294 PHYSIOLOGY. 



through their walls. The anatomist places the length of the capillary 

 at one-thirtieth of an inch. 



As previously stated, the capillary wall is formed entirely of a 

 simple layer of endothelial cells. They are flat, lance-shaped cells 

 joined edge to edge and represent the continuation of the intima of 

 the arteries. The outlines of the cells with their lines of junction may 

 be beautifully demonstrated by nitrate of silver staining. In the 

 capillaries stained by silver there is here and there to be seen between 

 the cells an increase in the amount of the intercellular substance. The 

 white blood-corpuscles when migrating from the blood-vessels pass 

 between the endothelial cells. 



Microscopical Examination. When a thin and vascular mem- 

 brane belonging to a living animal is placed in the field of the micro- 

 scope, the admirable spectacle observed for the first time in 1661, by 

 Malpighi, is seen : the blood is circulating in the capillary vessels. 

 For this examination, frogs present several parts which are suitable: 

 the interdigital membrane, mesentery, tongue, bladder and lungs. 



Differences in volume of the capillaries have much influence 

 upon the movement of the blood in their interior. In the widest 

 capillaries a rapid current takes place, and the corpuscles are carried 

 along with a velocity which does not permit distinguishing their 

 form clearly. In the smallest vessels, on the contrary, the corpus- 

 cles progress slowly. In fact, the slowness of the current and dis- 

 appearance of the pulse are the chief characteristics of the capillary 

 current. For, normally, the flow through the capillaries is in a 

 steady, constant stream. 



In the very smallest vessels the corpuscles are often at some 

 little distance from one another. They seem to advance with diffi- 

 culty and to rub against the walls of the vessels. According to 

 many observers, the corpuscles are sometimes obliged to bend out 

 of shape in order to traverse these narrow channels. 



At other times, in the midst of the intricacy of the vessels and 

 of the various directions of their current, two capillaries are seen to 

 join a third. Corpuscles coming along the two vessels alternately 

 pass into the single capillary, which receives them one by one, and 

 through which they pass in single file. Elsewhere may be seen a 

 pile of corpuscles, distinct from each other, and all of which pro- 

 gress with the same swiftness. All hasten and slacken their pace at 

 the same time. At other points a complete immobility is seen in 

 consequence of some temporary obstruction or of the contrary direc- 

 tion of the current; then all at once the corpuscles start off again. 



