152 



ADVANCED PHYSIOLOGY 





ward in the other direction, they fill and thus occupy the 

 whole calibre of the vein. 



Capillaries are delicate, thin-walled tubes made of cells 

 which are continuous with the lining epithelium of the arteries 

 and veins; Fig. 84. The capillaries are essentially the same 

 as the arteries with the muscle and connective tissue layers 

 absent. Through their thin walls the fluids of the blood 

 easily pass, and thus come in contact with the surround- 

 ing tissues. In. size, capillaries are, on the average, about 2^00 

 of an inch in diameter, and their numbers are countless. 



The finest needle cannot 

 pass through the skin with- 

 out puncturing some of 

 them, and the deeper lying 

 organs are supplied in the 

 same way as those on the 

 surface. 



Each of these blood tubes, 

 then, is especially fitted to 

 its place and function; the 

 arteries withstand the pow- 

 erful, unremitting driving 

 of blood into them by the 

 heart; the capillaries allow 

 the passage of the nutrient 

 fluids and gases into the 

 surrounding tissues, and also 

 take up waste fluids and 

 gases; the veins conduct 

 blood back to the heart, 

 open to their full diameter 

 all the time to permit the 

 easy flow of blood, yet col- 

 lapsible so as to prevent any tendency to the formation 

 of empty spaces. Of course, blood will not tend to flow 



FIG. 83. A VEIX CUT 

 OPEN 



To show the irregular valves 

 within. 



FIG. 84. 



A BIT OF 

 A CAPIL- 

 LARY 

 Showing it 

 to be made 

 of a single 

 layer of 

 epithelial 

 cells only. 



