4i8 



EXCRETION 



and on the other contraction of the necessary muscles for the expulsion 

 of the contents of the bladder. In the voluntary act these motor centers 

 are stimulated to activity by impulses coming from the higher cerebral 

 centers. 



THE STRUCTURE AND EXCRETORY FUNCTIONS OF THE SKIN. 



The skin serves, i, as an external integument for the protection of the 

 deeper tissues, and 2, as a sensitive organ in the exercise of touch, a subject 

 to be considered in the chapter on the Special Senses. It is also, 3, an im- 

 portant secretory and excretory organ; and 4, an absorbing organ. 5, It 

 plays an important part in the regulation of the temperature of the body by 



FIG. 304. Vertical Section of the Epidermis of the Prepuce, a, Stratum corneum, 

 of very few layers, the stratum lucidum and stratum granulosum not being distinctly 

 represented; &, c, d, and e, the layers of the stratum Malpighii, a certain number of the cells 

 in layers, d; and e showing signs of segmentation; layer c consists chiefly of prickle or ridge 

 and furrow cells;/, basement membrane; g, cells in cutis vera. (Cadiat.) 



controlling the loss of heat, i.e., a temperature-regulating function, discussed 

 in the chapter on Animal Heat. 



Structure. The skin consists principally of a vascular tissue named 

 the corium, derma, or cutis vera, and of an external covering of epithelium 

 termed the epidermis or cuticle. Within and beneath the corium are em- 

 bedded several organs with special functions, namely, sudoriferous glands, 

 sebaceous glands, and hair follicles; and on its surface are sensitive papilla. 



