HISTOLOGY OP EPITHELIAL AND CONNECTIVE TISSUES. 37 



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

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

 material entering the body must first traverse the epithelial 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 especi- 

 ally 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 en- 

 gaged in promoting the absorption of oxygen and the exhalation 

 of carbon dioxid. 



3. To form secretions and 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 excretory 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, 

 adipose, retiform, white fibrous, yellow elastic, cartilaginous and os- 

 seous. Notwithstanding their apparent diversity, they possess many 

 points of similarity. They have a common origin, developing from 

 the same embryonic material ; they have much the same structure, 

 passing imperceptibly into one another, and perform practically the 

 same functions. 



Areolar Tissue. This variety is found widely distributed through- 

 out the body. It serves to unite the skin and mucous membrane to 

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

 of blood-vessels, nerves, and lymphatics ; to unite into compact masses 

 the muscular tissue of the body, etc. Examined with the naked 

 eye, it presents the appearance of being composed of bundles of fine 

 fibers interlacing in every direction. In the embryonic state the ele- 



