EVOLUTION OF THE NERVOUS SYSTEM 



333 



the existence of an anatomical continuum throughout the whole nervous 

 system in these invertebrate animals. Apathy and Bethe have demon- 

 strated 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 



Nerve ClL_. 



FIG. 140. 



system. 



NerveCell 



Diagram of a reflex arc in a (neuro-fibrillar) invertebrate nervous 

 (BETHE.) The efferent paths are coloured red, the afferent black. 



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 pos- 

 sible, 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 condi- 

 tion 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 any time entering the body of the cell (Fig. 140). 



