THE CAPILLARIES. Z 6l 



and to remain stationary in the intervals; while, if the 

 debility of the animal is extreme, they even recede some- 

 what after each impulse, apparently because of the elasti- 

 city of the capillaries, and the tissues around them. These 

 observations may be added to those already advanced 

 (p. 132) 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 vessels; 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 

 greater than that with which it moves in the veins ; 



