CONVOLUTIONS OF THE CEREBRUM. 



69I 



fissure terminates below in the calcarine fissure (fig. 480, oc), and the latter runs backwards in 

 the occipital lobe dividing it into two branches, oc', oc". Between the parietooccipital and 

 calcarine fissures lies the wedge-shaped lobule termed the cuneus (fig. 480, Oz). The calcarine 

 fissure indicates on the surface the position of the calcar avis or hippocampus minor, in the 

 posterior cornu of the lateral ventricle. The dentate fissure or sulcus hippocampi (fig. 480, h) 

 marks the position of the elevation of the hippocampus major, or cornu ammonis, in the lateral 

 ventricle. The temporo -sphenoidal lobe terminates anteriorly in the uncinate gyrus, while, 

 running along the former and the occipital lobes, is the collateral fissure (occipitotemporal 

 sulcus), which marks the position of the eminentia collateralis in the descending cornu of the 

 lateral ventricle, while it also separates the superior from the inferior teinporo-occipital con- 

 volutions (T 4 and T s ).] 



Motor Centres. In 1870 Fritsch and Hitzig discovered a series of circum- 

 scribed regions on the surface of the cerebral convolutions, whose stimulation 



Fig. 480. 

 Median aspect of the right hemisphere. CC, corpus callosum divided longitudinally ; Gf, 

 gyrus fornicatus ; H, gyrus hippocampi ; h, sulcus hippocampi ; U, uncinate gyrus ; cm, 

 calloso-marginal fissure ; F, first frontal convolution ; c, terminal portion of fissure of 

 Rolando ; A, ascending frontal ; B, ascending parietal convolution and paracentral lobule ; 

 Pj', prsecuneus or quadrate lobule ; Oz, cuneus ; Po, parieto-occipital fissure ; o 1} trans- 

 verse occipital fissure ; oc, calcarine fissure ; oc', superior, oc", inferior ramus of the same ; 

 D, gyrus descendens ; T 4 , gyrus occipito-temporalis lateralis (lobulus fusiformis) ; T 5 , 

 gyrus occipito-temporalis medialis (lobulus lingualis). 



by means of electricity causes co-ordinated movements in quite distinct groups of 

 muscles of the opposite side of the body (fig. 483, I, II). 



Methods Stimulation. The surface of the cerebrum is exposed in an animal (dog, monkey) 

 by removing a part of the skull covering the so-called motor convolutions and dividing the 

 dura mater. "When the convolutions are fully exposed, a pair of blunt non-polarisable ( 328) 

 needle electrodes are applied near each other to various parts of the cerebral surface. We may 

 employ the closing or opening shock of a constant current, or the constant current may be 

 rapidly interrupted, the current being of such a strength as to be distinctly perceived when it is 

 applied to the tip of the tongue (Fritsch and Hitzig). Or, the induced current may be used, 

 also of such a strength that it is readily felt when applied to the tip of the tongue (Ferrier, 

 1873). The cerebrum is completely insensible to severe operations made upon it. 



The areas of the cerebral cortex, whose stimulation discharges the characteristic 

 movements, are regarded by some as actual centres, because the reaction-time after 

 stimulation of the centres and the duration of the muscular contraction are longer 

 than when the subcortical fibres which lead towards the deeper parts of the brain 

 are stimulated. Another circumstance favouring this view is, that the excitability 



