ORGANS OF THE SENSORI-MOTOR SYSTEM iii 



Both figures represent actual conditions : a receptor may be 

 the dendrites of one neurone (as in touch-receptors) or it may 

 be a chain of neurones (as in the retina of the eye). But in 

 actuahty the dendrons or conducting fibres of many receptors 

 are all bound up together to form a nerve. The nerve after 

 following a more or less prolonged path in the body enters into 

 the spinal cord, or brain, and then each of its fibres, or each of 

 the branches of a fibre, ends in a synapse. All these synapses 

 in which one, or several, nerves end constitute a ganglion. And 



Receptor 



^ Afferent 



GajzgHoriLc 

 centre ] 



Effector >^ >^^-^ 



Efferent' 



Ganglixjnijc " 

 centre 



Fig. 20. 



Effector 

 (twulqcIb fibre) 



I, Diagram of the simplest conceivable neuro-muscular mechanism ; 2, connections of a 

 receptor ; 3, the same, but the receptor is a chain of neurones ; 4, connections of neurones 

 in a ganglionic centre ; 5, the simplest conceivable reflex arc. 



as Fig. 21 shows, there are always other nerves, or nervous, or 

 conducting tracts entering into the same ganglion. A ganglionic 

 centre is therefore the junction, via a multitude of synapses, of 

 two, or many nerves. 



And all receptors are thus the terminations of afferent, or 

 ingoing, nerves that end in ganglia which are the predominant 

 structures in the spinal cord and brain of the vertebrate animals 

 or simply the cerebral, or segmental, or other ganglia of the 

 invertebrates. These structures we shall consider more fully 

 later on. 



