HYPERTROPHY 121 



that proliferation may be more active if the tissues are well fed, but 

 this does not show that the presence of food is a factor in the pro- 

 liferation except so far as it keeps the proliferating cells in their best 

 condition for growth. It is possible in many animals, more espe- 

 cially in some of the lower forms, to force them to grow rapidly by 

 supplying them with a large amount of food, and conversely by de- 

 creasing the food to delay the growth. While this shows that the 

 rate of growth is, within certain limits, a function of the amount of 

 food, there may be also other factors that enter into the result, and 

 in all cases there is an upper limit beyond which it is not possible to 

 make the animal grow any larger. 



That the presence of certain substances may bring about the 

 enlargement of a part must be admitted as probable. It has been 

 shown, for instance, that after the removal of certain lymphatic 

 glands other glands may become larger. This appears to be due to the 

 greater activity of the gland, brought about probably by the presence 

 of an increased amount of some specific substance. In this instance 

 the result can scarcely be due to a decrease in the physical resistance 

 to growth or to an increase in the blood flow, except so far as this 

 is brought about by the increased activity. It is, of course, possible, 

 even if it cannot be positively shown in the case of the lymphatic 

 glands, that a substance in the blood causes the hypertrophy in cer- 

 tain organs, while in others, as in the kidney, an increase in the blood 

 flow may be also a factor in its hypertrophy. 



The view held by several pathologists, that hypertrophy and 

 regeneration may be caused by the removal of a physical resistance 

 to growth, cannot be looked upon as a very probable hypothesis. 

 The experiments in grafting of hydra and lumbriculus show that 

 regeneration may still take place when the physical resistance has 

 been reestablished by grafting two pieces together. These results, 

 which are more fully described in a later chapter, demonstrate that 

 the growth is due to other influences. 



A comparison with the lower animals shows that proliferation 

 takes place when all but three of the factors considered in connection 

 with hypertrophy and regeneration in the higher forms have been 

 eliminated. These are, first, the action of substances that act either 

 directly or as counteracting some substance already present, as Zieg- 

 ler suggests ; second, an innate tendency in the organism to complete 

 itself; and, third, the use of the organ. It is impossible that the sec- 

 ond factor enters into the problem of hypertrophy. In those cases 

 in which regeneration takes place when a part of an organ is removed, 

 as in the case of the liver, for example, the result may possibly also 

 involve the second of the two factors, for the process is much like 

 that of morphallaxis in the lower animals. 



