DIGESTION.] PHYSIOLOGY, 109 



epidermis, and at last overflows at the mouth of the 

 tube over the skin. We call this fluid sweat or 

 perspiration. We call the tube with its knotted end 

 a gland ; and we call the act by which the colourless 

 fluid passes out of the blood capillaries into the canal 

 of the tube, secretion. We speak of the sweat 

 gland secreting sweat out of the blood brought 

 by the capillaries which are wrapped round 

 the gland. 



47. Now we can understand why the inside of the 

 mouth is red and moist. The mouth has a skin just 

 like the skin of the hand. There is an outside epi- 

 dermis, made up of cells and free from capillaries, 

 and beneath that a dermis or true skin crowded with 

 capillaries. Only the epidermis of the mouth is ever 

 so much thinner than that of the hand. The red 

 capillaries easily shine through it, and their moisture 

 can make its way through. Hence the mouth is red 

 and moist. Besides there are many glands in it, some- 

 thing like the sweat gland, but differing in shape; 

 these especially help to keep it moist. 



Because it is always red and moist and soft, the 

 skin of the inside of the mouth is generally not 

 called a skin at all, but mucous membrane, and the 

 upper layer is not called epidermis, but epithelium. 

 You will remember, however, that a mucous membrane 

 is in reality a skin in which the epidermis is thin and 

 soft, and is called epithelium. 



The mouth is the beginning of the alimentary canal. 

 Throughout its whole length the alimentary canal is 

 lined by a skin or mucous membrane like that of the 

 mouth, only over the greater part of it the epithelium is 

 still thinner than in the mouth, and indeed is made up 



