426 PRINCIPLES OF GEN ERA/. PHYSIOLOGY 



the afferent fibres from the muscle have been divided. It appears, then, that there is direct 

 evidence that the process of inhibition implies more than the mere cutting off of impulse*. 



In a somewhat different sense, however, inhibition may perhaps be regarded 

 as a kind of block. We have found reason to look upon an increase of permeability 

 of a membrane as an intimate part of the excitatory process, and inhibition, as the 

 opposite process, would thus be associated with decrease of permeability. But this 

 is obviously quite a different matter from blocking the propagated disturbance 

 itself. 



Other Contributions to the Theory of Inhibition. Although the protoplasm of 

 the neurone is probably a liquid, it contains various substances in the colloidal 

 state. Further, we have seen the necessity of the presence of electrolytes to 

 account for the electrical changes in nerve Now, if these electrolytes lower 

 surface energy, they will be adsorbed on the surface of the colloidal particles (see 

 page 55). This is the foundation of the theory of Macdonald (1905) as to the 

 nature of excitation and inhibition. This investigator points out (p. 335) how 

 the concentration of electrolytes in the external phase would be increased if any- 

 thing in the nature of aggregation or coagulation in the nerve colloids occurred. 

 The adsorbing surface would be diminished. Assuming that these electrolytes are 

 essential to excitation, it will be seen how a coagulation process would be associated 

 with excitation, while a greater dispersion than normal would be associated with 

 greater adsorption of electrolytes and inhibition (Macdonald, p. 348). This is a 

 brief account of this important theory. Details of its application require more 

 knowledge than we possess as yet of the phenomena taking place at the membranes. 

 If we place the seat of excitation and of inhibition of nerve cells at the synaptic 

 membrane, we may suppose that the system of colloids and electrolytes in question 

 either forms the membrane itself or is intimately associated with it. 



In discussing Hill's modified form of Nernst's equation for excitation, we saw (page 394) 

 that it contains a constant, C, which is connected in some way with adsorption (or dis- 

 appearance) of ions. Keith Lucas (1910, p. 243) shows that it is altered by removal of calcium. 

 It seems not unlikely that changes in the value of this factor, C, might be made use of to 

 investigate the hypothesis of Macdonald. 



With regard to the properties of the synaptic membrane, Keith Lucas (1911) 

 points out that it must present a* greater resistance to conduction than the axis 

 cylinder does ; it is thus similar to the junction between nerve and muscle or to a 

 narcotised region in nerve. There is thus a possibility that it might obliterate 

 very rapid, and therefore small (see Wedensky inhibition, page 420), nerve 

 impulses, so that they would not get through the synapse. Adrian (1912, p. 411) 

 calls attention to the fact that an impulse, if it has been able to pass a region 

 of decrement at all, recovers its full size on arriving in normal nerve 

 again ; so that, in order that the block mechanism above referred to may be 

 effective, the impulses must be reduced to zero in one of the synapses ; unless this 

 were the case, it would not matter how many separate regions of decrement the 

 impulse had to traverse. 



It cannot be said that any one of the theories suggested is a satisfactory one. 

 Perhaps each, when modified in accordance with certain important parts of the 

 others, has a part of the truth, and there may be particular fields in which aspects 

 of one theory have more part to play than in other fields. 



There are certain facts which admit of no doubt, and any theory must reckon 

 with these. The function of any particular nerve fibre depends on its termination ; 

 the nature of this termination determines whether it excites or inhibits. In the 

 case of smooth muscle or heart and certain nerve centres, which have an inherent 

 state of excitation, each element requires two nerve fibres to modify its state, 

 increase it or decrease it. In Langley's view, each of these nerve fibres ends in a 

 distinct receptive substance, whose qualities determine whether the effect is 

 excitatory or inhibitory. It appears also that, of the various nerve fibres forming 

 synapses with a particular nerve cell, each has its definite character of inhibiton 

 or of excitation. In such cases as those of reciprocal innervation, Sherrington 

 points out (1906, p. 105) that the inference must be made that each afferent nerve 

 fibre concerned in the reflex divides into two parts in the spinal cord, one set of 



