20 ZOOLOGY 



fat cell is entirely given over to the production of an 

 oily substance. The cells of the stomach wall secrete 

 small quantities of hydrochloric acid, which in greater 

 amount would be a violent poison. All cells give rise 

 to waste products, the results of their katabolic pro- 

 cesses. In general, we speak of the waste products as 

 excretions, of the useful products as secretions. The 

 marvel is, that different cells, all nourished in essen- 

 tially the same way, can secrete entirely different 

 substances, acid or alkaline, solid or liquid, accord- 

 ing to their appropriate function. The mother's milk 

 and the poison of the snake are equally products of 

 cell activity. 



Limitations io. So astonishing is this power of cells to take up 

 t( owersof ordinary nourishment and out of it elaborate the 

 ceils most extraordinary and unique substances, that we 



are prepared to believe that their ability is wholly 

 independent of the character of the food. This is not 

 really the case. No cell can change one of the chemical 

 elements into another, or produce a secretion contain- 

 ing a particle more of a given element than was con- 

 tained in the food. Thus, for instance, if the food of 

 babies is deficient in the element calcium, which goes 

 toward the formation of the hard parts of bones, the 

 result is the condition known as rickets. The ^an 

 who dilutes the milk may be responsible for rickety 

 children, whose bones become bent and deformed, 

 because they are deficient in lime. An abundance of 

 other substances will not make up for the deficiency. 1 



1 Certain authors state that rickets is not due so much to deficiency of 

 lime in the food, as to an abnormal state of the body in which the lime is 

 not adequately deposited. In the absence of lime-containing food, the body 

 can produce no lime ; but under certain conditions, though it is supplied, it is 

 not properly utilized. 



