756 



DISSECTION OF THE BRAIN. 



Cut down to 

 smaller oval 

 centre of the 

 hemisphere. 



Reflect 

 callosal con- 

 volution. 



Cingulum. 



Repeat dis- 

 section on 

 right side 



to show 

 collosal 

 fibres. 



The larger 

 oval centre 

 is deeper. 



Corpus 

 callosum : 



situation, 



extent and 

 form ; 



anterior and 

 posterior 

 ends ; 



fibres 

 transverse ; 



a few longi- 

 tudinal. 



Dissection. 



The student will now proceed to examine the parts in the interior 

 of the hemisphere, carrying the dissection from above downwards. 



Dissection. Supposing both hemispheres entire, the left is to be 

 cut off to the level of the calloso-marginal sulcus. When this has 

 been done, the surface displays a white central mass of a semi-oval 

 shape (centrum ovale minus) sending processes into the convolu- 

 tions, and surrounded by an irregular grey margin. In a fresh 

 brain this surface would be studded with drops of blood escaping 

 from the divided vessels. 



Next, the callosal convolution is to be divided transversely about 

 the middle, and the two pieces, taken in the fingers, are to be thrown 

 backwards and forwards. On its under surface will be seen a thin 

 band of white fibres, the dnyidum or covered band of Reil, which 

 bends downwards before and behind the corpus callosum. 



A similar dissection is to be carried out on the opposite side ; 

 but in this case the student should insert his fingers into the 

 calloso-marginal sulcus and peel off the marginal convolution, and 

 again he should do likewise with the callosal convolution. He 

 will find that the parts tear in the direction of the central white 

 fibres, and in this way he will obtain a good idea as to how the 

 fibres of the corpus callosum diverge into the convolutions above 

 its level. 



Finally both hemispheres are to be removed to the level of the 

 corpus callosum. 



Now a much larger white surface comes into view (centrum ovale 

 majus), and the white masses in the two hemispheres are seen to be 

 continuous, across the middle line, through the corpus callosum. 



The CORPUS CALLOSUM reaches from one half of the cerebrum to the 

 other, and forms the roof of the lateral ventricle in each hemisphere 

 (fig. 279, p. 768). Its central part, which is exposed in the longi- 

 tudinal fissure, is narrow, and measures about three inches in 

 length from before backwards. It is nearer to the anterior than 

 to the posterior end of the cerebrum, and is somewhat arched from 

 before backwards. On each side its upper surface is free for a 

 short distance beneath the callosal convolution, from which it is 

 separated by the callosal sulcus. 



In front, the corpus callosum is bent downwards, forming the 

 genu and rostrum ; and behind, it ends in a thickened part named 

 the splenium (fig. 274). 



The fibres of the corpus callosum are for the most part directed, 

 transversely, but on its upper surface there is a somewhat irregular, 

 narrow longitudinal band on each side close to the middle line (the 

 supra-callosal convolution or the striae longitudinales). Between the 

 two bands is a median groove or raphe. Farther out there may be 

 seen other longitudinal fibres belonging to the cinguluni, if that has 

 not been completely removed. The longitudinal striae are prolonged 

 downwards in front, and are connected with the sub-callosal convo- 

 lutions or the peduncles of the corpus callosum. 



Dissection. In order to see the thickness of the corpus callosum, 

 and to bring into view the parts in relation with its under surface, 



