EESPIRATION, AND ITS EFFECTS. 57 



possess its peculiar properties, its state of fluidity, and its dispo- 

 sition to coagulate : if the former did not exist, the blood would 

 be obstructed in the capillary vessels, and the vital functions 

 could not be carried on ; and, if deprived of its coagulating pro- 

 perty, no wounds could heal, or loss of substance be restored ; 

 but the most trifling cut would be the precursor of death. 



The quantity of blood contained in the body it is very difficult 

 to ascertain ; for if an animal be bled to death, a good deal will 

 still remain in the blood vessels. It has, however, been estimated 

 to be about one fifth the weight of the body ; and of this, about 

 three fourths is contained in the veins, and one fourth in the 

 arteries. In young animals there is more than in old ones ; as 

 in them the body must not only be sustained, but increased in 

 size. It is likewise more abundant in Avild animals than in 

 tame ones ; and in proportion to the vigour of the animal. 

 — Ed.] 



CHAP. XIII. 



ON RESPIRATION, AND ITS EFFECTS. 



[The process of respiration, which is carried on from the first 

 minute after birth to the last of existence, consists of two acts, 

 expiration and inspiration. The former, that of inhaling the at- 

 mosphere, is accomplished mostly by the diaphragm,' which, in 

 its relaxed state, is convex towards the chest. As its fibres 

 contract, the muscle flattens, and thus enlarges in a considerable 

 degree the cavity of the thorax. A vacuum is thus produced, 

 or rather a tendency towards it; for the air rushes into the 

 lungs, and the blood into the heart ; and, as the lungs are elastic 

 and spongy in their nature, they become closely adapted to the 

 enlargement of the chest, and prevent any vacuum from taking- 

 place between them and the sides of the thorax. 



The diaphragm is thus the chief agent in the act of inspiration, 

 although it is in some degree assisted by the intercostal muscles, 

 which raise the chest ; and, also, when the breathing is violently 

 excited by those muscles that in quadrupeds attach the fore ex- 

 tremities to the body. This may be witnessed in horses after 

 severe galloping, and also when labouring under inflammation 

 of the lungs, in which disease the animal seldom or never lies 

 down, but makes use of the fore legs as fixed points to assist in 

 enlargino: the chest. The air thus drawn into the luno-s traverses 

 throughout its internal surface, and, having fulfilled its office, is 

 forced out by the act of expiration. This part of the process is 

 effected chiefly by means of the elasticity of the lungs, which 

 acts as soon as the diaphragm becomes passive, assisted, however. 



