140 



STUDIES IN PHYSIOLOGY 



Structure of the Capillaries. In structure the capillaries 

 are extremely simple. At the points in the blood system 

 where arteries end and capillaries begin, connective, mus- 

 cular, and elastic tissues disappear. The walls of the capil- 

 laries are continuous with the inner lining of the arteries 

 and the veins, and are formed of a single layer of very thin 

 cells'. We have in this arrangement the best possible con- 

 ditions for the process of osmosis. Only the thin membrane 

 of ^he capillary wall separates tLe blood from the surround- 

 ing tissues, 



1 and an ex- 



change of ma- 

 terials be- 

 tween the two 

 is readily car- 

 ried on (Fig. 

 53). 



Flow of Blood 

 in the Web of 

 a Frog's Foot. 1 

 One can 

 easily watch 

 the flow of the 

 blood in the 



thin web of a frog's foot. If a 1 % solution of chloretone is 

 forced into the mouth of the animal and its body is wrapped 

 in a wet cloth, the frog will lie perfectly passive, and the 

 thin membrane can be spread out and examined with the 

 microscope. Under these conditions one can trace the blood 

 current from the arteries through the capillaries, into the 

 veins, and a most fascinating study it is. One can see in all of 

 the tubes a large number of oval-shaped red corpuscles, and 

 possibly here and there a more or less spherical white corpus- 

 cle. These are carried along in the transparent stream of 

 colorless plasma. In some of the larger vessels the current 

 1 See "Laboratory Exercises," No. 28. 



B AC 



FIG. 53. Capillaries 



A = surface view. C cross section. 



B = longitudinal section. d = interior of capillary. 

 e.c = cells forming wall of capillary. 

 n = nuclei of cells. 



