350 PRACTICAL ANATOMY. 



the space between them is designated the intercellular 

 passage. The air-cells are about one-twohundredth 

 inch in depth. The pulmonary capillaries form a close 

 net-work, the meshes of which are less than the diameter 

 of the vessels. Their walls are extremely delicate, con- 

 sisting of a single layer of endothelial cells, supported 

 on a skeletal basement membrane. In the septa, the 

 capillaries are often disposed in a single layer, exposing 

 them to the action of the air on both sides of the septum. 

 The pulmonary artery does not nourish the lung, which 

 depends for its nutrition on the blood which flows in the 

 bronchial arteries. 



THE ORGANS OF DIGESTION. 



The alimentary canal and its appendages comprise 

 the organs of digestion. It begins at the mouth and 

 terminates at the anus, and consists of a tube about 

 twenty-eight feet long, which undergoes many changes 

 in name and position, arrangement and form. 



The mouth is the commencement of the alimentary 

 canal ; it is an ovoidal cavity, containing the tongue, 

 teeth, and openings of the ducts of the salivary glands. 

 It is bounded above by the hard palate, laterally by the 

 cheeks, anteriorly by the lips, below by the mylo-hyoid 

 muscles; posteriorly it opens into the pharynx. The 

 lining mucous membrane is of the squamous variety and 

 rich in mucous glands. Opposite the second upper 

 molar is a papilla, which presents the opening of the 

 parotid duct ; on the floor of the mouth, at each side of 

 the fraenum linguae, is the opening of Wharton's duct, 

 the duct of the submaxillary gland. The ducts of the 

 sublingual gland, eight to fifteen in number, open on the 

 floor of the mouth. The gums are fibrous structures, 

 closely attached to the periosteum, and covered by the 

 lining mucous membrane, which is intimately adherent. 



