ATROPHY 123 



building up of an organ in more than an inverse ratio to its breaking 

 down. We are, however, familiar with phenomena of building up 

 due to an increase in the food supply. It might appear from this to 

 be more in accordance with what we find, to assume that the hyper- 

 trophy is solely due to an increase in the food supply ; yet there are 

 other facts known that show that an organ does not increase in size 

 simply because it gets more blood, and that this occurs only when 

 the organs have a greater functional activity. It is a safer conclu- 

 sion, I think, at present to assume that both the activity of the 

 organ and the increase in its supply of food acting together are 

 factors in the result. On the other hand we are so much in the 

 dark concerning the functioning and growth of organs that we can do 

 little more, as the preceding pages show only too clearly, than specu- 

 late in the vaguest sort of way as to what changes take place ; but 

 since the processes seem to be within reach of experimental methods 

 we can hope in the near future to learn more of how the pro- 

 cesses of hypertrophy are brought about. 



ATROPHY 







It would not be profitable to enter into a general discussion of 

 the many cases of absorption, or of atrophy of parts of the organism, 

 but a few examples may be given that have a general bearing on the 

 topics discussed in this chapter. The more noticeable cases arise 

 through disuse of an organ, as shown, for example, in the decrease 

 in size of the muscles of man when they are not used. Since this 

 may take place in a single group of disused muscles, when no such 

 change occurs in other muscles of the same individual that are in 

 use, the most obvious explanation is that the decrease is due directly 

 to disuse. Since the blood that goes to all the parts is the same, 

 the diminution cannot be ascribed to any special substance in the 

 blood. The flow of blood into the disused muscle is less than when 

 the muscle is used, and it might be supposed that atrophy is directly 

 caused by the lessened nourishment that the muscle receives. There 

 is also the possibility that the decrease is brought about by the 

 accumulation of certain substances in the disused muscle itself, but 

 since, in general, the breaking down of the muscle is most active 

 when it is usecl, it seems improbable that the result can be due 

 directly to this cause, unless indeed it could be shown that the sub- 

 stances produced by a disused muscle are different from those in an 

 active muscle. 



Lack of food, as is known, may cause organs to decrease, the fat 

 first disappearing, and then in succession in vertebrates, the blood, 

 the muscles, the glands, the bones, and the brain. Certain poisons 



