142 THE INTEGUMENT 



translucent cells. The granules so conspicuous in the granulosuni 

 are, in this layer, dissolved into a semifluid colloidal substance 

 called eleidin. (Fig. S5.) 



Stratuvi Corueum.— This is the superficial layer of the epidermis 

 composed of dead scale-like cells in which the eleidin has become 

 keratin and the nuclear regions appear as clear spaces. These cells 

 are constantly being shed or desquamated. 



In other regions of the skin, however, these four zones do not 

 normally appear and only the stratum germinativum and the 

 stratum corneum are distinct. 



Sweat Glands.— Distributed widely through the skin of some 

 mammals are the simple coiled tubular sweat glands. (Fig. 84.) 

 Their coiled portion lies well down in the reticular layer of the der- 

 mis, or even in the subcutaneous region. The excretory duct 

 formed by cuboidal cells runs a more or less straight course. In 

 the epidermis the duct is merely a spiral passageway between the 

 cells. The internal twisted secreting portion is composed of pyra- 

 midal cells, each with a basally placed nucleus and granular or 

 vacuolated cytoplasm. The free end of the cells adjacent to the 

 lumen may swell with secretion and become detached from the basal 

 nucleated part to fill the lumen. The secretion is watery and may 

 be slightly colored from pigment present in the cells. In cats, 

 rats, and mice the sweat glands are restricted to the toe pads; in 

 rabbits, to the region around the lips; in deer, to a region near the 

 base of the tail; in the hippopotamus, to the inside of the ears; and 

 in the cow, to the surface of the snout. 



These glands play a part in the regulation of body temperature 

 and the excretion of wastes. 



Hairs. — Hair development is similar to that of feathers and scales. 

 A hair originates from groups of cells in the deep layers of the 

 epidermis which grow down into the subjacent dermal connecti\e 

 tissue and produce an epithelial rod at an oblique angle with the 

 surface. From these epithelial cells surrounded b>- the connective 

 tissue of the dermis, shafts of hairs are formed and grow out beyond 

 the surface of the skin. The epithelial cells and surrounding con- 

 nective tissue of the skin form the hair root. (Fig. SG.) 



The root is surrounded by the hair follicle, a blind tube-like sac, 

 internally comi)()sed of epithelial sheaths, and surrounded by con- 

 nective-tissue sheaths. Indenting the base of the follicle there is 

 a knob-like extension of vascular connective tissue called the hair 

 papilla. Cells multiplying in the basal portion of the epithelium 



