EVOLUTION OF THE NERVOUS SYSTEM 



329 



of the whole animal acting on the marginal bodies. In the reaction 

 of an animal to external stimuli it must be an advantage if the 

 energies of the whole can be concentrated in defence of any one part 

 and be evoked by a stimulus applied at one point. Such a co-opera- 

 tion of the whole for the benefit of the part involves the existence 

 of direct paths from the stimulated point to all parts of the animal 

 if the reaction is to take place with any promptitude. In the medusa 

 we find a beginning of such ' long paths.' The general direction 

 of the fibres of the network is radial, and there is a concentration 

 of such fibres in the neighbourhood of the marginal bodies, so that 

 an excitation can pass more readily from a sense-organ to the manu- 

 brium than it can laterally along the circumference of the animal. 



A 



FIG. 137. Schema showing the utility of the multiplication of neurons and 

 their grouping in central ganglia. (CAJAL.) 



A, an ideal invertebrate with only cutaneous ' sensory ' neurons. 



B, invertebrate, such as a medusa, with sensory and motor neurons, 

 but no central nervous system. 



c, invertebrate (e.g. Annelid) in which the motor neurons are concen- 

 trated in central ganglia. 



a, sensory neuron ; 6, muscle ; c, motor neuron. 



Moreover, a stimulus which is too slight to excite a reflex contraction 

 of the muscular tissue may travel along the nerve tissue to each of 

 the marginal ganglia and arouse these to a discharge of motor impulses. 

 We have therefore in the medusa sensory cells of different sensibilities ; 

 central cells specially adapted to reacting to and reinforcing a nerve 

 stimulus started by a small change in the environment ; a general 

 nerve network propagating the excitatory changes in all directions, 

 but with special ease in certain directions ; and a reactive muscular 

 tissue which carries out movements at the end of the chain of excita- 

 tion, all the elements forming the chain being derived from epiblastic 

 cells. 



A further differentiation of a nervous system, such as that just 

 described, must in the first place involve the laying down of more 

 ' long paths ' and the collection of the special ' central ' cells into closely 

 connected masses (ganglia), so as to concentrate the control of the 

 reactions of the body, and to permit of the ready subordination of 

 every part to the needs of the whole. A special direction is given to 



