PRESIDENTIAL ADDRESS. 527 



tendency to break bounds, or when taking place behind resistant but distensible 

 bounds with a tendency to expand the region of activity. Thus it happens that 

 the excited cell tends to grow in size, whereas, on the other hand, the in- 

 hibited cell tends to diminish, and the resting cell to remain unaltered. 

 These several proceedings are possible so long as the surface membranes of the 

 cells, or of structures within them, which form bounds resistant to the pressure 

 of molecular activity, are at the same time porous to water molecules ; and this 

 we know is within limits true — namely, that the cell is enclosed by such semi- 

 permeable membranes. Thus when the excited nerve-cell grows in size, and the 

 region of molecular activity is thus increased, the materials within the cell are 

 diluted by an admission of water. 



Attention is now directed to the probability that there is some kind of 

 material in solution within the cell which takes no part in this increase of mole- 

 cular activity ; is, on the other hand, retarded in its motion by agglutination 

 into colloidal clusters, and may finally be precipitated. I, for my part, have no 

 hesitation in saying that there is every probability that this is indeed the 

 primary phenomenon of excitation, this precipitation. Leaving that point, how- 

 ever, alone, it is probable that this tendency towards precipitation occurs. 

 This material, precipitated and diluted, thus loses some of that mass-action 

 formerly holding in check its formation by the particular chemical reaction 

 that is always tending to produce still more of it. More of this material is thus 

 produced within the excited cell, and is in turn precipitated, and still more and 

 more. We may therefore think of these excited cells as laying down a structure 

 which I will ask your permission to describe as a cuticle. The nerve-fibre is the 

 cuticle of the nerve-cell. Once give it such a name, as is in part justifiable, and 

 no one will be surprised that these structures are pushed out to an extraordinary 

 distance from their parent cells, and that their length is measured not like other 

 details of cell-structure in thousandths of millimetres, but sometimes in metres, 

 and therefore on a scale with units one million times larger than usual. 



If we entertain this idea, that nerve-fibre growth is proportional to ex- 

 citation, we are prepared for the statement that the physical characters of the 

 cells within the nervous system and their relations to one another are all due 

 to their relative experience of incidents of excitation. We face the fact that 

 their chatnical work is of a universally monotonous type, a drearily slow and 

 respectable type, and that their physical features and arrangements are capable 

 of very simple explanation. 



Now structure is everywhere the outcome of function, and those functional 

 developments that lead to the growth and differentiation of structure contain the 

 most interesting and most fundamental problems of physiology. If it is thought 

 that the main relationships of parts within the nervous system are fixed from 

 an early date of development, it would then seem that to the physiologist the 

 nervous system is a place of very limited interest. But this is by no means the 

 case, the relationship of parts is by no means a fixture within the nervous system. 

 In' so far as it is fixed, it is the sign of the orderly action of circumstance upon 

 the structures of the body, and the result rather than the cause of the monotony 

 of existence. There is, however, no need to labour this point or to debate our 

 interest in this system. One portion of the nervous system is the seat of the 

 mind, a fact to which I will return later. The whole of it is the very essence 

 of the unity of the organism containing it. It is the rapid transmission of 

 physical states through its individual nerve-fibres, and the modifications in 

 transmission determined by passage into its constituent cells, which serve to weld 

 the actions of the several parts of the body into that phase of common action 

 which is suited to the necessities of the moment. 



That there is no moment during life when there are not many paths through 

 the central nervous system engaged in this business of transmission is a state- 

 ment of commonplace realised by all. There are not, however, in my opinion, a 

 sufficiently large number of persons sufficiently impressed by that greater truth, 

 discovered and analysed by Sherrington : that no path is thus busy without there 

 being at the same time some other path maintained in a condition of enforced 

 rest. Whenever the system is excited at one part it is also inhibited at another, 

 and it is this phenomenon that lies at the root of the harmonious effects produced 

 by this system, and forms the means whereby action suspends antagonistic action. 



