FUNCTIONS OF THE CEREBRAL HEMISPHERES 507 



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 experi- 

 ment 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 in 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, cere- 

 bellum, and mid-brain, and excite a tonic activity of these centres. 

 The normal attitude of the animal depends on the tonus thereby pro- 

 duced in certain muscles. Muscular tone is indeed a quality specially 

 found in certain groups of muscles. If the cerebral hemispheres 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 

 stimulation 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 most 

 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 totai action of the 

 cerebral hemispheres and that of the proprioceptive part of the 

 central nervous system. Whereas in the decerebrate animal there 

 is increased tonus in the masseters, in the neck muscles, the muscles 

 of the trunk, and the extensor muscles of the limbs, stimulation 

 of the cortex produces opening of the mouth, flexion of the fore limb 

 or of the hind limb, more easily than any other movements. That 



