332 



PHYSIOLOGY 



branches of the ganglion-cells and is transmitted by them along the 

 motor nerve fibre to the muscles. Opinions were long divided as 

 to the nature of the central granular material. It consists entirely 

 of a close felt-work of fibres, which may be regarded as processes 

 either of the sensory nerve fibres or of the nerve-cells. The typical 

 reflex arc in this case therefore is formed by two nerve -cells with their 



/ 



processes. Such a nerve-cell with its processes is spoken of as a 

 neuron. The first neuron, the recipient neuron, or receptor, is repre- 



&ANGLION - CHAIN 



FIG. 139. Diagram of nervous system of a segmented invertebrate (earth- 

 worm or crayfish). (From SCHAFBB, after RETZITJS.) 

 s', sensory cells ; s, afferent nerve-fibres ; TO, motor neuron ; i, central 



or intermediate cell. 



sented by the sensory cell with its 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 



