Rev. P. Keith oti the Susceptibilities of Living Structures. 171 



If nerves transmit impressions to the brain, liow is the 

 transmission effected? On this question physiologists are 

 divided. Some have supposed that the impression is trans- 

 mitted by means of a sort of vibratory or molecular motion 

 that is communicated to the extremity of the nerve, and pro- 

 pagated throughout its whole extent with great rapidity. This 

 opinion was adopted by Hartley in his celebrated work Oti Man; 

 but it is supported by no admissible proof, and has consequently 

 no legitimate claim upon our belief. Others think that the 

 impression is transmitted by means of a subtle and peculiar 

 fluid which the nerve secretes*, by means of a chemico-vital 

 process, and deposits in the longitudinal tube that it is sup- 

 posed to form, or which if it secretes not, it at least conducts, 

 — analogous to the electric or galvanic fluid, or to a fine 

 and subtle asther. This opinion may perhaps be correct; but 

 still it is only an opinion : we cannot receive it as an esta- 

 blished fact. Hence, though it is certain that nerves transmit 

 impressions, yet the actual mode of their transmission is al- 

 together unknown ; and the difiiculty attending the elucidation 

 of the subject is increased by the fact, that impressions made 

 upon organs that are even destitute of nerves are still found 

 to be conveyed to the brain. Thus injuries done to the ten- 

 dons, cartilages, or ligaments, are quickly followed by pain, as 

 well as inflammations incidental to other parts, where no nerve 

 enters, or rather where no nerve is known to enter. — Is this 

 pain produced in any way independent of the brain ? We 

 may rest satisfied that it is not: for if the brain is stupified 

 by wine, or by opium, or by any other means, the pain is no 

 longer felt. — Bichat thinks that where nerves are wanting, the 

 impression is transmitted by means of the operation of the 

 vital energyf . It is at least as good a conjecture as that of 

 Reil's nervous atmosphere, but it is no solution of the diffi- 

 culty. 



But though the mode of the conveying of impressions is not 

 known, yet the perception that we have of the impression 

 made is well known. It is a noble and exalted attribute, ele- 

 vating the subjects of the animal kingdom to the dignified 

 rank which they hold in the scale of earthly being, and en- 

 dowing them with the knowledfje of good and of evil. Yet 

 all animals are not endowed with this attribute m the same 

 degree. The lowest tribes of animals exhibit but slight indi- 

 cations of sensibility, and but a small proportion of brain. 

 Animals that are placed higher in the scale, exhibit stronger 

 indications of sensibility, and a larger proportion of brain. 



* f'uvicr, Rigur Ammal : Iiilmd. f Aiinl. (Icn. j). 188. 



Z 2 Man, 



