468 THE ANATOMY OF THE HORSE 



It is found by experiment that if venous blood is exposed to the 

 action of oxygen, through a thin membrane such as bladder, it absorbs 

 a portion of that gas, and changes its colour from dark red to a bright 

 scarlet. This is in accordance with the recognized laws of endosmose and 

 exosmose ; and as the blood circulates in very fine streams within the 

 vessels of the lungs, whose walls are much thinner than an ordinary 

 bladder, it may readily be understood that it is placed in more favourable 

 circumstances for this interchange of gases than when tied up in a large 

 mass within a comparatively thick membrane. On examining the structure 

 of the lungs, they are found to be made up of a pair of cellular sacs, com- 

 municating with the trachea, which admits air into them ; and these 

 sacs are furnished with a fine network of capillary vessels distributed on 

 their walls, and on those of the numei'ous cellular partitions of which they 

 are composed. Thus the blood, as it enters the lungs in a venous state, is 

 submitted under very favourable circumstances to the agency of atmospheric 

 air; it readily absorbs the oxygen while it gives off large volumes of 

 carbonic acid gas, the result of the combination of previously absorbed 

 oxygen with the carbon given off by the various organs of the body already 

 alluded to. 



The exact chemical changes which have taken place in the atmospheric 

 air exhaled from the lungs and in the blood itself are believed to be as 

 follows: — 1. A certain portion of oxygen has disappeared from the air. 

 2. It has received a considerable volume of carbonic acid. 3. It has 

 absorbed fresh nitrogen. 4. It has parted with some of the nitrogen of 

 which it was previously made up. The last two changes cannot readily be 

 demonstrated, but are inferred from the fact that, under varying conditions 

 of the body, the nitrogen in the exhaled air may be either above or below 

 the proper proportion. Besides these, the air also receives a considerable 

 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 physiologists 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 carbonic 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 reversed as it passes through the 

 lungs. 



