BLOOD VESSELS ^5 



vessel walls. The arteries in general have thicker and more elastic walls 

 than the veins, and tend to remain open after death; the thinner walls 

 of the veins are prone to collapse. 



DEVELOPMENT. In an early stage the blood vessels of the embryo 

 form a network in the splanchnopleure. In mammals, as in the chick 

 (Figs. 27 and 28, p. 40), the portion of the net nearest the median line 

 forms, on either side of the body, a longitudinal vessel, the dorsal aorta. 

 The part of the net folded under the pharynx constitutes successively 

 (beginning posteriorly) the mtettine veins, the heart, and the ventral norta, 

 and the latter are continuous in front of the pharynx with the dorsal aorta. 

 The heart first appears as two dilated vessels, one on either side, which 

 are parts of the general network. They are brought together in the median 

 line under the pharynx and fuse. At 

 first the heart pulsates irregularly, 

 but with the establishment of the 

 circulation, its beats become rhyth- 

 mical. The blood flows from the 

 general network through the veins to 

 the heart, and thence through the 

 arteries back to the net. All the 

 future vessels of the body are be- 

 lieved to be offshoots from the en- 



FIG. 159. 



dothelial tubes jUSt described. They Blood vessels from a rabbit embryo of 13 days. 

 . . developing as endothelial sprouts (en) from 



grOW OUt, aS ShOWn in Fig. I^Q, pre-existing vessels (b.v.); b.c., blood cor- 



puscle within a vessel. 



through the mesenchyma with which 



they often appear to be inseparably connected. The sprouts are at 

 first solid, but soon become hollow except at the growing tips. They 

 may encounter similar offshoots from the same or other vessels and 

 fuse with them. Through the anastomosis of such sprouts new capillary 

 nets are produced. 



The formation of a definite system of arteries and veins out of a general 

 network may be partly explained on mechanical principles. The vascular 

 outgrowths must take certain courses marked out by the epithelial struc- 

 tures. Thus in early stages they may grow between the somites, but not 

 into them, producing a series of segmental vessels; they pass around the 

 front of the fore-gut and up and down between its lateral outpocketings, 

 so that the regular system of aortic arches appears to depend upon these 

 epithelial obstructions; and they are guided along the under surface of the 

 developing brain in a very characteristic manner. Epithelial obstructions 

 therefore determine the position of the capillary plexuses. In each plexus 

 the favorable channels enlarge and become the main arteries and veins, 

 sending forth new branches and acquiring thick walls; whereas the 

 vessels in which the current is slow remain small or disappear. 



