PHYSIOLOGY. 



ject, so comes near to the absence of impressions. * Seda- 

 tive impressions' I distinguish from sensations ; they very of- 

 ten produce no sensation : thus, opium, alcohol, mephitic air, 

 applied to our bodies, induce a state of sleep ; they are known 

 to diminish the motions in general, and have got the appella- 

 tion of sedatives. With regard to the chief of them all, opium, 

 the question has been often put, quomodo opium facit dormire? 

 and the variety of theories offered by the mechanical physicians 

 has amounted to little more than that of the Galenists, quid 

 habet in se facultatem dormifaciendi. It has been alleged by 

 some, that opium coagulates, and by others, that it rarifies the 

 blood ; but we say, that opium produces its effect independently 

 of the fluids and of their circulation. Whatever difficulties 

 Dr. Haller has raised upon this subject, I say that the experi- 

 ments of Alston, Whytt, and Monro, our colleagues, upon 

 animals, after the circulation of the blood had ceased, are quite 

 conclusive ; that though opium acts slower, it most certainly 

 does act, after all motion of the fluids have ceased ; nay, that 

 it acts upon every separate and detached part, even when the 

 communication with the brain is destroyed, that it acts upon 

 the inherent power, so that we need not discuss the matter 

 whether it coagulates or rarifies the blood, as its direct operation 

 is upon the nervous power, the mobility, sensibility, and irrita- 

 bility of which, it destroys in every particular part to which it is 

 applied. 



" I go on to add another cause ' Evacuations? That evacu- 

 ations dispose to sleep has been commonly observed ; they can 

 go so far as to destroy the action of the brain, and produce de- 

 liquium animi, and therefore may induce sleep, or cause a ces- 

 sation of the animal functions to a certain degree. These eva- 

 cuations are so sudden in producing the deliquium animi, that 

 they cannot act entirely by destroying the secretions, but by 

 taking off the necessary tension of the brain. A certain degree 

 of impetus is necessary to support its action, which ceases on 

 drawing this off but a very little ; and we know that the cessation 

 of the action depends upon that cause, because, by the horizon- 

 tal posture, we obviate or recover it ; so that it is owing to causes 

 of a diminution of motion in the brain, and is not occasioned by 

 the diminution of secretions there. Some would refer the whole ef- 



