THE AILAIY HORSE, 29 



from forty to fifty, according to the length of the neck. It 

 succeeds the hirynx, runs down the neck, enters the thorax or 

 chest, and terminates at the base of the heart where it branches 

 into the right and left bronchi, which enter the hmgs and sub- 

 divide into branches termed bronchial tubes. These, becoming 

 gradually smaller as they divide, finally terminate in air cells. 

 The entire ramification, when isolated, has the appearance of 

 a tree, the trachea being the trunk, the bronchi and bronchial 

 tubes the branches, and the air cells the leaves. These struc- 

 tures are accompanied throughout by arteries, veins, and nerves. 



The thorax, or chest, is formed by the ribs, sternum, the 

 bodies of the dorsal vertebra*, the muscles between the ribs 

 (intercostal), and the diaphragm. It contains the lungs, heart, 

 large blood vessels, the trachea, esophagus, and a number of 

 nerves. The thorax is lined by two serous Jiiemhranes, the right 

 and left 1)1610X1, each pleura lining one-half the thorax and 

 enveloping the structures contained therein. A serous mem- 

 brane is a thin glistening structure and lines a closed cavity. 



The lungs, the essential organs of respiration, are light, 

 spongy organs of a conical shape, situated in the thoracic 

 cavity. (Healthy lungs float in water.) 



The diaphragm or midriff is the muscular partition which 

 separates the thorax from the ahdominal cacity or belly. 



DIGESTIVE ORGANS. 



(Plate VI.) 



The digestive organs consist of the mouth, pharynx, esopha- 

 gus, stomach, intestines, and anus, all lined with mucous mem- 

 brane. Together they form the alimentary canal through 

 which the alimentary matter (food) is subjected to the special 

 actions which adapt it to the purpose of nutrition. 



The mouth is an irregular cavity, containing the organs of 

 taste and the instruments of mastication (chewing or grind- 

 ing). It is situated between the jaws, its long diameter follow- 

 ing that of the head, and is pierced by two openings — the an- 

 terior, for the introduction of food, and the posterior, through 

 which the food passes into the pharynx. It is bounded in front 

 b}' the lips and laterally by the cheeks; the roof is formed by 

 the liard palate; the floor is occupied by the tongue, while the 

 rear boundary is the soft palate. The mucous membrane 

 covers the Avliole free surface of the mouth and its accessories 

 except the teeth. The lips are the organs of touch as well as 



