HIGH SCHOOL ZOOLOGY. 



13 



which layers indeed exist in tlie skin of all Vertebrates. Jt is 

 chiefly but not exclusively in the latter that the pigment is 

 contained. If the epidermis be removed by scraping, the ex- 

 posed surface of the corium will be observed to be rough with 



A. B. 



Fiy. 2. — A, Cycloid Scale from Lake Hcrrinf,'. B, Ctenoid Scale from Rock Bass. "/j. 



papillae, so that the smoothness of the surface is due to 

 these interpapillary spaces being filled up with the epidermis. 

 The papillae are of importance as the channels through which 

 nerves and nutriment from the blood-vessels reach the epidermis 

 fi'om the corium. 



6. Minute Structure of the Skin. 



General Remarks on Histology. — That branch of the study of the 

 structure of animals which deals with the minute elements of the various 

 organs, and which requires the microscope and other tools in tlie course 

 of its investigations, is termed Histology. Each organ of the body 

 is built up of tissues, and each tissue is formed of ultimate elements 

 named cells arranged in a characteristic way. Thus the skin, which is 

 a complicated organ performing very different functions, is comjjosed 

 chiefly of two kinds of tissues — the epithelial tissue of the epidermis, 

 and the connective tissue of the corium. The former may fairly be con- 

 sidered its most characteristic tissue, hut both are necessary elements 

 in its structure. It is still more complex, however, in virtue of the pre- 

 sence of both muscle and nerve tissue, so that this one organ of the 

 body contains tissues of all the four categories under which histolopipts 

 arrange the component parts of the animal body. 



