296 PHYSIOLOGY 



two processes in the granular material. The second neuron is formed by the 

 ganglion-cell with its finely branched dendritic processes in the granular 

 matter and its motor axon. which passes into the muscle fibres. 



As to the manner in which the impulse passes from the branches of one 

 cell into those of the other, opinions are still divided. The question will 

 have to be more fully considered when we come to deal with the vertebrate 

 nervous system. Many believe that there is no anatomical continuity between 

 the two neurons, and that the excitatory change is transmitted by a mere 

 contiguity, a change in one set of nerve-endings exciting a corresponding 

 change in another set of nerve-endings in immediate contact with them. 

 By certain methods however it is possible to show the existence of an anato- 

 mical continuum throughout the whole nervous system in these inverte- 

 brate animals. Apathy and Bethe have demonstrated the presence of a 

 continuous system of neurofibrils (much smaller than an individual nerve 

 fibre), which, starting in a sensory cell, pass into a network of fibrils forming 

 the greater part of the central granular matter. From this network neuro- 

 fibrils run along the dendrites into the ganglion cells, forming there a small 

 network through the centre of which a neurofibril is continued down the nerve 

 processes again, and passes out along the motor nerve to end in a network 

 of fibrils among the muscle fibres. In a system so constituted it is evident 

 that, although an excitatory process passing along a given fibril may find 

 certain paths easier than others, and so maintain a constant prescribed 

 path through the nerve system, yet it will be possible, by sufficiently increas- 

 ing the strength of the excitatory process, to cause it to travel in all direc- 

 tions in the central nervous system and to evoke in this way a general 

 activity of all parts of the body, a condition in fact found to obtain in the 

 normal animal. It is significant that, although a great number of fibrils 

 pass into the bodies of the ganglion cells, yet in many cases, especially in 

 crustaceans, fibrils are to be found sweeping from the neuropilem or nerve 

 network of the granular substance into a nerve process, and thence into 

 its motor axon without at any time entering the body of the cell (Fig. 1 : ( .n. 



