PHVSIOLOGY OF MUSCULAR EXERTION. 299 



in which they are embedded. Nevertheless these veins form 

 but a small portion of the lungs. There is another set of 

 vessels, equally minute and numerous, for taking the blood 

 back to the heart. In its passage through the lungs, the blood 

 is exposed to the air, which acts upon it, though covered from 

 actual contact. The blood is thus changed in composition. It 

 is purified, losing something or gaining something, and is 

 ready again to perform the duties of which it had previously 

 become incapable. In this state it is collected from the lungs, 

 and taken to the cavity in the left side of the heart, whence it 

 is sent by another set of tubes to be distributed over the 

 body. These are termed arteries ; as they pass into the sub- 

 stance of parts, their ultimate arrangement can not be traced. 

 In the hidden recesses, the blood performs its functions. 

 There it produces changes on the tissue, and is itself changed. 

 It suffers some deterioration, or alteration, which can not be 

 rectified till it reaches the lungs, to which the veins collect 

 and carry it. 



The blood is in constant motion. It is not all altered at 

 one time, nor at one place At some particular places the 

 alteration may be greater than at others ; but the best and the 

 worst are mingled together on their road to the heart. Under 

 ordinary circumstances, the purification keeps pace with the 

 deterioration. Both go on simultaneously, and to an equal 

 degree. But in some cases the equilibrium is deranged. 



Muscular Exertion produces at least four important 

 changes. It quickens the circulation ; it quickens the breath- 

 ing ; it increases the formation of heat ; and it produces per- 

 spiration. The muscles are the active instruments of motion. 

 They act by alternate contraction and relaxation ; their active 

 state is that of contraction. Thev shorten, and their ends 

 being fixed to different bones, motion takes place from the 

 joints. The animal wills to move, and the muscles instantly 

 produce the motion desired. The direction and velocity, the 

 force and duration of the motion, are regulated entirely by 

 the will of the animal. But, in order that the muscles may 

 obey, it is an indispensable condition that ihey have an abun- 

 dant supply of pure blood. In action they consume more than 

 at rest. 



Quickness uf the Circulalioji is therefore a necessary con- 

 sequence of muscular exertion. The muscles demand more 

 blood ; and the heart hastens to furnish it by performing 

 double, treble, or more than treble its usual number of str(;kes. 

 When the horse is at rest, the heart contracts from thirty to 



