58 



On the Structure of the Cerebral Hemispheres, [June 17, 



described, a brief account is given of the intraventricular thalamus and 

 C. striatum. 



When the taenia semicircularis is removed, and the edge of the C. 

 striatum pushed back, large rounded cords of fibres are seen radiating 

 outwards in all directions from the thalamus with the fibres of the crus, 

 posteriorly slender flat bands of fibres curve backwards from the narrow- 

 ing extremity of the C. striatum to dip down between them (together with 

 fibres apparently belonging to the taenia) ; they can be traced through the 

 fan of radiating fibres to the extraventricular C. striatum. Anteriorly the 

 soft grey matter of the C. striatum fills the spaces between the diverging 

 cords ; but no distinct origin of fibres in the mass of grey matter is here 

 met with. 



The plan of construction of the frontoparietal portion of the hemisphere 

 seems to be as follows : — 



The C. callosum divides into two main planes of fibres, one of which 

 turns up to the margin of the great longitudinal fissure, the other passes 

 onward to the supramargiual gyrus of the fissure of Sylvius. The radiat- 

 ing central fibres approach the under surface of these at the acute angle, 

 and pass obliquely between them before the ascending and descending 

 planes have well separated from each other, the central as well as the 

 callosal fibres going mainly to the margins of the hemisphere. An angle 

 is thus left along the axis of the frontal and parietal lobes, which is occupied 

 by a vast longitudinal system of fibres, some of which have already been 

 mentioned as entering this part of the hemisphere from the temporo-sphe- 

 noidal lobe. Large bands turn upwards and then forwards from the 

 parallel and angular gyri, that from the parallel gyrus running forward 

 close behind the ascending callosal lamina; other fibres turn forwards 

 from the annectent gyri, and more anteriorly from the posteroparietal lobule ; 

 still furtherforwards some of these fibres coming from behind bend upwards, 

 and end in the parietal convolutions ; while others start in the same gyri, 

 and pass forwards, the principle of construction being apparently simple, 

 but the details extremely intricate. At the decussation the central and 

 callosal fibres are worn into a compact inextricable mass, aud the difficulty 

 of following the different sets is increased by the fact that the central 

 fibres are not transverse in direction like those of the C. callosum, but 

 mostly very oblique backwards or forwards, as may best be seen by exa- 

 mining the bands radiating under the C. striatum from the thalamus ; 

 this necessitates corresponding obliquity in the fissures through which the 

 central fibres penetrate the C. callosum. A few fibres from the under 

 surface of the C. callosum turn inwards to the centres ; but the statement 

 of Gratiolet that all the fibres of this commissure can be traced from the 

 central radiations on one side to the convolutions on the other, is not 

 confirmed. 



The detailed dissection of the parietofrontal convolutions need not be 

 given here. It will be sufficient to mention that the posteroparietal and 



