THE CONNECTIVE TISSUES. 49 



protecting them from the injurious influences of moisture, air, 

 dust, microorganisms, etc., which would otherwise impair their 

 vitality. Wherever continuous pressure is applied to the skin, 

 as on the palms of the hands and soles of the feet, the epithelium 

 increases in thickness and density, and thus prevents undue 

 pressure on the nerves of the true skin. The density of the 

 epidermis enables it to resist, within limits, the injurious influence 

 of acids, alkalies, and poisons. 



2. To promote absorption. Inasmuch as the skin and mucous mem- 



branes cover tne surfaces of the body, it is obvious that all 

 nutritive material entering the body must first traverse the epi- 

 thelial tissue. Owing to their density, however, the epithelial 

 cells covering the skin play but a feeble role as absorbing agents 

 in man and the higher animals. The epithelium of the mucous 

 membrane of the alimentary canal, particularly that of the small 

 intestine, is especially adapted, from its situation, consistency, 

 and properties, to play the chief role in the absorption of new 

 materials into the blood. The epithelium lining the air-vesicles 

 of the lungs is engaged in promoting the absorption of oxygen 

 and the exhalation of carbon dioxid. 



3. To form secretions ancL. excretions. Each secretory gland con- 



nected with the surfaces of the body is lined by epithelial cells, 

 which are actively concerned in the formation of the secretion 

 peculiar to the gland. Each secretory organ is similarly provided 

 with epithelial cells, which are engaged either in the production 

 of the constituents of the excretion or in their removal from the 

 blood. 



2. THE CONNECTIVE TISSUES. 



The connective tissues, in their collective capacity, constitute 

 a framework which pervades the body in all directions, and, as the 

 name implies, serve as a bond of connection between the individual 

 parts, at the same time affording a basis of support for the muscle, 

 nerve, and gland tissues. The connective-tissue group includes a 

 number of varieties, among which may be mentioned the areolar. 

 adrgose, retiform, whitejibrou&, yellow^eiastic, cartilaginous and 

 osseous. Notwithstanding their apparent diversity, they possess 

 many 1 points of similarity. They have a common origin, developing 

 from the same embryonic material; they have much the same struc- 

 ture, passing imperceptibly into one another, and perform practically 

 the same functions. 



Areolar Tissue. This variety is found widely distributed 

 throughout the body. It serves to unite the skin and mucous mem- 

 brane to the structures on which they rest; to form sheaths for the 

 support of blood-vessels, nerves, and lymphatics; to unite into com- 



4 



