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 development 

 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 continuity between nerve-ending and con- 

 tractile substance being subserved by undifferentiated protoplasm. 

 There is therefore no difficulty in conceiving 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 con- 

 ception we have formed above of the evolution of a nervous system 

 from a continuous conducting protoplasmic network, by a process of 

 facilitation or ' Bahnung ' 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 dis- 

 cussed both for vertebrates and invertebrates. In the case of the 

 latter, evidence in favour of the continuity of neuro-fibrillss 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 



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