TACTILE SENSE. 



727 



(a) Stratum Corneum. This is the superficial horny layer and 

 consists of several layers of horny scales, without any nuclei. The 

 layers are separated from one another by narrow clefts containing air. 

 They are in a process of desquamation. The variable thickness of 

 the epidermis is chiefly dependent upon the thickness of this outer 

 layer. The stratum corneum is of greater thickness on the palm of 

 the hand and fingers, and sole of the foot. 



(b) The stratum lucidum is clear and transparent and consists 

 of a few layers of clear cells which contain but the remains of nuclei. 



(c) Stratum Granulosum. Under this is the (d) rete muco- 

 sum, or rete Malpighii. This layer consists of strata of nucleated, 

 protoplasmic, epithelial cells. In the colored races these contain 

 pigment. Among the fair races this layer of the skin of the scrotum 

 and anus contains pigment-granules. The deeper cel!s are more or 

 less polyhedral, while the deepest ones are columnar. These last are 

 placed vertically upon the papillae and are provided with spherical 

 nuclei. Granular leucocytes or wandering cells are occasionally found 

 between these cells. 



The superficial layers of the epidermis are continually being 

 thrown off, w r hile new cells are just as rapidly being formed in the 

 deep layers. Within them there occurs a proliferation of the cells of 

 the rete Malpighii. Many of the cells exhibit the changes of karyo- 

 kinesis. No pigment is formed within 'the epidermis itself. But in 

 brunettes and colored races pigment granules of melanin exist within 

 the cells of the lowermost layers of the stratum Malpighii. The 

 pigment-granules owe their presence here to their having been car- 

 ried thither by leucocytes from the subcutaneous tissue. This ex- 

 plains how a piece of white skin transplanted to a colored person 

 becomes black. 



2. The Coriiim, or cutis vera, is a dense network of fibrous con- 

 nective tissue admixed with elastic fibers. Its entire surface is studded 

 with numerous papilla, the largest of which are upon the volar surface 

 of the hand and foot. The majority of the papillae contain a looped 

 capillary. In some regions of the surface of the body they contain 

 touch-corpuscles. The papillae are arranged in groups whose disposi- 

 tion varies in the several parts of the body. 



The lowermost connective-tissue layers of the corium gradually 

 merge into the subcutaneous tissue. Its arrangement is such as to 

 leave spaces which contain, for the most part, cells of fat. The sub- 

 cutaneous connective tissue, composed of ordinary connective tissue, 

 is soft, and is rich in adipose cells, vessels, nerves, and lymphatics. 



