THE CAUSATION OF DISEASE. 313 



being in 110 way heightened ; but when the exciting cause 

 is some shock, sudden, or even, it may be, instantaneous, it 

 cannot be the mal-E which sustains it. It must, therefore, be 

 kept up by some permanent alteration of S, and we have 

 thus actual proof of the important part which mere atomic or 

 molecular alterations play in disease. 



The fact that disease is self-sustaining — that it continues 

 after removal of the exciting cause — is conclusive proof that 

 there is a more or less permanent alteration in some tissue or 

 tissues. Hence, the great test of the existence of a purely 

 functional lesion is to ascertain whether or not the disease 

 continues after the removal of the exciting cause and under 

 a perfect system of hygiene. If it does continue, there must 

 be some more or less permanent abnormality of S, and the 

 disease is not purely functional ; but if it does not, it does not 

 follow that there is no structural alteration whatever, for there 

 may be a temporary imperfection in cell structure, as in opium 

 poisoning or in profound exhaustion. When, however, the 

 functional disturbance is only temporary, we may, as already 

 observed, disregard the change in S. 



Let us take an example. A man is so over-worked and 

 jaded that he can do nothing for himself. He is suffering 

 from functional disease, and few would doubt that in such a 

 case there is serious molecular alteration in some of the nervous 

 tissues ; but after a few hours' rest and proper food complete 

 recovery occurs. In such a case the alteration has been very 

 temporary, and one cannot designate it a lesion, seeing that 

 the centres have the power of reverting almost at once to 

 their proper state if they be allowed a fair chance. But if 

 this over- work be continued for a long period, the centres 

 may get into a condition which is not curable by a few hours' 

 rest and proper food ; months of most careful treatment, entire 

 cessation of work, change of scene, and so forth, may be needful 

 to effect recovery ; nay, so serious may the disturbance be, that 

 complete recovery will not occur under the most favourable 

 conditions. Here, then, we have to deal with a distinct lesion, 

 undiscoverable indeed, but nevertheless there. 



I have spoken much of nerve centres, as though they consti- 

 tute the tissues chiefly affected in functional disorders, and this 



