222 THE CAT. [chap. viii. 



effected immediately by the fresh air last taken in, but only by it 

 after it has become diffused (according to the laws of gaseous 

 diffusion) through the stationary air previously respired. It is 

 effected bv this stationary air because, as we shall shortly see, only 

 a small part of the air which the lungs contain, is introduced or 

 expelled at each respiratory movement. This stationary air is then 

 the direct agent in effecting the exchange, and it is on this account 

 that the air last expelled in each act of expiration, is the part most 

 loaded with carbonic acid. Thus both external and internal 

 respii-ation are processes which go on continuously, and without 

 any such intermissions as those which take place in that alternating 

 action which popularly goes by the name of "breathing," but 

 which is merely a process of introducing into the body that material 

 by which respiration, or true " breathing," can be effected. 



§ 2. The process of respiration in the cat is something more than 

 this mere interchange of gases, since whatever be the dryness of 

 the air inspired, the air given forth in its breathing is nearly 

 saturated with moisture, so that much water is thus given out from 

 the body daily. Moreover, however cold may be the air taken into 

 the body in breathing, the animal's breath as it is given forth is 

 always hot, having become heated by the internal heat of the body. 

 This heat is due to a process of chemical change taking place, as 

 lately mentioned, in the innermost parenchyma of the body. Now, 

 wherever chemical combination takes place, heat is [evolved, and 

 therefore those intimate processes of life which are effected by 

 internal respiration have been described as a sort of slow combustion. 

 External respiration then is the indirect cause of the heat of the 

 cat's body, as may be shown by the increase of heat produced in it, 

 by increased rapidity of breathing. Nevertheless, that it is only an 

 indirect cause, is proved by the fact that the body may not only 

 continue warm for some time after death, but that, under special 

 circumstances, its temperature may temporarily increase. 



This process of heat generation can go on as long ps food is 

 supplied, and the temperature of the body can be maintained at an 

 even heat of about 100° Fahr., in spite of a very lo\y external 

 temperature in the winter season. The animal thus being always 

 " warm-blooded." 



By these chemical changes in the recesses of the tissues, not only 

 is nutrition effected, but also the waste products of the wear and 

 tear of life are removed by the introduced oxygen decomposing those 

 products and converting them into soluble crystalloids or into gases 

 — changes which enable them either to pass out readily at the lungs, 

 or else through the skin or through organs hereafter to be described, 

 namely the kidneys. 



The function of respiration as generally understood,^ i.e., external 



respiration, is effected by a certain set of organs, which form two 



■ categories. The first category includes the accessor// onjam of 



rc-tpi ration, or parts which convey air into the body, and which again 



expel it when it has* done its work. The organs of the second 



