FUNCTIONS OF THE CEREBRAL HEMISPHERES 449 



SMELL AND TASTE 



The course of the fibres from the. olfactory lobe may be used to throw 

 light upon the localisation of olfactory sensation in the cerebral cortex. 

 There is a great divergence between different animals in the degree to which 

 the olfactory sense is developed, and with this divergence we find corre- 

 sponding variations in the development of certain portions of the brain. 

 In those species with highly developed olfactory sense the following parts 

 of the brain show special growth : 



(1) The olfactory lobe, including the olfactory bulb, and the olfactory 

 tract. 



(2) The posterior part and the inferior surface of the frontal lobe. 



(3) The hippocampal gyrus and the dentate convolution. 



(4) A convolution termed the gyrus supracallosus and forming that part 

 of the gyrus fornicatus closely encircling the corpus callosum. 



(5) The anterior commissure. 



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 bf 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 connexions 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 cere- 



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