6/2 FUNCTIONS OF THE CEREBRUM. [CH. XLVIII. 



the pons, K in the pyramid of the bulb, and p in the crossed and 

 direct pyramidal tracts of the cord. 



Sensory areas. Stimulation of these produces no direct 

 movements, but doubtless sets up a sensation called a subjective 

 sensation ; that is, one produced in the animal's own brain, and 

 this indirectly leads to movements which are reflex ; thus on 

 stimulating the auditory area there is a pricking up of the ears ; on 

 stimulating the visual area there is a turning of the head and eyes 

 in the direction of the supposed visual impulse. That such move- 

 ments are reflex and not direct is shown by the long period of 

 delay intervening between the stimulation and the movement. 



Extirpation of a sensory area leads to loss of the sense in question. 



The rougher experiments performed by nature in the shape of 

 diseases of the brain produce corresponding results. 



Some diseases are of the nature of extirpation. 



An instance of this is cerebral haemorrhage. If the haemorrhage 

 is in the region of the internal capsule, it cuts through fibres to 

 the muscles of the whole of the opposite side of the body, as they 

 are all collected together in a narrow compass, and the condition 

 obtained is called hemiplegia. The varieties of hemiplegia are 

 numerous, according as motor or sensory fibres are most affected, 

 and in one variety of hemiplegia, called crossed hemiplegia, the face 

 is paralysed on one side of the body, the limbs on the other ; this 

 is due to injury of the nerve-tracts in the bulb subsequent to the 

 crossing of the fibres to the nucleus of the seventh nerve, but above 

 the crossing of the pyramids (just below the asterisk in fig. 505). 



If now the haemorrhage occurs on the surface of the brain, a 

 much more limited paralysis, called monoplegia, is the result ; if 

 the arm area is affected, there will be paralysis of the opposite 

 arm ; if the leg area, of the opposite leg ; if a sensory area, there 

 will be loss of the corresponding sense. 



Some diseases, on the other hand, act as the induction currents 

 do in artificial stimulation ; they irritate the surface of the brain ; 

 such a disease is a tumour growing in the membranes of the 

 brain ; if the tumour irritates a piece of the motor area, there will 

 be involuntary movements in the corresponding region of the 

 body ; these movements may culminate in the production of 

 epileptiform convulsions commencing in the arm, leg, or other 

 part of the body which corresponds to the brain area irritated. 

 It is these cases of " Jacksonian Epilepsy " which have given the 

 best results in surgery ; the movement produced is an indication 

 of the area of the brain which is being irritated, and the surgeon 

 after trephining is able to remove the source of the .mischief. 

 If the area of the brain which is irritated is a sensory area, the 



