300 HANDBOOK OP PHYSIOLOGY. 



The flii id naturally secreted by the serous membranes appears to b( 

 identical, in general and chemical characters, with very dilute liquoi 

 sanguiuis. It is of a pale-yellow or straw-color, slightly viscid, alkaline, 

 and on account of the presence of albumen, coaguiable by heat. This 

 similarity of the serous fluid to the liquid part of blood, and to the fluid 

 with which most animal tissues are moistened, formerly led to the belie! 

 that it was a- simple translation; but lleidenhain has concluded from 

 experiments that the process of separation is one of secretion, dependent 

 upon the vital activity of the endothelial cells. There is reason for sup- 

 posing that the fluids of the cerebral ventricles and of the arachnoid sac 

 are likewise secretions; for they differ from the fluids of the other serous 

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

 ity, but in that they seldom receive the tinge of bile when present in the 

 blood, and are not colored by madder, or other similar substances intro- 

 duced abundantly into the blood. 



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

 of 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 edge and processes of the synovia! 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 sub- 

 stances taken into it. They are soft and velvety, and extremely vascu- 

 lar. 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 perios- 

 teum ; 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. 



