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 anatomical continuum throughout 

 the whole nervous system in these invertebrate 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 neurofibrils 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 increasing the strength of the excitatory process, 

 to cause it to travel in all directions 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. 139). 



