3 62 



NAT URL 



[September 14, 191 1 



and it is thus not , surprising that the chemical reactions 

 are in the main the same in every cell, no matter how 

 multifold they may be. The physical experiences of the 

 liver-cells are similarly the same for each cell, and we are 

 not surprised that in physical appearance there is as 

 monotonous a similarity between all the cells in the liver 

 as there is a monotonous chemical similarity between all 

 the cells in the nervous system. In the nervous system, 

 however, there is no monotony in the physical character of 

 rh< cells. It is a notable physical fact that the cells of the 

 nervous system have diverse shapes and sizes, and still 

 more so that these are such as to bring them into a kind 

 of physical relationship observed in no other epithelial 

 organ. It is a notable physical fact that cells originally 

 separated by considerable distances are brought into close 

 contact by a growth of processes, and that they are in this 

 way arranged into chains forming definite paths for the 

 transmission of physical influence through this system. 



Before attempting to explain the manner in which 

 physical conditions give rise to this arrangement, I must 

 briefly sketch the differences in physical state which may 

 be met with in these cells. Thus there are the states of 

 excitation, of rest, and of inhibition. I may simplify 

 matters by saying that there are reasons for considering 

 excitation as associated with an increase in pressure, either 

 due to a temporary increase of particles in motion within 

 the solutions of the cell or to some acceleration in the 

 motion of particles initially present. In rest these particles 

 are in their normal quantity and have their normal mot ; on. 

 During inhibition the particles are decreased in number, or 

 have a retarded motion. Associating excitation with an 

 increase, inhibition with a diminution, and rest with 

 normal degrees of molecular activity, we shall not be far 

 away from the facts. 



Everyone is aware that increased molecular activity is 

 associated with a 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 inhibited 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 molecular 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 'saving that there is 

 • very probability that this is indeed the primary pheno- 

 menon of excitation, this precipitation. Leaving that 

 point, however, 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 vour permission to 

 d. scribe as a cuticle. The nerve-fibre is the cuticle of the 

 -cell. Once give it such a name, as is in part justifl- 

 abli . and no "lie will be surprised that these strui tui 



d out to an extraordinary distance from their parent 

 cells, and that their length is measured not like other 

 details of roll-structure in thousandths of millimetres bul 

 sometimes in metres, and ther.efori on a ;cale will, 'units 

 one million times larger than usual. 



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



proportional lo excitation, we are prepared for tin- state- 



al 1I1- physical characters of the cells within the 



Mn and their relations to one .moil,,,- are all 



'" their relatiyi - >?pi i ii m e of i,,, idents of exi itation. 



NO. 2185, VOL. 87] 



We face the fact that their chemical work is of a uni- 

 versally 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 fur., 

 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 develop- 

 ment, 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 thp 

 nervous system is the seat of the mind, a fact to which 1 

 will return later. The whole of it is the very essen, 

 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 i 

 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 statement of common- 

 place realised by all. There arc not, however, in my 

 opinion, a sufficiently large number of persons sufficiently 

 impressed bv 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. 



When considering the influence of states of excitation 

 upon the growth and arrangement of structures within this 

 system, it follows then that I cannot afford to omit some 

 proper consideration of the manner in which this pheno- 

 menon of simultaneous inhibition may be explained, and of 

 its influence on the growth and arrangements of structures. 

 To get a clearer view of this process we must think in 

 detail_ of the probable nature of the structures involved in 

 the_ simplest case of transmission through the system. It 

 is indeed a simple thing to form a picture of the track 

 entering the system, the structure called the afferent 

 neurone. Here we have a long length of cuticle, or nerve- 

 fibre, _ stretching right from the surface where it is liable 

 to stimulation by change in circumstance, or — more com- 

 plicated case, but very usual one — by the maintenance of 

 circumstance. This afferent neurone is mainly cuticle. It 

 is true that its cell-body is placed like a hump somewhere 

 on its back, but this is no more than an index that it is 

 never inhibited. Thus from the site of change of circum- 

 stance right into the nervous system transmission is of the 

 simplest kind, since all we know of this nerve-fibre is that 

 it transmits most of the excitations it receives at a rapid 



I and without loss from one end to the other. We can 



therefore see the excitation planted by it into everv cell 

 with which it comes in contact within the system'. By 

 some of its branches it plants this excitation into nerve- 

 cells, whose nerve-fibres pass out to reach the site of 

 action. It is a simple matter again to picture this first 

 set of efferent neurones as receiving an excitation which 

 thi '\ then transmit. That there is a certain complexity in 

 the process is a fact with which we are not al 

 concerned. 



But now, what about the site of antagonistic action, the 

 parts that are held in a state of enforced rest? To them 

 also lead perfectly similar efferent neurones, incapable of 

 producing any other effect in the site of antagonistic action 

 than that of exciting it or transmitting excitations 

 it. We must therefore conclude that it is this 

 second sel of efferent neurones that are inhibited and main- 



