340 PHILOSOPHY OF ZOOLOGY. 



last, bv a contractile movement, propels it at intervals 

 through the pulmonic artery, to its remotest subdivisions. 

 When the auricle relaxes, it receives the blood from the 

 venae cavte ; and, when it contracts, the ventricle relaxes to 

 receive its contents. When the ventricle contracts, the 

 blood is thrown into the artery, and suddenly enlarges its 

 dimensions. These motions, which are sensible to the eye 

 and touch, have been denominated the Pulsation, or beat- 

 ing of the heart and arteries ; those vessels being termed Ar- 

 teries into which the ventricles discharge themselves. 



The number of pulsations of the heart, in a given time, 

 varies according to the species, and in the same individual,, 

 according to circumstances connected with food, exercise, 

 or the state of the mind. The same individual, likewise, ex- 

 hibits remarkable variations in this respect, according to age*. 



Climate, likewise, is supposed to exercise a considerable 

 influence on the frequency of the pulsations, their number 

 being greatest in warm climates* 



Before proceeding to consider the aeration of the blood, 

 it is necessary to make some inquiry into its mechanical 

 structure, and chemical constitution. When blood, newly 

 taken from an animal, is sufficiently diluted, and placed on 

 the stage of the microscope, it is observed to consist of a 



* BLUMENBACH found the pulsations of the heart of a new born infant, 

 while placidly sleeping, amount to 140 in a minute. 



Towards the end of the first year, about 124 



second 110 



third and fourth, 96 



"When the first teeth begin to drop out, 8ft 

 At puberty, 80 



At manhood, - 75 



About sixty, ... 60 



In those more advanced, scarcely two were found alike. The Institutions 



of Physiology, p. 58. 



