NERVOUS SYSTEMS, PERIPHERAL AND CENTRAL 481 



be described in the next chapter. The same cortical point, after rest, yields 

 very nearly the same result as it did on previous occasions ; but, if it be stimulated 

 immediately after a previous response, the result is usually found to be reversed ; 

 that is, a point giving excitation after rest gives inhibition if stimulated again 

 after an excitatory response. Suppose, again, that a point gives extension of the 

 elbow, and that then another point which gives flexion of the elbow is stimulated 

 and finally stimulation of the extension point is repeated. It is usually found 

 that the effect is reversed, giving flexion. In the decerebrate preparation, 

 stimulation of an afferent nerve of the limb observed causes contraction of 

 the flexors and inhibition of the extensors. With the cerebrum intact, the 

 action of the cortical flexion point is augmented by stimulation of such an 

 afferent nerve, and the effect of a cortical extension point is reversed to flexion ; 

 so that, if the latter point were being stimulated and giving its normal extension 

 effect, stimulation of the afferent nerve may reverse the effect to flexion, but 

 the result depends, much on the relative strengths of the two stimulations. If two 

 antagonistic cortical points are stimulated, there is some indication of algebraical 

 summation of the opposed actions. The general conclusion is drawn that one 

 of the special functions of the cortex is to reverse the factors of purely spinal or 

 decerebrate reflexes, when necessary. 



In connection with these results, the work of Osborne and Kilvington (1910) is 

 of interest. One of the nerve cords of the left brachial plexus was cut, and its 

 central stump joined to the peripheral stump of the corresponding cord of the 

 right side. After time for regeneration, ten months, stimulation of the right 

 motor area gave movements of both paws, although normally it gives movements of 

 the left only. The left motor area was dead. A point of importance is that the 

 natural co-ordinated movements of the limbs appeared to be quite normal, so that 

 the conclusion is justified that the motor centres of the cortex can change their 

 function. The part of the left motor area in the above experiment must have 

 been assumed by that of the right side. Kennedy (1914) performed similar 

 experiments. He joined together the nerves of the fore limb of the dog in such 

 a way that both extensors and flexors were supplied by the same nerve, while 

 the antagonist nerve was eliminated. After regeneration, the respective cortical 

 centres were stimulated. That of the eliminated nerve was inexcitable, according to 

 the usual rule. The other centre, which would normally have produced either flexion 

 or extension only, according to the point excited, caused contraction of both antago- 

 nists, and at no part of the centre could contraction of either group alone be obtained. 



Some experiments by Burnett (1912) serve to illustrate further points in the 

 function of the cortex. The behaviour of frogs from which the cerebral 

 hemispheres had been removed was much more machine-like and predictable than 

 that of normal ones, although, on casual observation, there was not much 

 difference to be detected, so long as they were not exposed to any new conditions. 

 The normal and the decerebrate frogs were kept together in the same vivarium, 

 and if flies were put in, the normal frogs were more skilful and accurate in 

 capturing them. On the other hand, if a frog of each kind was removed, placed 

 under a glass jar on the table, and flies added, the decerebrate frog captured them 

 all in a few minutes, while the normal frog spent all his time in trying to escape 

 from the holder or in a crouching position, apparently inhibited by fear. 



THE CEREBELLUM 



The cortex of the cerebellum seems to be a supreme centre on the sensory 

 side. With this view of Edinger, the work of Horsley and Clarke (1908) is in 

 accord. No direct result is to be obtained from electrical stimulation. The 

 motor centres in correspondence with the cerebellar cortex are in the dentate and 

 other "basal" nuclei. 



MEMORY AND ASSOCIATION 



Since no fresh neurones are formed during the life of an animal, and when the 

 cell body of a neurone is destroyed no regeneration occurs, it will be obvious that 



3* 



