THE BASAL GANGLIA. 



715 



per cent, of the length of the whole line. The direction of the fissure is downwards and for 



wards, and the long axis of the fissure forms, with the average mesial line, an angle of 67 



the angle opening forwards. Its 



average length is 3f inches.] 

 [The fissure of Slyvius is found 



by drawing a line from the ex- 



ternal angular process of the frontal 



bone backwards to the occipital 



protuberance, taking the nearest 



route between these two points. 



A point, 1 i ncn backwards from 



the angular process along this line, 



marks the origin of the fissure ; 



while a straight line drawn to the 



centre of the parietal eminence 



marks the course of its posterior 



limb. The parieto-occipital fissure 



will be two inches behind the upper 



end of the Rolandic fissure (A. W. 



Bare). 



[Corpus Callosum. It is usually 



stated that the corpus callosum 



connects the convolutions of one 

 side of the brain with those on the 



other, i.e., that it is an inter- 

 hemispherical commissure. D. J. 

 Hamilton, however, is of opinion 

 that it is not an inter-hemispheric 

 commissure, but is due to "cortical 

 fibres coming from the cortex 

 cerebri to be connected with the 

 basal ganglia of the opposite side. 

 On this view, the "corona radiata," 

 as usually understood, consists only 

 of the fibres which pass from the 

 cerebral peduncle directly up to the 

 cortex on the same side, and are 

 contained in the posterior division 

 and knee of the internal capsule. 

 They correspond to the motor 

 pyramidal tracts. Hamilton main- 

 tains that all the other fibres of 

 the internal capsule pass into the 

 crossed callosal tract, and, instead 

 of running directly up to the cortex 

 on the same side, cross in the 

 corpus callosum to the cortex of 

 the opposite side. Beevor, relying 

 on the examination of the brain of 

 monkeys, by Weigert's method, 

 denies that any fibres of the corpus 

 callosum pass into the external or 

 internal capsules, and he supports Fi 

 the old view that the corpus cal- 

 losum is a commissure between the 

 two hemispheres. ] 



Erb observed a case of its almost 

 complete destruction without any 

 considerable effect on motility, 

 co-ordination, sensibility, (reflexes, 

 senses, speech, or any marked im- 

 pairment of intelligence. 



Fig. 499. 



498. The fissures of Rolando and Sylvius are marked 

 as broad dark lines. The shaded circles mark approxi- 

 mately the motor areas. 1, lower extremity; 2, 3, 4, 5, 6, 

 and a, b, c, d, upper extremity ; 7, 8, 9, 10, 11, oro- 

 lingual muscles (A. W. Hare). Fig. 499. Head, skull, 

 and cerebral fissures. OPr, occipital protuberance ; EAP, 

 external angular process ; SF, Sylvian fissure ; A, its 

 ascending limb ; FR, fissure of Rolando ; PE, parietal 

 eminence ; MMA, middle meningeal artery ; TS, tip of 

 temporo-sphenoidal lobe ; B, Broca's convolution ; IF, 

 inferior frontal sulcus ; POF, parieto-occipital fissure ; 

 IPF, intra-parietal sulcus {A. W. Hare). 



379. BASAL GANGLIA 

 MID BRAIN. [The corpus striatum consists of two parts, an intra- ventricular 

 portion projecting into the lateral ventricle, the caudate nucleus, and an extra- 



