CIRCULATORY CHANNELS 



159 



end. P. 



II. v: 



FIG. 142. Developing blood vessels in the embryonic 

 connective tissue of a rabbit, bl.v., blood vessels con- 

 taining young blood cells ; end.p., endothelial pro- 

 cesses. (From "SxOHR's Text-book of Histology" by 

 LEWIS.) 



all parts, with a simple endothelium resting on the surrounding connective 

 tissues. In section, these cells appear as a line marked at intervals 

 by the thin sections of their disk-shaped nuclei. Only on the large 

 arterial trunk are some of the immediately surrounding connective- 

 tissue cells developed into a single layer of muscle fibers placed in a 

 circular position. 



The tunicates show also 

 a weakly developed blood 

 vascular system. It re- 

 sembles that of Amphioxus 

 closely, and in both cases we 

 must remember that if the 

 animals were larger and the 

 blood vessels consequently 

 stronger, they would show 

 a more definite and charac- 

 teristic structure. 



Turning to the vertebrate 

 animals, we find the best 

 developed and best known 



forms of blood-channel structure. We shall study the walls of the 

 vessels in man with some reference to the Amphibian forms. 



The mammal blood-vascular system begins as 

 a system of solid connective-tissue cords which, 

 almost immediately that they are formed, become 

 hollow (Fig. 142). This leaves some question as to 

 whether they were intercellular or intracellular in 

 origin. Their subsequent development into a tube 

 composed of many cells united into a cylindrical 

 cover makes it appear that they were intercellular 

 spaces from the beginning. 



Figure 143 shows a transection of a capillary 

 in an Amphibian. It serves well to demonstrate a 

 case of a capillary wall whose circumference is com- 

 posed of but one endothelial cell. All larger 

 vessels show more than one such cell in section. 



This covering of endothelium (Fig. 144) is all 

 that a blood vessel actually would need to retain 

 the blood if the pressure were always low. But owing to the weight 

 of the blood and the great pressures that it is put to in driving it on its 

 course, the wall of the tube is strengthened by various connective-tissue 

 elements developed in the neighboring cells. Also the flow of blood 

 has to be diminished at times in most localities, and this is done by 



r. N. c. 



FIG. 143. Transection 

 of a capillary from a 

 tadpole's tail, w.bl.c. 

 and r.bl.c., a white and 

 a red blood cell sur- 

 rounded by the thin 

 wall which here con- 

 sists of a single endo- 

 thelial cell, showing its 

 nucleus. 



