PHENOMENA OF THE CAPILLARY CIRCULATION. 287 



and fibrous coats, the movement is distinctly remittent, even 

 in their most minute ramifications. The blood moves in 

 them with much greater rapidity than in either the capil- 

 laries or veins. They become smaller as they branch, and 

 carry the blood always in the direction of the capillaries. 

 The veins, which are relatively larger than the arteries, carry 

 the blood more slowly, and in a continuous stream, from the 

 capillaries toward the heart. In both these vessels the cur- 

 rent is frequently so rapid, that the form of the corpuscles 

 cannot be distinguished. Only a portion of the white cor- 

 puscles occupy the still layer, the rest being carried on in the 

 central current. 



The circulation in the true capillaries is sui generis. Here 

 the blood is distributed in every direction, in vessels of nearly 

 uniform diameter. The vessels are generally so small as to 

 admit but a single row of corpuscles, which move almost like 

 beings endowed with volition. In a single vessel, a line of 

 corpuscles may be seen moving in one direction at one mo- 

 ment, arid a few moments after taking a directly opposite 

 course. Spallanzani, in one of his observations, describes the 

 following phenomenon. Two single rows of corpuscles, pass- 

 ing in two capillary vessels of equal size, were directed to- 

 ward a third capillary vessel, formed by the union of the two 

 others, w r hich would itself admit but a single corpuscle. The 

 corpuscles in one of- these vessels seemed to hold back until 

 those from the other had passed in, when they followed in their 

 turn. 1 When the circulation is natural, the movement in 

 the capillaries is always quite slow compared with the move- 

 ment in the arterioles, and is continuous. Here, at last, the 

 impulse of the heart is lost. The corpuscles do not neces 

 sarily circulate in all the capillaries which are in the field ot 

 view. Certain vessels may not receive a corpuscle for some 

 time, but after a while one or two corpuscles become engaged 

 in them, and a current is finally established. Many inter- 

 esting little points are noticed in examining the circulation 



1 SPALLANZANI, Experiences sur la Circulation Paris, 1808, p. 177. 



