80 ZOOLOGY. 



toise), and the batrackia (frogs, salamanders, &c.), the thorax 

 is not formed so as to act as a suction pump, and accordingly 

 these animals swallow the air by a sort of deglutition. 



OF EXHALATION AND THE SECRETIONS. 



144. We have seen how the nutrient matter is distri- 

 buted to all parts of the body by means of the blood ; we 

 have now to examine how the matters contained in the 

 general mass escape from it, whether into the interior or 

 directly from the body. 



145. Nutrition we have seen to be effected in two ways; 

 either, 1st, by the simple absorption into the body of matters 

 requiring no modification : 2nd, by the more complex function 

 of digestion : the first is almost mechanical ; the second par- 

 takes more of a chemical character. 



In like manner nature, in freeing the body of the useless 

 substances and the necessary results of vital actions, employs 

 two methods of expulsion, exhalation, and secretion. The 

 first of these processes is almost physical, and depends on the 

 permeability of the tissues; by the second, peculiar substances 

 are selected from the blood, and eliminated, as it were, in 

 order to be expelled the body ; to effect this, certain organs 

 are required, and these are called secreting organs. The 

 process bears to simple exhalation the relation which diges- 

 tion does to absorption. 



. EXHALATION. 



146. The walls of the bloodvessels are permeable to 

 liquids. By this means, water, gases, and the thinner parts 

 of the blood generally, may pass through these walls by 

 exhalation; but they do not admit of the passage in this 

 way of the globules, or denser parts of the blood. This is 

 proved by injecting prussiate of potass into the veins of a 

 living dog ; the salt injected may be soon after detected in 

 the thorax and abdomen, mingled with the fluids, which are 

 constantly exhaled on the surface of serous membranes. 

 Spirituous and other liquors are perceivable in the breath soon 

 after being taken. 



147. Mechanism of Exhalation. Exhalation is purely 

 a physical phenomenon, however it may be modified by the 

 presence of life. Thus, if into the vascular system of an animal, 



