CENTRAL NERVOUS SYSTEM. 241 



of the olfactory impulses is from the olfactory area in the nose to the olfactory- 

 bulb of the same side, thence via the olfactory tract to its termination in front 

 of the anterior perforated space, one branch of the tract passing directly into 

 the substance of the gyrus fornicatus at this point, and the other going into 

 the more lateral portion represented in man by the temporal end of the gyrus 

 hippocampi. The cortical areas, together with the olfactory lobe and tract, 

 form the rhinencephalon of the comparative anatomists. It has been shown, 

 nevertheless, by Hill 1 that in anosmic mammals the fascia dentata alone 

 varies with the development of the olfactory apparatus. The experimental 

 pathological evidence is very meagre in relation to these nerves, but, on the 

 other hand, the anatomical evidence is of the best. 2 



D. Localization of Cell-groups in the Cerebral Cortex. 



The foregoing section has brought to light the fact that groups of incom- 

 ing impulses find their way to the cerebral cortex. The significance of this 

 is evident only when in response to those impulses arriving at the cortex 

 others leave it, and finally affect some expressive tissue or instrument by the 

 aid of which we can interpret them. Since the cerebral hemispheres with 

 their cortex become increasingly developed as we pass up the mammalian 

 series, it naturally follows that the pathways connecting the cortex with the 

 lower parts of the system are correspondingly increased. Using the reactions 

 of the expressive tissues as a guide, it is our present purpose to trace the 

 impulses in those cases in which the cortex forms part of the path. We turn, 

 therefore, to the study of those parts of the cerebral cortex the direct stimu- 

 lation of which produces impulses that pass to cell-groups lying more or less 

 caudad in the central system. 



Earlier Observations. — It was demonstrated by Fritsch and Hitzig in 

 1870 3 that if a constant current is applied to the surface of the dog's cere- 

 brum, it is possible, by jnterrupting it, to obtain movements of the limbs 

 and face when the electrodes arc placed on the parts of the cerebral cortex 

 about the sulcus cruciatus. The reaction varies according to the place of 

 stimulation, a constant relation subsisting between the two. From this time 

 on, active investigations of the relations thus suggested have been pursued, 

 both by stimulating small areas in the cortex of various animals, including 

 the monkey and man, and by the removal of various parts of the cerebral hemi- 

 spheres and cortex, together with the study of the effects of pathological lesions 

 in man. The results following removal of the parts are complicated by the 

 transitory effects of the lesion, and can best be treated by themselves later on. 

 The results following the stimulation of the cortex arc the simplest, and will 

 next be described. 



Stimulation of the Cortex. — The common method of experiment is to 



1 Philosophical Transactions of the Royal Society, 1893, vol. clxxxiv. 



2 For a description of the very complicated pathways associating the olfactory bulb with 

 the other portions of the eerehrum, the reader is referred t<> Barker's The Nervous System, 1899. 



3 Archiv fur Anatomie und Physiologie, INTO. 



Vol.. II.— 16 



