NERVOUS SYSTEMS, PERIPHERAL AND CENTRAL 467 



be developed at that end of an animal which is most exposed to the complexity of 

 outer influences. The cerebral ganglia have, no doubt, been formed in relation to 

 these manifold sensory impressions, and long association neurones have been 

 developed in order that slight influences, able to affect the delicate receptors in the 

 head but not the less sensitive ones elsewhere, may affect distant parts of the organism. 



As we proceed upwards, we rapidly gain complexity and efficiency by the 

 development of these long association neurones, which are indeed the only 

 fundamental difference between the higher and lower invertebrates. In the 

 vertebrates, the primary motor neurones have their cell bodies in the central 

 mass, like the invertebrates, but the primary sensory neurones, instead of having 

 their cell bodies in or near the surface, have undergone a change in situation, so 

 that the cell body is placed, as it were, on a side branch of the nerve fibre close to 



