138 THE INVERTEBRATA 



precise but, since one efferent neurone can serve several afferent, 

 more economical of fibres than the nerve-net. Usually, moreover, 

 the system is complicated, and in the highest animals it is enormously 

 complicated, by the branching of axons, which increases the number 

 of efferent fibres an afferent fibre can affect, and by the intervention, 

 in the central nervous system, of intermediate neurones between 

 those which are directly afferent and efferent. By this the number 

 of afferent fibres which an efferent fibre can serve is increased. For 

 the efficient working of this system it is essential that impulses should 

 pass over it in one direction only, and thus should not leak from one 

 path to another and affect organs for which they were not destined. 

 That is provided for in the following way. Where the terminal 

 branches of an axon meet the dendrites of another neurone the two 

 are not continuous but interlace without joining, making what is 

 called a synapse. Their discontinuity makes an obstacle, over which 

 impulses can pass in one direction only, from axon to dendrites. The 

 mode of passage of impulses from the one to the other and again from 

 efferent neurone to effector cell is not at present known. It is perhaps 

 an electrical process, but it involves the production of a chemical 

 that probably affects the sensitivity of the recipient cell. In that case 

 the transmission of a nervous impulse includes a process which 

 recalls that other mode of communication, mentioned above, in 

 which the chemical messengers known as hormones are distributed 

 through the vascular system. 



Stimuli received from the nervous system, like other stimuli, may 

 inhibit as well as cause activity. This is very important, because when 

 an action is to be performed activity which hinders it must be 

 abolished. Thus, for instance, a contracting muscle may by a reflex 

 inhibit contraction in an opposing muscle: the circular and longi- 

 tudinal muscles of the earthworm are an example of such a system 

 (p. 261). A similar end is obtained in a different way in the muscles 

 which open and close the claws of crabs and lobsters, where each 

 muscle fibre has two nerve fibres, one excitatory and the other in- 

 hibitory, and impulses from the central nervous system pass simul- 

 taneously to the excitatory fibres of one muscle and the inhibitory 

 fibres of the other. Further, one neurone may inhibit another, and 

 thus inhibition may be effected not only through but in the central 

 nervous system. 



Concerning the way in which the central nervous system, which 

 in the lowest animals that possess it is merely a relay station where 

 impulses from the principal sense organs are multiplied and distri- 

 buted, later takes on inhibitory functions, and later still develops 

 "functional units" for co-ordination, and concerning the control of 

 the latter by the brain, something is said on pp. 198, 261, 448 



