256 THE HUMAN BODY 



of a nature to be conveniently controlled by the nervous system. 

 Their control is vested in substances known as hormones, which 

 are produced in special tissues differentiated for the purpose, and 

 are conveyed by the internal medium to all parts of the Body, 

 being thus sure of reaching the structures upon which their in- 

 fluence is to be exerted. Finally, the environment which is fa- 

 vorable for the life and growth of the body-cells is also favorable 

 for the life and growth of foreign and harmful organisms. That 

 the Body is subject to invasion by such organisms is only too 

 well known, and but for the system of defense which the internal 

 medium affords these invasions could not fail to be even more 

 disastrous than they are. 



We can summarize the functions of the internal medium as 

 follows : 



1. To convey to all the living cells their needed supplies of 

 food material and gases. 



2. To convey away from the body-cells the waste materials gen- 

 erated by their activities. 



3. To distribute heat uniformly over the Body and provide for 

 getting rid of the excess. 



4. To convey from the regions where they are produced to those 

 where they are used the special substances, hormones, which 

 regulate many bodily processes. 



. .5. To defend the Body against the inroads of disease-producing 

 micro-organisms. 



The Blood. In the Human Body the internal medium is 

 primarily furnished by the blood which, as every one knows, is a 

 red liquid very widely distributed over the frame, since it flows 

 from any part when the skin is cut through. There are in fact 

 very few portions of the Body into which the blood is not carried. 

 One of the exceptions is the epidermis or outer layer of the skin: 

 if a cut be made through it only, leaving the deeper skin-layers 

 intact, no blood will flow from the wound. Hairs and nails also 

 contain no blood. In the interior of the Body the epithelial layers 

 lining free surfaces, such as the inside of the alimentary canal, 

 contain no blood, nor do the hard parts of the teeth, the cartilages, 

 and the refracting media of the eye (see Chap. XV), but these 

 interior parts are moistened with liquid of some kind, and unlike 

 the epidermis are protected from rapid evaporation. All these 



