ATROPHY OF ORGANS FROM LACK OF SPACE 261 



structure, in nutrition, and in functional activity, 

 but the succession of these three events varies with 

 the nature of the exciting cause. 



Eeduction begins with structure when the exciting 

 cause is lack of space, due, for instance, to increase 

 in another organ (atrophy from lack of space). 



Atrophy begins with function when an organ has 

 become useless (atrophy from lack of utility). This 

 uselessness may arise from two causes ; the function 

 may be no longer useful to the individual or to the 

 species, or it may be assumed by another organ. 



Lastly, atrophy may begin with a diminution in 

 the supply of nutritive materials (atrophy from lack 

 of nutrition). This defective nutrition may be the 

 result of a general cause such as feebleness of the 

 whole organism, or it may be due to the hypertrophy 

 of another organ. 



1. Atrophy from lack of space. 



Cases of this kind are rare. 



1. Development of the teeth. Among animals, the 

 development of the teeth furnishes an excellent 

 example. The number of the teeth in human 

 beings is reduced compared with the number 

 present in Lemurs and in Platyrhine Monkeys. 

 These have six grinding teeth while in man five 

 is the maximum number. Our posterior molar, 

 however, appears late in life ; it is smaller than 

 the others, so that it may be useless for chewing ; 





