventral division of a somatic 



intersegmental artery. 

 V.E.D. Branch to ventral enteric diver- 



ticulum. 



V.V. Vitelline vessels. 

 V.So. Ventral somatic anastomosis. 

 V.Sp. Ventral splanchnic anastomosis. 



THE SEGMENTAL 

 ARTERIES AND 

 THEIR ANAS- 

 TOMOSES. 



The main longi- 

 tudinal trunks are 

 the primitive aortae. 

 The descending aorta 

 is formed, in the 

 greater part of its 

 extent, by the fusion 

 of the dorsal parts of 

 the primitive aortae, 

 and from it the inter- 

 seg mental, lateral, 

 and ventral arteries 



In a typical portion of the body 

 from the dorsalsurface of the primi- 

 tive dorsal aorta, i.e. from the dorsal 

 longitudinal trunk, and runs later- 

 ally and ventrally in the tissues 

 developed from the somatic meso- 

 derm ; it is distributed to the body 

 wall, including the vertebral column 

 and its contents, and is termed 

 a somatic intersegmental artery. 

 A second vessel arises from the side 

 of the primitive dorsal aorta ; it is 

 distributed to the structures de- 

 veloped from the intermediate cell 

 mass, viz., the suprarenal gland, the 

 kidney, and the ovary or the testis, 

 and it is accordingly termed a 

 lateral or- an intermediate visceral 

 artery. The third artery, which 

 is known as the splanchnic artery, 

 springs from the ventral surface of 

 the aorta. It runs in the tissues 

 developed from the splanchnic 

 mesoderm, and supplies the wall 

 of the alimentary canal. 



The somatic intersegmental 

 arteries form, in the early embryo, 

 a regular series of paired vessels 

 throughout the cervical, thoracic, 

 lumbar, and sacral regions. It is, 



arise in pairs, 

 of the embryo there are three arteries on each side. One arises 



MS 



P.D.A 



VED.(Hy) 



VV. 



FIG. 841. DIAGRAM OF THE ARTERIES IN THE KEGION CAUDAL 

 TO THE UMBILICUS. 



Cd.A.A. Caudal aortic arch. 

 D.Sp. Dorsal splanchnic ana- 

 stomosis. 



Middle sacral artery. 

 Parietal branch from 



caudal arch. 

 Primitive dorsal aorta. 

 Primitive ventral aorta. 



M.S. 

 Pa.C. 



P.D.A. 

 P.V.A. 



So.S.A. Somatic iutersegmeiital 



arteries. 



Sp.S.A. Splanchnic arteries. 

 V.E.D. (Hy). Branch to a ventral 



enteric diverticulum. 

 Vi.C. Visceral branch from the 



caudal arch. 



V.V. Vitelline vessels. 



however only in the thoracic and lumbar regions that their 



