694 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



from the skiu and muscles. It has been suggested, to be sure, that separate 

 localities were the seat for the dermal and muscular sensations. Ferrier l in- 

 dicated the limbic lobe, especially the hippocampal gyrus, while Horsley and 

 Schafer 2 argued for the gyrus fornicatus. At present the weight of evidence 

 is in favor of the location of the centres for dermal and muscular sensations 

 in the same area as that from which the muscles of the trunk and limbs can 

 be made to contract. 3 Both in monkeys and in man defects in sensation are 

 not permanent after limited lesions of the cortex, but, as suggested by Mott, 

 the wide distribution of the incoming impulses would explain this result. 



Thus the entire portion of the cortex to which a definite function can be 

 assigned must be looked upon as made up of fibres which bring impulses into 

 it and cell-bodies which by their discharge send impulses to other divisions of 

 the central system as well as to other parts of the cortex itself. All parts of 

 the cortex having assigned functions give rise on stimulation to movements, 

 but in the case of the movements aroused by the stimulation of the sensory 

 areas, so called, they involve the contractions of only those muscles controlling 

 the external sense-organ, as the eyeball, external ear, tongue, and nostrils, and, 

 though physiologically important, and in the case of the eye at least reaching 

 a high degree of refinement, they are quantitatively very insignificant as com- 

 pared with the responses to be obtained from stimulating the " motor region," 

 from which contractions of the larger skeletal muscles are obtained. Hence 

 the significance of the usual terms " sensory " and " motor " in describing the 

 respective regions. 



Multiple Control from the Cortex. It has been found that stimulation 

 of the cortex in the region of the frontal lobes marked "eye" (Fig. 184) was 

 followed by movements of the eye. Schafer 4 has shown that very precise 

 movements of the eye also follow the stimulation of the temporal and 

 various parts of the occipital cortex. Since the efferent fibres which control 

 the muscles concerned start from the cell-groups of the third, fourth, and 

 sixth cranial nerves, it would appear most probable that in both parts of the 

 cortex there are located cells the neurons of which pass to those groups and 

 are capable of exciting them. An alternative hypothesis namely, that the 

 impulses which produced the movements when the occipital region was 

 stimulated, travelled first to the cortical cells in the frontal lobe and thence 

 by way of them to the efferent cell-groups was at one time considered, for 

 the latent period of contraction of these muscles was less by several hun- 

 dredths of a second when the stimulus was applied in the frontal region than 

 when applied elsewhere. The experiments of Schafer show, however, that 

 when the occipital and frontal lobes are separated from one another by a sec- 

 tion severing all the association fibres, still the reactions can be obtained by 

 stimulation in the former locality, showing that the connections of the two 



1 The Functions of the Brain, 1876. 



2 Philosophical Transactions of the Royal Society, 1888, vol. cxxix. 

 s Mott : Journal of Physiology, 1894, vol. xv. 



4 Proceedings of the Royal Society, 1888, vol. xliii. 



