OUTLINE OF THE STRUCTURE OF THE HORSE. 57 



mals, as well as those produced by the human voice. In the 

 horse, however, these vocal sounds are few, so that this func- 

 tion is not an important one. The larynx is situated imme- 

 diately behind and below the nasal cavities. It consists of 

 five strong cartilages, united together by ligaments. One of 

 these cartilages is that remarkable little valve-like appendage 

 called the epiglottis (see 2 in cut). This somewhat resembles 

 a heart in shape, and is so attached that, when the animal 

 swallows, it shuts downward and backward, so as to entirely 

 close the opening to the larynx. Thus the food and water, 

 in their passage to the stomach, are prevented from entering 

 the lungs, but go onward into the oesophagus, or gullet (see 

 10 in cut), after which the elastic muscles of the epiglottis in 

 an instant throw it back to its original position, and the 

 windpipe is open again. 



Next below the larynx comes the trachea, or windpipe, 

 (see 6 in cut,) which is a flexible tube, made up of about fifty 

 incomplete, cartilaginous rings, connected together by a strong, 

 elastic membrane. It terminates in the bronchi, or two 

 bronchial tubes, of which the right is the more capacious, 

 corresponding with the difterence in size of this lobe of the 

 lungs. These tubes again divide and subdivide, like the 

 branches of a tree, into lesser tubes, still called bronchial, 

 which finally open into the air-cells of the lungs. As they 

 thus continue to divide, they diminish in size, of course, until 

 at last their diameter is only the one twenty -fifth part of an 

 inch. 



The philosophy of respiration we have already explained, 

 in connection with the circulation. The lungs operate on 

 the same principle as a pair of bellows. By the action of 

 the inspiratory muscles — situated in the thorax and abdo- 

 men — the cavity of the chest is expanded, when the air 

 rushes in to fill the vacuum. The muscles then contract, and 

 the air, laden with its foul gases, is forcibly expelled. They 

 are under the control of the will only to a limited degree. 



