388 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



Arteries and veins (fig. 309) are larger than the capillaries and 

 they have their walls strengthened outside of the intima by layers 

 of smooth muscle fibres (muscle wall) and connective tissue, mostly 

 elastic (adventitial wall) (fig. 309). Since the arteries are subjected 



Fig. 308. — Embryonic circulation of snapping turtle, Chelydra, showing relations of 

 allantois, after Agassiz and Clarke, a, right auricle; al, allantois; av, allantoic vessels; 

 c, caudal vein; da, dorsal aorta; h, hypogastric artery; j, jugular; /, liver; oa,oy, omphalo- 

 mesenteric artery and vein; pc, post-cardinal; sc, subcardinal vein; uv, umbilical vein; w, 

 WolfiSan body; y, yolk sac; 3-6 aortic arches. 



to greater pressure than the veins their walls are relatively much 

 thicker, but in other respects the two are much alike, except that 

 valves to prevent the backflow of the blood, may occur in the veins, 



Fig. 309. — Diagram of artery or vein. At the left the intima alone; covered in the 

 middle by the muscularis, and at the right with the adventitia added. 



especially those which are vertical in the normal position of the 



animal (legs). 



It has been suggested, with much plausibility, that the main blood-vessels 

 are the remnants of the segmentation cavity, which elsewhere has been obliter- 

 ated by the increase of the mesoderm. .\s will be recalled (p. 16) the meso- 

 thelium grows toward the middle line above and below the digestive tract, thus 



