273 THE HORSE. 



inferred from the fjict that, under varying conditions of the body, 

 the nitrogen in the exhaled air may be either above or below the 

 proper proportional. Besides these, the air also receives a con- 

 siderable quantity of moisture, and some organic matters, which 

 in certain cases are largely increased. The changes in the blood 

 are not so fully known ; but it is now the general opinion of phy- 

 siologists that the formation of carbonic acid does not take place 

 in the lungs, but that the blood arrives there surcharged with it 

 already made, and not with carbon, as was formerly believed. The 

 action chiefly consists in the excretion of this carbonic acid, and 

 in the absorption of oxygen, which is stored up for the several 

 purposes for which it is required in the course of its circulation 

 through the body. Magnus demonstrated by experiment that 

 arterial and venous blood contain very different quantities of car 

 bonic acid, oxygen, and nitrogen in a free state, for on obtaining, 

 by means of the air-pump, a volume of the gas contained in each 

 kind of blood, and analyzing them, he found them to be made up 

 as follows : — 



Arterial. Venous. 



Carbonic acid 62-3 71-6 



Oxygen 23-2 15 3 



Nitrogen 14-5 13-1 



It appears, therefore, that in passing through the capillaries, the 

 gas in the arterial blood loses about eight per cent, of oxygen, and 

 receives about nine per cent, of carbonic acid, which action is re- 

 versed as it passes through the lungs. 



CHAPTER XVII. 

 THE ABDOMINAL AND PELVIC VISCERA. 



The Abdomen and its Contents — Physiology of Digestion — Absorp- 

 tion — Structure of Glands and Physiology of Secretion — Depu- 

 ration and its Office in the Animal Economy — The Stomach — 

 The Intestines — Liver — Spleen — Pancreas — Kidney — Pelvis — 

 Bladder — Organs of Generation, Male and Female. 



THE ABDOMEN AND ITS CONTENTS. 



Lying immediately behind the thorax, from which they 

 are separated only by the diaphragm, are the important organs of 

 digestion, and the space in which they are closely packed is called 

 the abdomen. This part is capable of being distended downwards 

 and sideways to an enormous extent, or of contracting till the lower 



