90 THE CAUSATION OF DISEASE. 



at once that this pulling-to-pieces will take place in the inverse 

 order of the putting-together — that the dissolution will be a 

 more or less exactly inverted evolution. Often enough dissolu- 

 tion is by no means an exact inversion of evolution. At death, 

 for instance, rapid dissolution occurs ; but when, let us say, a 

 full-grown man dies, we do not find him passing through the 

 several phases of youth, childhood, and foetal life, finally becom- 

 ing an ovum again. There is, however, a great difference 

 between the dissolution which occurs after death and that which 

 attends disease : while in the one case evolution is completely 

 undone, in the other it is only partially undone, for disease 

 being, as just now observed, a vital process — i.e., the tissue 

 undergoing dissolution being still alive — it must have, if we 

 may so put it, a certain amount of evolution still left in it — 

 the structure has not been pulled to pieces to its very founda- 

 tions. Now, as dissolution proceeds from stage to stage, one 

 would naturally expect the tissues to be unbuilt in an inverse 

 order to that in which they were put together — from roof to 

 foundation, so to say ; for if the process commenced at the 

 foundation, all analogy leads us to assume that complete disso- 

 lution would at once occur. These are the grounds on which 

 I have concluded that the principle for which we are now con- 

 tending follows from a priori reasoning. 



I am not for one moment contending that the dissolution 

 in disease is always an exact inversion of evolution, but merely 

 that there is a tendency to such inversion. No one, for instance, 

 would assert that in pneumonia or nephritis the lungs and 

 kidney pass inversely through the various phases of embryonic 

 development. Although the principle applies more or less to 

 the organ as a whole, it is in the study of the individual 

 elements of the tissues that we have the best proof of its truth. 

 When the morbid process leads to rapid dissolution, it is im- 

 possible to trace carefully the various phases of the backward 

 process, but when it takes place by slow and steady steps this 

 may be sometimes done. 



No disorder presents a more favourable opportunity for this 

 study than progressive muscular atrophy. In the first place, 

 the disease is a slow and, for the most part, steady one ; then, 

 again, the process limits itself to a specialized tract of cells ; 



