334 



INTERNAL ANATOMY OF HOESE. 



forms animal heat. It also exercises on the organs a stimulat- 

 ing action, without which the animal tissues cannot manifest 

 their true properties. The organs that carry out this art of 

 respiration in mammalia are the lungs. They take in oxygen 

 from the surrounding air and pass out in return CO2. These 

 organs of respiration, so essential to animal life, are closely 

 shut in, in the thoracic cavity, and are connected with the air 

 by, first, a cartilaginous tube (fig. 192, Tr) arising in the back 

 of the pharynx, and, secondly, by the two nasal cavities. These 

 latter open into the back of the mouth by two openings, the 

 so-called posterior nares, and to the air by the nostrils. 



The whole organs of respiration may be divided into four 

 sections — (1) the nasal cavities; (2) the larynx; (3) trachea 

 and bronchi ; (4) the lungs. 



In the horse we shall notice very large nostrils : owing to the 



Pig. v.y 



-Median Longitudinal Section of Head and Upper Part of 

 Neck of Horse. (After Cliauveau, ) 



A, Nasal septum ; T, tongue, with liard palate .above ; Sp, soft palate ; Fs, frontal 

 boue and sinus; Fn. posterior nares: Ep, epiglottis; C, cerebrum; Ch, cerebellum; 

 S. occipital bone ; L, larynx ; Tr, trachea ; G, oesophagus ; iip.C, spinal chord ; Oyi and 

 C'yii, atlas and axis. 



structure and size of the soft palate (fig. 192, Spi) at the back of 

 the mouth, the horse can only breathe through the nasal cavities, 



