PRESIDENTIAL ADDRESS. 529 



excited by identically the same character of transmitted change that inhibits the 

 intermediate neurones. 



Now it would be a simple matter to show that all these points might be dealt 

 with adequately on the assumption that nerve-cells invariably contained a mix- 

 ture of two materials, existing in different proportions in different cells, each 

 of which was forced into a diametrically opposite physical state to the other as 

 the result of changes in physical conditions of the kind transmitted by nerve- 

 fibres. 



It is of interest then that there is definite reason to suppose that within nerve- 

 cells there are always two substances which seem to have their states diversely 

 affected by different conditions. One of them is the characteristic constituent of 

 what I have been irreverently terming the cuticle, the nerve-fibre ; and the 

 other a complex material which apparently represents the primary product of 

 nuclear activity, and is spoken of as the material of Nissl. It may seem a 

 weak point in my use of the term that this cuticle-stuff is found within the cell- 

 body. Perhaps so, but perhaps also not so; the point is not worth discussing. 



The point really worth discussion is as to whether it is true that these sub- 

 stances are affected in diametrically opposite ways by the same change, just as 

 if, for example, one of them was possessed of acid and the other of basic 

 characters ; so that the basic was precipitated, and the acid dissolved by the 

 addition of an alkali : since if they exhibit any opposite behaviour in the presence 

 of the transmitted excitation, then it is indeed probable that their admixture 

 is responsible for many of the orderly vagaries of transmission through nerve- 

 cells. I am proceeding as if this is really true to a consideration of its influence 

 on the development of nerve-cells. 



Imagine a developing afferent neurone in contact with two other neurones, 

 hut by different extents of it's surface, so that it transmits a larger quantity of 

 change to the one than to the other. In both cases it affects an algebraical 

 sum of opposing properties, and we might think of it as effecting a compression 

 and an expansion. Now let there be the slightest difference in the force required 

 to compress and to expand, and it might readily happen that the effect of a 

 minimal dose might be to produce an algebraical sum in favour of compression, 

 whilst a maximal created a general effect of expansion. One of these cells then 

 might be habitually excited and grow a cuticle traversing considerable distances 

 in the central nervous system ; whereas the other is inhibited until the accumu- 

 lation of charges previously received add up to the dose required to tip the 

 algebraical sum in favour of excitation, and then first commences the growth 

 of a short nerve-fibre. 



This, however, involves the assumption that these cells of both classes store 

 up all the transmitted energy they receive, that they do not leak, do not trans- 

 mit, and thus grow their nerve-fibres from the effects of accumulation. Within 

 certain limits this supposition is sound, since we are familiar with that sum- 

 mation which is a leading feature in nerve-cell conduction. Below a certain 

 definite quantity of charge they do not leak, and are found by a second 

 impulse arriving some little time after an apparently ineffectual predecessor in a 

 new state, so that the new-comer is effectual. Now if no new-comer arrives 

 in time we must suppose the energy due to the first as having affected the growth 

 of the cell in one direction or another — that is to say, in one direction if it pro- 

 duced the change characteristic of excitation, and the other if producing to a 

 minimal degree the change characteristic of inhibition. It is legitimate, too, 

 to suppose these limits as set by the capacity and extent of excitable contacts, 

 The larger the extent of contact the sooner and the more effectual must be the 

 leakage. Thus we may readily picture the excited neurone as growing more 

 and more cuticle until this growth is checked by the number, extent, and capa- 

 city of the excitable contacts made in course of growth. When a certain 

 measure of growth has occurred we may suppose that residual charges below the 

 margin of leakage are now only just sufficient to maintain the district of cuticle 

 that has been laid down. We have therefore encountered the limits of growth 

 of the nerve-fibre. 



As for the second cell, which we have considered as mainly inhibited. In it 

 the ma.ss-action of the products of nuclear change is diminished and we must 

 think of it as enlarging its cell-body by an increased nuclear activity; possessed 



1911. M M 



