118 CHAPTER 13. 



is veiy thin, in the mouth it is comparatively thick, whilst in the stomach 

 and intestines it is found in folds. Everywhere it is soft and velvety in 

 texture and abundantly supplied with blood-vessels, nerves, and glands. 

 In the digestive tract and in the spleen, liver, and kidneys it is liberally 

 furnished with absorbents. The secretions from its glands vaiy accord- 

 ing to the part in which they are situated. 



Mucous membrane is everywhere furnished with an external layer of 

 Epithelium, situated on the basement membrane, and beneath this is a 

 structure of vascular tissue of variable thickness, which in different parts 

 presents either outgrowths in the form of papillae or villi, or depressions 

 and involutions in the fomi of glands or follicles. 



In mucous membrane the tissues essential to the production of a 

 secretion exist in their most simple form, namely a simple membrane 

 having on one surface blood-vessels and on the other a layer of cells. 

 The secretion of the membrane, called mucus, is elaborated in these cells. 



228. Villi. 



Villi are the eminences seen on mucous membrane, and when magnified 

 appear like the pile of velvet. 



Villi are extremely vascular and sensitive, and are largely concerned 

 in the process of absorption. 



They are plainly seen on the surface of the tongue. 



229. Epithelium. 



Epithelium consists of a layer of cells arranged in one or in many 

 superposed layers on a basement membrane. The epithelial tissue covers 

 the free surface of the body, i. e. the skin and mucous membrane ; and 

 one variety of it is found lining serous and synovial membranes. In 

 these cases it receives the name of Endothelium. The functions of epi- 

 thelium are secreting, protective, and sensorial. In this membrane the 

 nutrient blood-vessels are contained, whilst in the cells the secretion de- 

 rived from them is elaborated. There are several varieties of Epithelium 

 in different parts of the body. 



230. Serous memhrane. 



Whilst Mucous Membrane lines all surfaces that communicate directly 

 or indirectly with the air. Serous Membrane on the other hand lines all 

 closed cavities, i. e. those excluded from the air, with perhaps one excep- 

 tion, namely, the opening of the Fallopian tube into the abdominal 

 cavity ; and it also surrounds or envelopes all organs which move in the 

 performance of their functions. It secretes a halitus or fluid for the 

 purpose of preventing friction between the different parts. 



Serous membrane is formed of fibro-cellular tissue, intenvoven so as to 

 constitute a membrane ; the free surface of which is covered with a single 

 layer of flattened endothelial cells. It is abundantly supplied with 

 nerves, arteries, and blood-vessels. 



Serous membranes are of two kinds, viz. 1st, those which line visceral 



