328 HANDBOOK OF PHYSIOLOGY. 



sacs not only in being pellucid, colorless, and of much, less specific 

 gravity, but in that they seldom receive the tinge of bile when pres- 

 ent in the blood, and are not colored by madder, or other similar sub- 

 stances introduced abundantly into the blood. 



It is also probable that the formation of synovial fluid is a process of 

 more genuine and elaborate secretion, by means of the epithelial cells on 

 the surface of the membrane, and especially of those which are accumu- 

 lated on the edges and processes of the synovial fringes; for, in its pecu- 

 liar density, viscidity, and abundance of albumen, synovia differs alike 

 from the serum of blood and from the fluid of any of the serous cavities. 



(2) Mucous Membranes. 



The mucous membranes line all those passages by which internal 

 parts communicate with the exterior, and by which either matters are 

 eliminated from the body or foreign substances taken into it. They are 

 soft and velvety, and extremely vascular. The external surfaces of 

 mucous membranes are attached to various other tissues; in the tongue, 

 for example, to muscle; on cartilaginous parts, to perichondrium; in the 

 cells of the ethmoid bone, in the frontal and sphenoidal sinuses, as well 

 in the tympanum, to periosteum; in the intestinal canal, it is connected 

 with a firm sub mucous membrane, which on its exterior gives attach- 

 ment to the fibres of the muscular coat. The mucous membranes line 

 certain principal tracts Gastro-Pulmonary and Genito-Urinary; the 

 former being subdivided into the Digestive and Eespiratory tracts. 



1. The Digestive tract commences in the cavity of the mouth, from 

 which prolongations pass into the ducts of the salivary glands. From 

 the mouth it passes through the fauces, pharynx, and oesophagus, to the 

 stomach, and is thence continued along the whole tract of the intestinal 

 canal to the termination of the rectum, being in its course arranged in 

 the various folds and depressions already described, and prolonged into 

 the ducts of the intestinal glands, the pancreas and liver, and into the 

 gall-bladder. 



2. The Respiratory tract includes the mucous membrane lining the 

 cavity of the nose, and the various sinuses communicating with it, the 

 lachrymal canal and sac, the conjunctiva of the eye and eyelids, and the 

 prolongation which passes along the Eustachian tubes and lines the 

 tympanum and the inner surface of the membrana tympani. Crossing 

 the pharynx, and lining that part of it which is above the soft palate, 

 the respiratory tract leads into the glottis, whence it is continued, 

 through the larynx and trachea, to the bronchi and their divisions, 

 which it lines as far as the branches of about -^ of an inch in diameter, 

 and continuous with it is a layer of delicate epithelial membrane which 

 extends into the pulmonary cells. 



