NASAL CAVITIES. 419 
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 proportional. 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 
analysing them, he found them to be made up as follows :— 
Arterial. Venous. 
Carbonic/acid! eam 1) eine 0253 716 
Oa Gg Gao og ou 2H 15°3 
WHORES GGG on G6 OO ce UES 1371 
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. 
MECHANISM OF THE PULMONARY APPARATUS. 
ALTHOUGH THE WHOLE of these parts are not contained within the 
thorax, it will be convenient to examine them together, since they all 
mutually bear upon each other both in health and disease. 
THE PULMONARY APPARATUS of the horse consists of four parts—Ist, 
The nasal cavities, destined to prepare the air for entering the larynx ; 
2d, Of the larynx, which acts as a portal or guard against the admission 
of noxious matters floating in it; 3d, Of a set of tubes, consisting of the 
trachea and bronchi, which convey the air from the larynx to the air- 
cells ; and 4thly, Of the air-cells themselves, where the changes are effected 
in the blood, for which the lungs are specially designed. 
THE NASAL ORIFICES AND CAVITIES. 
THE NASAL ORIFICES in the horse and ass differ from those of the other 
domestic animals, and also from the human nostrils, in being the sole 
means of admitting air to the lungs. The ox, sheep, dog, cat, &. can 
breathe either through the nostrils or the mouth, but the horse is pre- 
vented, by the formation of his soft palate, from drawing in air through 
his mouth, and hence he requires nostrils of a size calculated to admit an 
extra supply of air. The orifices or nostrils consist of an oblong opening 
on each side of the nose, separated from each other externally by the skin 
covering the cartilaginous ale, which encircle three-fourths of the opening. 
These ale, together with the septwm, which divides the two nostrils ver- 
tically, constitute the five cartilages of the nose, all being lined by the 
Schneiderian membrane, upon which the nerves of smell are freely dis- 
CEQ 
