

CONDITIONS AFFECTING THE MOTOR CENTRES. 701 



being considerably modified. Stimulation of sensory nerves diminishes it ; thus, 

 the curve of contraction of the muscles becomes lower and longer, while the 

 reaction-time is lengthened simultaneously. Only when, owing to strong stimula- 

 tion, the reflex muscular contractions are vigorous, the excitability of the cortical 

 centres appears to be increased. Specially noteworthy is the fact that, in a certain 

 stage of morphia-narcosis, a stimulus which is too feeble to discharge a contrac- 

 tion becomes effective at once, if immediately before the stimulus is applied to the 

 cortical centre, the skin of certain cutaneous areas be subjected to gentle tactile 

 stimulation. When strong pressure is applied to the foot, the contractions become 

 tonic in their nature, so that all stimuli, which under normal conditions produce only 

 temporary stimulation, now stimulate these centres continuously. If, during the 

 tonic contraction, one gently strokes the back of the foot, blows on the face, gently 

 taps the nose, or stimulates the sciatic nerve, suddenly relaxation of the muscles 

 again occurs. These phenomena call to mind the analogous observations in 

 hypnotised animals ( 374). Another very remarkable observation is, that when 

 either owing to a reflex effect, or to strong electrical stimulation of a cortical centre, 

 contraction of the corresponding muscles is produced, then feeble stimulation of the 

 same centre, but also of other centres, suppresses the movement. Thus, we have 

 the remarkable fact that, according to the strength of the stimulus applied to the 

 motor apparatus, we can either produce movement or suppress a movement already 

 in progress (Bubnoff and Heidenhain). 



[Excision of the Thyroid affects the nerve-centres. After thyroidectomy (twenty-four hours) 

 the tetanus obtained by stimulating the cortex is greatly changed. It ceases when the stimu- 

 lating current is shut off, as suddenly as that observed on stimulating the corona radiata. In 

 more advanced cases, the tetanus is soon exhausted, and is often followed by clonic epileptoid 

 spasms. In the latter stages, after thyroidectomy, there may be only a feeble tetanus, or none 

 at all, on stimulating the motor areas, so great is the state of depression of function of these 

 centres (Horsley).] 



[Warner has directed attention to visible muscular movements apart from those studied in 

 epilepsy, chorea, athetosis and including attitude, gait, movements of the eyeballs, position of 

 the hand, and posture in general, &c. as expressive of states of the brain and nerve-centres.] 



376. SENSOEY CORTICAL CENTRES. [There must be some connection 

 between the surface of the brain and the afferent channels through which sensory 

 impulses pass inwards, and although the channels for sensory impulses are, perhaps, 

 not so definitely localised as those for voluntary motion, still we know that sensory 

 impulses for the opposite half of the body travel upwards through the posterior 

 third of the posterior limb of the internal capsule (fig. 500, S), to radiate in all 

 probability into the occipital and temporo-sphenoidal lobes. Parts of these convolu- 

 tions are sometimes spoken of as " sensory centres " or " psycho-sensorial " areas.] 



[The same methods have been applied to the investigation of these centres, viz., stimulation 

 and extirpation. Stimulation. Ferrier found that electrical stimulation of the angular gyrus 

 (monkey) caused movements of the eyeballs towards the side, with sometimes associated move- 

 ments of the head, but he regarded these as reflex movements, so that for this and other reasons 

 he, in his earliest contributions, considered the angular gyrus and adjacent parts as the ' ' centre 

 for vision. " On stimulating the first temporo-sphenoidal convolution, the monkey pricked the 

 opposite ear, the pupils dilated, while the head and ears turned to the opposite side, it exhibited 

 movements similar to those caused by a loud sound ; these movements are also reflex pheno- 

 mena, so that he located the "auditory centre" in this region, and on somewhat similar 

 grounds. As the result of inferences from the stimulation and extirpation of other parts, he 

 referred the centres for smell and taste to the tip of the temporo-sphenoidal lobe, and for touch 

 to the hippocampus major, but all these statements have not been confirmed.] 



[Goltz experimented on dogs by washing away the cortex cerebri, and found that when a 

 sufficient amount of the grey matter is removed, and after recovery from the immediate effects 

 of the operation, there is a peculiar defect of vision and other sensory defects, but so far Goltz 

 has not found that there is any difference in this respect between removal of the anterior and 

 posterior lobes of the dog's brain. The dog is not blind, as it can see and use its eyes to avoid 

 obstacles, but it seemed as if the animal failed to recognise food or flesh as such, when placed 

 before it ; while exhibitions, which, before the operation, greatly excited the dog, ceased to do 



