SECRETION. 



of the other serous sacs not only in being pellucid, colorless, and of much 

 less specific gravity, but in that they seldom receive the tinge of hil* 

 when present in the blood, and are not colored by madder, or other 

 similar substances introduced abundantly into the blood. 



Synovial Fluid: Synovia. 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 espe- 

 cially of those which are accumulated on the edges and processes of the 

 synovial fringes; for, in its peculiar density, viscidity, and abundance of 

 albumin, 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 as in the tympanum, to periosteum; in the 

 intestinal canal, it is connected with a firm submucous membrane, which 

 on its exterior gives attachment 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 

 Respiratory 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 Eusta- 

 chian tubes and lines the tympanum and the inner surface of the mem- 

 brana 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 mem- 

 brane which extends into the pulmonary cells. 3. The Genito-urinary 

 tract, wh:'ch lines the whole of the urinary passages, from their external 

 orifice to the termination of the tubuli uriniferi of the kidneys, extends 

 also into the organs of generation in both sexes, and into the ducts of the 

 VOL. I. 21. 



