PHYSIOLOGY. 



the system ; perhaps this is not explained : the most common 

 view is, that opium carries in its substance both a sedative and a 

 stimulant power ; but this is by no means clearly proved, and I 

 think that another explanation may be given that the stimu- 

 lant power of opium arises from the vis medicatrix naturae, from 

 that power of the constitution by which it has a tendency to 

 obviate any sort of injury applied to it, and can obviate the effects 

 of these sedative and debilitating powers, as well as any others. 

 Were our reasonings of this kind confined to opium, there might 

 be some difficulty, but it appears to be the case with every se- 

 dative or debilitating power which we can apply ; thus, acids, 

 according to direct experiments, are undoubtedly sedative 

 powers, yet these are, in many instances, also stimulant ; neu- 

 tral salts, although for the most part sedative, are sometimes di- 

 rectly stimulant, and even the refrigerant are, at the same time, 

 stimulant. It is proved especially in the operation of cold. It 

 will be admitted that heat is the foundation of vital power, and 

 that cold is of a directly contrary nature, and, if in a certain de- 

 gree, will destroy life entirely. This is a matter not questioned ; 

 but within this degree, cold acts, for the most part, as a stimu- 

 lant. How the same power of fire, if I may so speak, should, 

 in its opposite degrees, be stimulant or sedative, is difficult to 

 explain, except by taking the last effect as not depending upon 

 the direct operation, but upon that condition of our economy by 

 which it resists debilitating powers. I have formerly called it a 

 reaction, but this term is not proper when applied to a body 

 which is not an agent, as against an active power." 



CXXII. As the mechanism of the brain, suited to its sever- 

 al functions, is not at all perceived (CXX.) ; and, at the same 

 time, as very few of these functions are carried on without sen- 

 sation and volition, it must appear from this and many other 

 considerations, that the mechanism of the brain (CXVII.) 

 would not be sufficient for the purpose, without being united 

 with a sentient principle or mind that is constantly present in 

 the living system. " This would require a good deal of ex- 

 planation or detail, which I shall avoid, referring you to my 

 late colleague Dr. Whytt, the whole of whose ' Essay on the 

 Vital and Animal Motions 1 is to prove this. But we differ 

 from other physicians with regard to the extent of the powers 



