456 Edivard Livingston Youmans. 



at the great organic laws of waste and repair, of digestion, 

 nutrition, and respiration. The brain, in its functional 

 exercise, was found to depend, equally with all other living 

 parts, upon these processes. The discovery of the minuter 

 structure of the brain resulted from the application of the 

 perfected microscope. Its grey matter was found to con- 

 sist of cells, and the white substance of fibres of amazing 

 minuteness the cells being regarded as the sources of 

 nerve power, while the fibres serve as lines for its dis- 

 charge. 



When a tolerably clear conception of the structure of 

 the nervous system had been reached, physiology imme- 

 diately propounded the question of its mode of action. 

 The first decisive response was made a number of years 

 ago, by Sir Charles Bell, who found that there are two 

 great systems of nerves, which perform different functions ; 

 one conveying impressions from the surface of the body to 

 the centres, and another transmitting impulses from the 

 centres to the muscles, and thus controlling mechanical 

 movement. This discovery was of the gravest importance. 

 It had been contemptuously asked, What has anatomy to 

 do with mind ? Bell silenced this cavilling forever by 

 showing that it first revealed a definite mental mechanism, 

 and traced out some of the fundamental conditions of the 

 working of mind. 



A few years later, Dr. Marshall Hall made another very 

 important step in determining the organic conditions of 

 mental activity, by the discovery of the independent action 

 of the spinal cord. It had hitherto been held that the 

 brain was the sole seat of nervous power. All impressions 

 were supposed to be conducted directly to it, and all man- 

 dates to the muscles to issue from it ; and as the brain was 

 the seat of consciousness and volition, these operations were 

 thought to be essentially involved in every bodily action. 

 But Dr. Hall demonstrated that the spinal cord is itself a 



