136 



THE CIRCULATION. 



FIG. 52. 



Capillaries in the web of the frog's 

 foot magnified. 



the periodic motion, arid to remain stationary in the intervals ; 

 while, if the debility of the animal is extreme, they even re- 

 cede somewhat after each impulse, 

 apparently because of the elastic- 

 ity of the capillaries, and the tis- 

 sues around them. These obser- 

 vations may be added to those 

 already advanced (p. 114) to 

 prove that, even in the state of 

 great debility, the action of the 

 heart is sufficient to impel the 

 blood through the capillary ves- 

 sels. 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, ob- 

 served that the movement of the 

 blood in the capillaries became 

 obviously pulsatory, the pulsa- 

 tions 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 multiplication 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 sectional 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 correspondence in the rapidity of the circulation in them. 

 It is quite true, that the force with which the blood is propelled 

 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 ; but this force has to 

 overcome all the resistance offered in the arterial and capillary 

 system the heart, itself, indeed, must overcome this resist- 

 ance ; so that the excess of the force of the blood's motion in 



