ANATOMY. 27 



The trachea, or windpipe, is a cylindrical, flexible tube con- 

 sisting of a series of incomplete cartilaginous rings, numbering 

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

 the larynx, 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 lungs and subdivide into branches 

 termed bronchial tubes. These, becoming gradually smaller as they 

 divide, finally tenninate 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 structures are accompanied throughout by arteries, 

 veins, and nerves. 



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

 the dorsal vertebrae, the muscles between the ribs (intercostal), and 

 the diaphragm. It contains the lungs, heart, large blood vessels, the 

 trachea, esophagus, and a nmnber of nerves. The thorax is lined by 

 two serous membranes, the right and left pleura, each pleura lining 

 one-half the thorax and enveloping the structures contained therein. 

 A serous membrane is a thin glistening structure atid lines a closed 

 cavity. 



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

 Gleans 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 abdominal cavity or belly. 



DIGESTIVE ORGANS. 



(Plate VI.) 



The digestive organs consist of the mouth, pharynx, esophagus, 

 stomach, intestines, and anus, all lined with mucous membrane. 

 Together they form the alimentary canal through which the ahmen- 

 tary matter (food) is subjected to the special actions which adapt it 

 to the purpose of nutrition. 



The mouth is an iriegular cavity, containing the organs of taste 

 and the instruments of mastication (chewing or grinding). It is 

 situated between the jaws, its long diameter following that of the 

 head, and is pierced by two openings— the anterior, for the introduc- 

 tion of food, and the posterior, through which the food passes into 

 the pharynx. It is bounded in front by the lips and laterally by the 

 cheeks ; the roof is formed by the hard palate; the floor is occupied 

 by the tongue, while the rear boundary is the soft palate. Opening 

 into the mouth are ducts leading from the salivary glands: the 

 parotid, submaxillary, and sublingual glands. The mucous mem- 

 brane covers the whole free surface of the mouth and its accessories 



