ADJUSTMENT THE NERVOUS SYSTEM 123 



cells of the outer skin. The muscle cells are evidently stimulated 

 to contraction by mechanical influences on the amoeboid epithelial 

 cells, which may in a sense be regarded as receptors, but of a very 

 simple kind. In the sea-anemone there are more highly specialised 

 cells in the outer epidermis, which have long, thin projections 

 inwards, forming, with other similar fibres, a felt work between the 

 outer layer and the muscular layer. These fibres ultimately end 

 on muscle cells at a greater or less distance from the cell giving 

 origin to them. Since they serve to elicit muscular movements at 

 a distance from the point stimulated, they may with justice be 

 called nerve fibres and, together with their epidermal cell bodies, 

 form primitive receptor or afferent neurones. There is still no 

 indication of nerve centres. The next stage is met with in the 

 earthworm and elsewhere, and is the beginning of the synaptic 

 system, which enables so much advance in adjustment and co- 

 ordination to be made. We find that the nerve fibre does not 

 proceed straight to a muscle cell, but it enters a nervous mass or 

 "ganglion," and forms a synapse with processes of a neurone, 

 whose cell body is found here. The axon of this neurone passes 

 to a muscle cell, and is hence called a motor neurone. The advan- 

 tage of such an arrangement is that the same muscle can be put 

 into action from different sources, since more than one afferent 

 neurone can form a synapse with it. Thus commences what is 

 called the principle of the "final common path;' where the neurones 

 supplying a particular muscle serve as a common channel for the 

 many reflexes in which this muscle takes part. In the earthworm 

 there are also association neurones. Here the afferent fibre does 

 not form at once a synapse with the motor neurone, but with 

 another one which is entirely confined to the nerve centre. The 

 axon of this neurone ends either directly on a motor neurone^ or 

 only after the interposition of one or more further association 

 neurones, which may end in a more distant part of the nervous 

 system. As complexity and variety of adjustments increase, we 

 find a more and more copious growth of these association neurones, 

 extending to a greater and greater distance, so that the organism 

 becomes a connected whole. Thus the general arrangement is that 

 of a series of alternative loops or arcs (P., pp. 468 and 478), by 

 which an impulse received, say in the foot, may either pass across 

 as a spinal reflex in a neighbouring part of the spinal cord or by 

 various other paths in the brain itself, including the cortex of the 

 cerebral hemispheres. 



There is a circumstance with respect to the receptor neurones 

 in the vertebrate in which they differ from those of the invertebrate. 

 It was mentioned above that cells in the epidermis of the latter 

 organisms become specialised so as to act as more sensitive 



