540° PHILGSOPHY OF ZOOLOGY. 
last, by 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 
venee cave ; 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 denomimated 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 toage*. 
Climate, likewise, is supposed to exercise a considerable 
influence on the frequency of the pulsations, their number 
being greatest in warm climates. 
Betore proceeding to consider the aeration of the blood, 
it 1s 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 

* Buiumenszacn 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 . 
110 
third and fourth, 96 
When the first teeth begin to drop out, 86 
At puberty, - - - 80 
At manhood, = = = 15 
- - 60 
In those more advanced, scarcely two were found alike.x—The Institutions 
of Physiology, p. 58. 

—_- second 




About sixty, - 
