no ANIMAL BEHAVIOUR 



Whatever it is, there is no doubt that it is a physical process 

 (that is, it is not a thought, or idea, or psychosis). In general, 

 then, the physical disturbance that is initiated by some agency 

 that reacts with a receptor organ travels along a chain of neurones, 

 that is, a nervous conductor, or nervous tract, or simply a nerve, 

 until it reaches a " nerve-centre." 



41. ON GANGLIONIC CENTRES 



There need not be any anatomical structure called a " centre." 

 In many lower animals, such as the medusae, or even in the walls 

 of the alimentary canal of the higher animals, the nervous system 

 consists of a continuous nervous netw^ork and the receptors are 

 connected with this by short nervous paths, while the effector 

 organs are similarly attached. Of such a nature is the primitive 

 animal nervous system or " nerve-net." But in all the higher 

 animals there is also a system which is differentiated into peri- 

 pheral and central parts : this is the synaptic nervous system. 

 There are two halves of the peripheral system, (i) the afferent 

 nerves^ which begin in the receptors and lead into the central 

 ganglia and (2) the efferent nerves, which begin in the central 

 ganglia and lead out from the latter to the effector organs. 



41 fl. The Anatomical Conception of the Synapse. If we 

 trace the axon from a single receptor w^e find that it terminates 

 centrally in the spinal cord, or brain, or in other nervous centre, 

 or ganglion (Fig. 20, i). The termination is in a synapse. 



At the synapse two bunches of dendrites come into relation 

 with each other so that the branches of the arborizations inter- 

 digitate with each other but do not actually touch each other. 

 That is, between the ingoing, or afferent neurone and the out- 

 going, or efferent neurone there is tissue, or material which is 

 non-nervous and the impulse, in passing through the ganglion 

 jumps, so to speak, from one set of dendrites to another. 



This is the very simplest conception of a synapse and it is 

 probably only a convenient fiction. What we actually have is 

 indicated in Fig. 20, No. 4. That is, a nerve-fibre entering a 

 centre has synaptic connections with several, or many other 

 fibres entering or leaving the centre. 



416. Ganglionic Centres. Thus receptors are the termina- 

 tions of nerve-fibres : Fig. 20, 2 and 3. 



