162 



ELEMENTARY PHYSIOLOGY 



skin. If we thrust a needle through the epidermis without pene- 

 trating the dermis beneath, we feel no pain and shed no blood; 

 but, as soon as the dermis or true skin is injured, we feel a sharp 

 sensation of pain, and more or less blood flows 



The internal cavities of the body are all lined with a soft, 

 pinkish membrane called the mucous membrane; and this is 

 continuous with the skin at the margins of the lips, the nostrils, the 

 eyelids, &c. 



The deeper portion of the epidermis, which connects the outer, 

 horny layer with the true skin, is softer and less transparent. It is 



made up of minute cells, some of 

 which contain granules of pigment 

 or colouring matter. Healthy skin 

 is always more or less pinkish in 

 colour. This is not due to the pig- 

 ment cells, but to the presence of 

 blood capillaries in the dermis, 

 the colour of the blood being 

 seen through the transparent epi- 

 dermis. We sometimes describe 

 the skin as 'dark' or 'fair,' 'tawny' 

 or ' blonde ' ; and in these in- 

 stances the differences in the 

 tint are due to the amount of 

 pigment in the cells of \\\& pigment 

 layer. In the European this 

 amount is generally small ; it is 

 greatest in the skin of the negro. 

 Fig. 150. Vertical Section through The pigment layer of the skin is 

 the Skin of a Negro. Magnified also called the rete mucosum 

 250 diameters. (mucous net), and the Malpighian 



a, dermis, or true skin ; be, undermost layer la/VCr. 

 of the epidermi ; b represents the dark " 



d > epidermis> ^ The cells of the rete mucosum are 

 nourished by the blood which circu- 

 lates in the dermis. They are also being continually pushed outward by the 

 growth of new cells beneath ; and, as they approach the surface, the pigment 

 disappears, and they become gradually more and more horny, being, in fact, 

 converted into horny scales which take the place of those which are continu- 

 ally worn off by friction from the outer layer. 



The epidermis, being impermeable to moisture, serves to 

 protect the living tissues beneath it against the absorption of 

 poisons. When it is perfect, poisonous substances may be freely 

 handled ; but these substances are readily absorbed into the blood 

 when the cuticle is cut, or when a small portion of it has been torn 

 off. 



