NERVOUS SYSTEMS, PERIPHERAL AND CENTRAL 477 



intraspinal neurones and synapses between (1) and (2) and between (2) and (3), 

 but the arrangement described is the simplest possible for such a reflex as the one 

 in question, having its receptors and effectors in parts of the body distant from one 

 another. 



The motor neurone (3) is the final common path, and the rest of the arc up 

 to it from the receptor is the afferent arc. 



The complete mechanism of the scratch reflex includes, of course, also a 

 cutaneous sense organ, which intensifies such a stimulus as the bite of a flea, for 

 example, so as to produce a propagated disturbance in the receptor neurone ; and 

 at the other end, the fibres of the flexor muscle, the actual effector, are a part of 

 the mechanism. 



There are some important general characteristics of reflex action which will 

 best be deferred till the following chapter. 



FUNCTIONS OF THE "BRAIN" 



The increase in complexity and effectiveness of the central mechanism, due to 

 the addition of longer intermediate neurones, as well as the addition of more and 

 more intermediate neurones to form cross connections, has been referred to above. 

 At the anterior end of an animal, we find the gradual evolution of the "head" 

 with its specialised and elaborate receptor system, notably that of the " distance 

 receptors," as they are called by Sherrington. These enable the organism to be 

 affected by occurrences which do not themselves come into actual contact with it ; 

 such occurrences are those affecting organs sensitive to light or to sound. 



In connection with these distance receptors, the highest part of the central 

 nervous system, the " brain," is developed. This part of the system has control 

 over all the rest. In ourselves, we know that it is associated, in some way, with 

 consciousness. 



So far is it the case that the brain is to be regarded as the central ganglion of 

 the distance receptors, that, as Langendorff has pointed out (Sherrington, 1906, 

 p. 349), a blinded frog is like one with its cerebral hemispheres removed ; a shark 

 without olfactory lobes behaves as if it had lost its fore-brain. 



Now Pavlov has worked out a method, that of " conditioned reflexes" to be 

 described in the next chapter, by which it is possible to investigate the mechanism 

 of the highest centres without appeal to consciousness. Results of great value 

 are to be obtained by this method. Pavlov himself regards the introduction 

 of psychological modes of expression as obscuring the problems. No doubt, the 

 physiologist is not, as such, concerned with phenomena of consciousness, and the 

 introduction of a method in which they can be omitted is a great advance. At 

 the same time, the phenomena of the higher sense organs necessitate the use of 

 methods of investigation involving the introduction of consciousness, although it is 

 used as an indicator only. 



The methods of comparative psychology may also be referred to as illustrating the 

 possibility of investigation of the higher functions of the nervous system without the necessary 

 introduction of consciousness. The book by Margaret Washburn on "The Animal Mind" 

 (1909) may be consulted by those interested. 



A description, in any detail, of the functions of the higher parts of the central 

 nervous system would require much more space than can be given in a book with 

 the scope of the present one. Fig. 147, compared with Fig. 143 (page 468), may 

 serve to give some idea of the kind of connections that are found in the higher 

 nerve centres. There are, however, a few facts of general interest that may with 

 advantage be given here. 



Those functions which we know as " mental " have their seat in the cerebrum, 

 especially in the cortex or pallium, if the philosophers will pardon the use of the 

 word " seat " in this connection. It is, then, at first sight, a somewhat surprising 

 fact that, by stimulation of certain limited regions of the cortex, definite, localised 

 movements of limbs, face, trunk and so on can be evoked. The area which has 

 these properties is known, for convenience, as the " motor area." Arising from 

 this we have the system of long neurones known as the "pyramidal tract," con- 



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