28 



PHYSIOLOGY. 



successive generations thus following each other until the end is ac- 

 complished for which they were first produced. 



OF CELLS DEVELOPED ON FREE SURFACES. 



Of these we have examples in the cells composing the epidermis 

 and epithelium, two structures which are essentially alike in their 

 origin, mode of development, situation, and individual history; they 

 differ, however, in the purposes which they respectively serve in 

 the economy. 



The epidermis is the cellular covering of the external surface, 

 the epithelium is the corresponding covering of the internal cavities. 

 They both consist of cells, which are developed from the subjacent 

 membrane, which are nourished by its vessels, and which are, after 

 a time, cast off from its free surface, to be replaced by a succeeding 

 generation. 



The epidermis is not permeated by either vessels or nerves, but 

 consists solely in a congeries of nucleated particles arranged in 

 laminse. Those that lie deepest and rest immediately on the cutis 

 are small granules scattered in a homogeneous matrix. Those of 

 the next layer are rounded cells of transparent membrane, in which 

 similar granules can be seen. In the succeeding layers these cells 

 are more and more compressed as they are nearer to the surface ; 

 and on the surface they are mere flattened scales. 



The superficial scales are being constantly shed ; and new ones 

 are as constantly formed below from germs, which are supplied by 

 the basement membrane. The soft layer immediately in contact 

 with the true skin was formerly called rete imicosum, but it is, now 

 proved to consist of the same elements with the ordinary epidermis. 

 The epidermis varies in thickness in different parts, according to the 

 amount of pressure or friction to which it is subjected. 



The use of the epidermis is to protect the true skin from mecha- 

 nical injury, and from the irritating effects of exposure to atmo- 

 spheric vicissitudes. 



Mixed up with the epidermic cells, we find the Pigm.e?it cells, or 

 those which contain the peculiar colouring matter of the races. The 

 black colour is caused by the presence within them of a number of 

 flat or oval granules, which are transparent, and exhibit an active 

 movement when set free from the cell. The chemical nature of this 

 pigment is unknown ; it includes, however, a larger proportion of 

 carbon than most other organic substances — every 100 parts con- 

 taining 58^ of carbon. 



Exposure to light increases the development of these pigment cells, 

 as is seen in freckles, tan, and the swarthy hue obtained by expo- 

 sure in tropical climates. The latter is doubtless the cause of the 

 blackness of the negro's skin, operating through successive gene- 

 rations. 



