EXCRETION". 481) 



suffer from intestinal worms, or other such irritation. Generally the 

 afferent impulse which calls in to action the desire to micturate is excited 

 by over-distention of the bladder, or even by a few drops of urine 

 passing into the urethra. This passes -up to the lumbar centre (or cen- 

 tres) and produces on the one hand inhibition of the sphincter and on 

 the other hand contraction of the necessary muscles for the expulsion of 

 the contents of the bladder. 



The Structure and Functions of the Skin. 



The skin serves (1), 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 important secretory and excretory, and (4), an absorb- 

 ing organ, already noticed, p. 425; while it plays an important part in 

 (5) the regulation of the temperature of the body. (See 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 epithe- 

 lium termed the epidermis or cuticle. Within and beneath the corium 

 are imbedded several organs with special functions, namely, sudoriferous 

 glands, sebaceous glands, and hair follicles ; and on its surface are sensi- 

 tive papillae. The so-called appendages of the skin the hair and nails 

 are modifications of the epidermis. 



Epidermis. The epidermis is composed of several strata of cells of 

 various shapes and sizes; it closely resembles in its structure the epithe- 

 lium of the mucous membrane that lines the mouth. The following 

 four layers may be distinguished in a more or less developed form: 1. 

 Stratum corneum (fig. 310, a), consisting of superposed layers of horny 

 scales. The different thickness of the epidermis in different regions of 

 the body is chiefly due to variations in the thickness of this layer; e.g., 

 on the horny parts of the palms of the hands and soles of the feet it is 

 of great thickness. The stratum corneum of the buccal epithelium 

 chiefly differs from that of the epidermis in the fact that nuclei are to 

 be distinguished in some of the cells even of its most superficial layers. 



2. Stratum lucidum, a bright homogeneous membrane consisting of 

 squamous cells closely arranged, in some of which a nucleus can be seen. 



3. Stratum granulosum, consisting of one layer of flattened cells 

 which appear fusiform in vertical section : they are distinctly nucleated, 

 and a number of granules extend from the nucleus to the margins of 

 the cell. 



4. Stratum MalpigJiii or Rete mucosum consists of many strata. 

 The deepest cells, placed immediately above the cutis vera, are columnar 

 with oval nuclei: this layer of columnar cells is succeeded by a number 

 of iayers of more or less polyhedral cells with spherical nuclei; the cells 



