THE CAPILLARIES. 173 



intervals ; while, if the debility of the animal is extreme, 

 they even recede somewhat after each impulse, apparently 

 because of the elasticity of the capillaries and the tissues 

 around them. These observations may be added to those 

 already advanced (p. 142) to prove that, even in the state 

 of great debility, the action of the heart is sufficient to 

 impel the blood through the capillary vessels. Moreover, 

 Dr. Marshall Hall having placed the pectoral fin of an eel 

 in the field of the microscope and compressed it by the 

 weight of a heavy probe, observed that the movement of 

 the blood in the capillaries became obviously pulsatory, 

 the pulsations being synchronous with the contractions 

 of the ventricle. The pulsatory motion of the blood in the 

 capillaries cannot be attributed to an action in these ves- 

 sels ; for, when the animal is tranquil, they present not 

 the slightest change in their diameter. 



It is in the capillaries, that the chief resistance is offered 

 to the progress of the blood ; for in them the friction of 

 the blood is greatly increased by the enormous multipli- 

 cation of the surface with which it is brought in contact. 

 The velocity of the blood is also in them reduced to its 

 minimum, because of the widening of the stream. If, as 

 Professor Miiller says, the sectional area of all the branches 

 of a vessel united were always the same as that of the 

 vessel from which they arise, and if the aggregate sec- 

 tional area of the capillary vessels were equal to that of 

 the aorta, the mean rapidity of the blood's motion in the 

 capillaries would be the same as in the aorta and largest 

 arteries ; and if a similar correspondence of capacity existed 

 in the veins and arteries, there would be an equal cor- 

 respondence in the rapidity of the circulation in them. It 

 is quite true, that the force with which the blood is pro- 

 pelled in the arteries, as shown by the quantity of blood 

 which escapes from them in a certain space of time, is 

 much greater than that with which it moves in the veins ; 

 but this force has to overcome all the resistance offered in 



