SEC. 1. MAIN GENEBAL FACTS OF THE CIRCULATION. 



1. The Capillary Circulation. 



If the web of a frog's foot be examined with a microscope, 

 the blood, as judged of by the movements of the corpuscles, is seen 

 to be passing in a continuous stream from the small arteries 

 through the capillaries to the veins. The velocity is greater in the 

 arteries than in the veins, and greater in both than in the 

 capillaries. In the arteries faint pulsations, synchronous with the 

 heart's beat, are occasionally visible ; and not unfrequently varia- 

 tions in velocity and in the distribution of the blood, due to 

 causes which will be hereafter discussed, are witnessed from time 

 to time. 



The flow through the smaller capillaries is very variable. 

 Sometimes the corpuscles are seen passing through the channel 

 in single file with great regularity; at other times, they may 

 be few and far between. Sometimes the corpuscle may remain 

 stationary at the entrance into a capillary, the channel itself being 

 for some little distance entirely free from corpuscles. Any one 

 of these conditions readily passes into another ; and, especially with 

 a somewhat feeble circulation, instances of all of them may be seen 

 in the same field of the microscope. It is only when the vessels 

 of the web are unusually full of blood that all the capillaries can be 

 seen equally filled with corpuscles. The long oval red corpuscle 

 moves with its long axis parallel to the stream, frequently rotating 

 on its long axis and sometimes on its short axis. The flexibility and 

 elasticity of a corpuscle are well seen when it is being driven into a 



