SECTION IV 



NATURE OF THE CONNECTION BETWEEN 

 NEURONS 



THE study of the development of the central nervous system in higher 

 animals has shown that this system is made up of neurons, the connections 

 of which determine the possible paths of impulses in the adult cord. The 

 first stage in the development of the neuron is a single cell without processes, 

 and it is only by the growth of these processes out from the cell that the 

 spinal cord becomes capable of serving as an aggregate of conducting paths. 

 Moreover the deferred acquisition of an influence of one neuron on the next 

 neuron in the line of impulse, or at any rate on the peripheral tissue which 

 receives the end arborisation of its axon, is shown by the fact that entire 

 destruction of the spinal cord in the embryo at an early stage in its develop- 

 ment does not prevent in any way the development of the voluntary muscles 

 (Harrison) ; although, after birth, a severance of the connection between 

 spinal cord and skeletal muscle leads to a rapid degeneration and atrophy 

 of the latter. In the muscle-nerve preparation there is an apparent break 

 of structure at the, termination of the nerve in the muscle fibre, any con- 

 tinuity between' nerve-ending and contractile substance being subserved 

 by undifferentiated protoplasm. There is therefore no difficulty in con- 

 ceiving a propagation across a similar nerve-ending or synapse, between 

 the axon of one neuron and the cell body or dendrites of another neuron. 

 If however the conception we have formed above of the evolution of a 

 nervous system from a continuous conducting protoplasmic network, by 

 a process of ' facilitation ' attended by histological differentiation, be 

 correct, we should expect to find in the fully developed brain and spinal 

 cord some traces at any rate of continuity throughout the whole system of 

 neurons. The question as to the existence of anatomical continuity from 

 neuron to neuron has been hotly discussed both for vertebrates and in- 

 vertebrates. In the case of the latter, evidence in favour of the continuity 

 of neuro-fibrillae from sensory surface to reacting tissue is very strong. 

 Many observers, especially Apathy, Bethe, and Held, have described a 

 similar continuity in the nervous system of mammals. The last-named 

 observer regards this continuity as a product of later development and as due 

 to a process of concrescence occurring between the axon terminations and 

 the bodies of the nerve cells with which they come in contact. It is. easy 

 to show the existence of a fibrillar structure both in the nerve cell and in the 



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