ANATOMY AND PHYSIOLOGY OF THE HOESE. 



145 



chest may be dilated: first, by the dia- 

 phragm ; and secondly, by the intercostales, 

 which elevate the ribs. In natural respira- 

 tion, the horse breathes chiefly through the 

 aid of the diaphragm. Should the respira- 

 tion become quickened, the intercostales are 

 employed, and, when the respiration is labo- 

 rious, the axillary muscles of the abdomen, 

 back, and sides, are brought into use. 



The glottis is opened dm-ing inspkation 

 by the muscles of the larynx. 



The expulsion of the air from the lungs 

 constitutes expiration. This takes place 

 as soon as the air which has been ex- 

 pired has parted with its oxygen, and re- 

 ceived in return a certain quantity of car- 

 bonic acid gas and vapor. In regard to the 

 elasticity of the lungs, it is now demon- 

 strated that they possess no inherent power 

 of elasticity other than that common with 

 all membranous textures. Hence, if an 

 opening be made in the sides of the chest, 

 the lobes on this side collapse in consequence 

 of the pressure of air from without. 



We have next to inquire what changes 

 have, in the meanwhile, been eflected in the 

 blood by the action of the air to which it 

 has been subjected in the lungs. A visible 

 alteration, in the first place, is produced in 

 its ciilor, which, from being of a dark pur- 

 ple, nearly approaching to black, when it 

 arrives at the air-cells by the pulmonary 

 arteries, has acquired the bright, intensely 

 scarlet hue of arterial blood, when brought 

 back to the heart by the pulmonary veins. 

 Li other respects, however, its sensible 

 qualities do not appear to have undergone 

 any material change. Judging from the 

 changes produced on the air which has 

 been in contact with it, we are warranted in 

 the inference that it has parted with a 

 certain quantity of carbonic acid and of 

 water, and that it has in return acquired a 

 certain proportion of oxygen. Since it has 

 been found that the quantity of oxygen 

 absorbed is greater than that which enters 

 into the composition of the carbonic acid 

 evolved, it is obvious that at least the excess 

 of oxygen is directly absorbed by the blood ; 



19 



ana this absorption constitutes, no doubt, 

 an essential part of its arterialization. 



It has been much disputed whether the 

 combination which seems to be effected be- 

 tween the oxygen of the air and the carbon 

 furnished by the blood, occurs during the 

 act of respiration, and takes place in the 

 air-ceUs of the lungs, or whether it takes 

 place in the course of circulation. On the 

 first hypothesis, the chemical process would 

 be very analogous to the simple combustion 

 of charcoal, which may be conceived to be 

 contained in the venous blood in a free 

 state, exceedingly divided, and ready to 

 combine with the oxygen of the air, and 

 imparting to that venous blood its charac- 

 teristic dark color ; while arterial blood, from 

 which the carbon had been eliminated, 

 would exhibit the red color natural to blood. 

 On the second hypothesis, we must suppose 

 that the whole of the oxygen, which disap- 

 pears from the au' respired, is absorbed by 

 the blood in the pulmonary capillaries, and 

 passes on with it into the systemic circu- 

 lation. The blood becoming venous in the 

 course of the circulation, by the different 

 processes to which it is subjected for sup- 

 plying the organs with the materials re- 

 quired in the exercise of their respective 

 functions, the proportion of carbon which it 

 contains is increased, both by the abstrac- 

 tion of the other elements, and by the addi- 

 tion of nutritive materials prepared by the 

 organs of digestion. The oxygen, which 

 had been absorbed by the blood in the lungs, 

 now combines with the redundant carbon, 

 and forms with it either oxide of carbon, 

 or carbonic acid, which is exhaled dming a 

 subsequent exposure to the air in the lungs. 

 Many facts tend strongly to confirm our 

 belief in the latter of these hypotheses. 



OF THE LARYNX.* 



The larynx is the organ producing the 

 voice of the animal. 



Situation. — It is joined to the top of the 

 trachea (or windpipe), and is placed in the 

 throat, between the posterior and broadest 

 * Percivall's Hippopathology. 



