56 AMERICAN FARMER'S HORSE BOOK. 



delicate, yet resists external violence with sufficient strength 

 as not to be easily broken. When minutely examined, it is 

 found to be composed of a countless number of very small, 

 irregularly-shaped compartments, called lobules, each en- 

 tirely distinct, and all without any communication one with 

 another. What is known as broken wind is occasioned bj 

 the rupture of the walls of some of these little compart- 

 ments. Each lobule receives one of the terminating branches 

 of a bronchial tube, and is again broken up into a cluster 

 of air-cells, on the walls of which the capillary branches of 

 the pulmonary arteries and veins are spread out. The in- 

 numerable air-cells are lined with a thin, attenuated mem- 

 brane, through which the blood, in passing through the 

 lungs, appropriates the oxygen, the life-giving principle of 

 the air, and gives off the poisonous carbonic acid gas which 

 the venous circulation has brought back from all parts of 

 the system. Carbonic acid gas is highly destructive to ani- 

 mal life, and is that element which chiefly makes the ex- 

 haled breath so peculiarly offensive. 



The nasal cavities subserve the purposes of respiration, by 

 modifying the condition of the impure air, so as that it may 

 be taken into the delicate air-cells of the lungs without in- 

 jury. They warm the air, if it is too cold ; they moisten it, 

 if it is too dry. In the horse, the nostrils are remarkable as 

 furnishing the sole means of admitting air to the lungs, such 

 being the formation of his soft palate that breathing through 

 the mouth is impossible ; yet he is enabled, by considerable 

 effort, to expel the air through the mouth in the operation 

 of coughing. The nostrils are lined with what anatomists 

 designate the Schneiderian membrane, whose appearance, and 

 especially its color, is an invaluable test for detecting the 

 presence, and tracing the course, of fever in the system. 



From the nostrils the air passes first into the larynx, or 

 throat; thence into the trachea, or windpipe; and, finally, 

 through the bronchial tubes into the lungs. 



Besides its services in respiration, the larynx (see 4 and 5 

 in last cut) forms the vocal sounds uttered by different ani- 



