FUNCTIONS OF THE CEREBRAL HEMISPHERES 449 



The olfactory lobe is connected almost exclusively with the cerebral 

 hemispheres of the same side. Ferrier found that electrical excitation of 

 the hippocampal region causes contortion of the lip and nostril on the same 

 side, i.e. a reaction such as that actually induced in these animals by applica- 

 tion of an irritative or pungent odour direct to the nostril. Ablation ex- 

 periments have not yielded very definite evidence on the question of localisa- 

 tion of the olfactory sense. So widespread are the connections of the olfac- 

 tory tract throughout the brain that it would be extremely difficult, if not 

 impossible, to extirpate all those parts which receive fibres from this tract. 

 It is usual to regard the sense of taste as associated with that of smell, but 

 here again experiment and clinical evidence have yielded very little that is 

 definite. 



GENERAL CHARACTERISTICS OF CORTICAL MOTOR FUNCTIONS 

 The motor phenomena, which may be observed as the result of artificial 

 excitation of the motor and sensory areas in the cortex, constitute a very 

 small fraction of the activities which must be associated with the cerebral 

 hemispheres. An animal with its cerebral hemispheres intact differs 

 markedly from a decerebrate animal in the variety of combined movements 

 which it may exhibit, either spontaneously or in response to external stimuli. 

 When, however, we excite the motor areas directly, we obtain movements 

 which are practically identical with those which we may elicit from a bulbo- 

 spinal animal by appropriate peripheral stimulation. The movements thus 

 excited from the skin may be looked upon as variations from the tonic postural 

 activity of the musculature of the body. We have seen that from the end- 

 organs subserving deep and muscular sensibility (the proprioceptive system), 

 as well as from the labyrinth, impulses are continually arising which travel 

 up to the spinal cord, bulb, cerebellum, and mid-brain, and excite a tonic 

 activity of these centres. The normal attitude of the animal depends on the 

 tonus thereby produced in certain muscles. Muscular tone is indeed a 

 quality specially found in certain groups of muscles. If the cerebral hemi- 

 spheres be removed, as by a section through the crura cerebri or in front of 

 the mid-brain, this postural tonus is increased and the animal enters into the 

 condition of ' decerebrate rigidity.' Destruction of one labyrinth diminishes 

 the tone on the same side of the body ; section of all the afferent nerves from 

 a limb abolishes the tone in that limb, so that its posture thereafter depends 

 entirely on gravity. 



The movements which are excited in such animals by cutaneous stimula- 

 tion involve as a necessary factor inhibition of the postural tone as well as 

 co-operative inhibition of the antagonistic muscles. In the same way 

 excitation of the motor area of the cortex has as its mos.t essential feature 

 an inhibitory action on the postural tonus in addition to its excitatory action 

 on the muscles concerned in the movement. A certain antagonism is 

 evident between the total action of the cerebral hemispheres and that of the 

 proprioceptive part of the central nervous system. Whereas in the decere- 

 brate animal there is increased tonus in the masseters, in the neck muscles, 



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