528 TRANSACTIONS OF SECTION I. 



\Yhen 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 phenomenon 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 pic- 

 ture 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 

 complicated 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 circumstance 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 pace 

 and without loss from one end to the other. We can therefore see the excitation 

 planted by it into every 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 they then trans- 

 mit. That there is a certain complexity in the process is a fact with which we 

 are not at present 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 towards it. We must therefore 

 conclude that it is this second set of efferent neurones that are inhibited and 

 maintained in a condition of enforced rest. How then does the change trans- 

 mitted into the several branches of the afferent neurone, having the same 

 character as it invades every branch, succeed in causing diametrically opposite 

 conditions in two groups of perfectly similar efferent neurones ? There is but 

 one answer to this question, namely that transmission into the second group 

 must be through some intermediate mechanism which reverses the character of the 

 change. Now I have no hesitation in naming definite structures in the nervous 

 system as being alone those to which we can impute this reversal, namely 

 certain intermediate neurones which have a way of being interpolated between 

 afferent and efferent neurones. Such neurones are seen in the cord sometimes 

 sending their main nerve-fibre towards efferent neurones placed on the other 

 side of the cord, and in the cerebellum the large cells of Purkinje are seen to be 

 approached by afferent nerve-fibres in this double fashion ; one set reaching them 

 directly, the other set indirectly through intermediate neurones. We shall 

 then picture neurones with short nerve-fibre processes as placed in these paths 

 that are inhibited, and as sometimes responsible for this singular reversal of the 

 transmitted excitation. 



In this connection, too, we must deal briefly with another fact observed by 

 Sherrington, that certain drugs, tetano-toxin and strychnine, affect these inter- 

 mediate mechanisms in such a way that they lose their power of reversing the 

 character of change transmitted through them. When these drugs are applied 

 to any part of the nervous system action and antagonistic action are simultaneous 

 consequences, and the stronger wins. Of the greatest interest, too, is the fact 

 that this disturbance of the process of reversal may be obtained in a graduated 

 manner by the application of such drugs in varied strengths of solution. It 

 is thus clear that there is nothing peculiar about the nerve-fibre portion of these 

 intermediate neurones, since when given excitations to transmit they transmit 

 them, although it is so frequently their normal business to transmit inhibitions. 

 Clear, too, that their cell-bodies frequently inhibited like those of the efferent 

 neurones may also with a slight modification of condition tend towards excita- 

 tion, or, as a matter of fact, be excited, again like the efferent neurones. There 

 is no difference discoverable here between these two sets of cells other than 

 a difference of degree. The one salient fact demanding explanation is this 

 difference under normal conditions in which the efferent neurones are seen as 



