80 



The Vertebrate Organ Systems 



keratin, an insoluble protein. This layer is continuously shed. In 

 most mammals, shedding is a continuous, sort of self-cleaning process ; 

 while in snakes and lizards the entire corneum is shed periodically. 



The Dermis. — The corimn or dermis is much thicker than is the 

 epidermis. It is formed chiefly of tightly packed masses of connective 

 tissues. This is the portion of the skin that is tanned for making 

 leather, while the epidermis is discarded. 



A few layers can be seen within the corium. Lying in the deeper 

 parts is the subcutaneous layer which has large numbers of fat cells 

 within a loosely woven mass of connective tissue. This is the chief 

 insulating layer of the skin. It has a rich blood supply, often con- 

 taining a high percentage of the total volume of blood in the body. 

 Some of the fibers of this layer are interwoven with the connective 

 tissue sheaths below the skin, thus more or less fastening the skin 

 into place. 



STRATUM CORNEUM. 

 STRATUM LUCIDUM. 



STRATUM GBANULOSUM 



STRATUM GERMINATIVUM. 



SEBACEOUS GLAND. 



FOaiO-E. 

 BULB 



HAIR PAPILLA. 



>. EPIDERMIS 



^ DERMIS 



.ERECTOR 

 MUSCLE 



.SWEAT 



GLAND 



.PRESSURE 



RECEPTOR 



Fig. 21. — Diagrammatic section of human skin. 



In those areas of the skin that are subjected to much pressure, 

 another layer, the papillary, may be seen within the corium. This layer 

 is thrown into folds and contains within it two types of papillae (finger- 

 like projections), nutritive and sensory. The epidermis folds into tiny 

 ridges formed by these papillae. Between this papillary layer and 

 the subcutaneous layer, a third part may be distinguished at times. 

 This is the reticular layer. On the whole, however, the layers of the 

 dermis are not so pronounced as are those of the epidermis. 



Within this layer of the skin are located the many blood vessels, 

 glands, and nerve endings which have been mentioned previously. 



