RESPIRATION 177 



respiration, another the beat of the heart, another deglutition. 

 The centres were dependent one on another ; each regulated 

 lower centres, and was governed by those above it, in this 

 bureaucratic scheme. We know nothing of any function of 

 nerve-cells other than that of transmitting impulses. All that 

 we know about nerve-cells is that fchey place afferent and 

 efferent routes in communication, and interpose resistance 

 into nerve- circuits. Every nerve-cell of the grey matter of the 

 brain and spinal cord gives off processes which ramify. The 

 ultimate twigs into which a branch divides are in connection 

 with other sets of twigs derived from the end-branchings of 

 nerve-fibres or processes of other nerve-cells. A nerve-fibre 

 is but the axis-cylinder process of a nerve-cell. Impulses en- 

 counter resistance in passing along the neuro-fibrillae (cf. Fig. 22) 

 contained in the twig-connections of the ramifying processes of 

 nerve-cells . There is no reas on for supposing that anything like 

 the same resistance is offered to the passage of impulses along 

 the fibrillae where they lie within the stout branches of the cell- 

 processes or within the body of the cell. It is easy to make a 

 pictorial representation of such a mechanism. Imagine a 

 model of the stem of a tree made by binding together a large 

 number of wires ; its branches as containing small groups of 

 wires ; the ultimate twigs as separate wires. Carry wires from 

 the roots of one tree to the branches of another. Trees so 

 constructed might be taken as representing nerve-cells. We 

 have not as yet succeeded in demonstrating the isolated neuro- 

 fibrillae as they pass over from the end-twigs of a nerve-fibre to 

 the end-twigs of a nerve-cell branch, but we have abundant 

 reason for believing that they do so pass, and that the resistance 

 to the passage of a nerve-impulse is interposed in this neutral 

 or junctional zone. This resistance has to be overcome. It 

 is overcome by the summation of impulses. All nerve- 

 inipulses are vibratory The first vibrations fail may to get 

 through ; but if the vibrations continue, they exert a cumula- 

 tive effect. After a time they overcome the resistance ; sen- 

 sory impulses flow through the centre into motor channels. 

 In this way we endeavour to explain the rhythmic discharge 

 through the respiratory and other centres. It has not been 

 found possible to determine the source of all the afferent 

 impulses which reach the centre. Respiration continues after 



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