LECT. XVII. VELOCITY OF THE CIRCULATION. 323 



Although the numbers may be far from expressing the 

 absolute velocity of the circulation in different vessels, they 

 are, nevertheless, sufficient to prove that the velocity dimi- 

 nishes in proportion to the distance from the heart, and to 

 the smallness of the sections of the vessels. Notwithstand- 

 ing the remarkable augmentation in the sum of the sections 

 of the branches, in comparison with those of the trunks, it 

 is certain that the velocity of the circulation is diminished, 

 and becomes much less than that which it would have been 

 if the partial sections had been united and formed into a 

 single vessel. This diminution of velocity is occasioned 

 by the friction of the fluid against the sides of the vessels, 

 by the large folds, the numerous curves, and the resistance 

 of the liquid column put in motion. On account of the 

 great number of anastomoses, between the ultimate arterial 

 trunks and the extremities of the arterial and venous capil- 

 laries, and which are especially remarkable in the lungs, 

 the velocity of the blood suffers less diminution than it 

 would otherwise do. In this way the lengths of the small 

 tubes are diminished as much as possible, and precisely in 

 proportion as the sum of the sections of the ramifications 

 exceeds the sections of the trunk from which they arise. 



A beautiful object of experimental physiology, is the 

 microscopic examination of the capillary circulation of the 

 lungs of the salamander, or that of the mesentery and the 

 claw of the frog. We perceive the globules of blood 

 moving with more or less rapidity within small vessels, with 

 a velocity which varies according as the section of the 

 vessels is greater or less. 



Pressure on the Blood. I must now speak of the pres- 

 sure which the blood sustains in the vessels in which it 

 circulates. The investigation of this subject has engaged 

 the attention of physiologists of all times, and they have 

 pursued different ways in their experiments. Such, also, 



