EVOLUTION OF THE NERVOUS SYSTEM 



295 



or subcesophageal, ganglion gives origin to ten pairs of nerves which are distributed 

 to the mandibles, to the jaws or maxillae, to the maxillipedes, and to the branchial 

 appendages of the latter. 



When we investigate the structural basis of such a nervous system we 

 find that, as in medusa, the starting-point of the reflex arc is in certain 

 neuro-epiihelial cells (Fig. 138) lying on the surface of the body. These cells 

 are spindle-shaped, and have one short process passing to the surface, and a 

 long process which runs in a nerve fibre or collection of fibres towards the 

 ganglion of the segment. Arrived at the ganglion it divides into two 

 branches, which pass towards the two ends of the body and become lost in 



Nerve Cell. 



NerveCell 



FIG. 139. Diagram of a reflex arc in a (neuro-fibrillar) invertebrate nervous system. 

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



the granular material forming the inner part of each ganglion. The ganglia 

 themselves consist internally of this ' punctuated substance ' or granular 

 material, and externally of a capsule of ganglion- cells. Each of the ganglion- 

 cells sends one thick process towards the centre, which rapidly divides, 

 some of the branches passing into the granular material, while one branch 

 passes outwards in a nerve to end in a network of fine fibrils within the 

 muscles on the surface of the body. The nervous impulse excited in the 

 sensory cell on the periphery travels therefore up a nerve fibre into the granu- 

 lar substance of the ganglion. From this granular substance it is collected 

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

 the motor nerve fibre to the muscles. The central granular material 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 represented by the sensory cell with its 



