770 



DISSECTION OF THE BRAIN. 



anterior and 

 posterior 

 ends ; 



pulvinar ; 



external 

 geniculate 

 l>ody ; 

 structure. 



Dissection 

 of fornix. 



Origin of 

 fornix. 



Dissection. 



Midbrain. 1 



on the outer side it is separated from the lenticular nucleus by the 

 posterior limb of the internal capsule (fig. 278, icp). 



The anterior end of the optic thalamus hounds the foramen of 

 Monro. The posterior end is much larger, and projects above the 

 superior quadrigeminal body (fig. 277, h) and cms cerebri, being 

 covered by the pia mater : behind and internally it forms a con- 

 siderable prominence called the pulvinar ; and below and outside, 

 appearing at present as if it were a part of the optic thalamus, there 

 is a smaller oval elevation named the external geniculate body (fig. 

 281, p. 772). 



In the section that has been made on the right side (fig. 278), 

 the optic thalamus is seen to be composed of dark grey matter ; 

 but it appears white on the upper surface, the grey suKst;mce 

 being here covered by a thin medullary layer. A faint white line, 

 which bifurcates in front, divides the grey mass into three portions 

 a small anterior, a larger internal and an external nucleus. 



Dissection. The origin of the fornix in the optic thalamus 

 may now be followed out. As a preparatory step the anterior 

 commissure, the front of the corpus callosum, and the commissure 

 of the optic nerves should be cut along the middle line, so that the 

 fore parts of the hemispheres can be separated from one another. 

 On the left hemisphere the anterior pillar of the fornix is to be 

 traced downwards through the grey matter of the third ventricle 

 to the corpus albicans, and thence upwards into the optic thalamus. 

 This can readily be done by following down the pillar of the 

 fornix and scraping away the overlying soft grey substance. 



The ANTERIOR PILLAR OF THE FORNIX is joined below the 

 foramen of Monro by the fibres of the tsenia semicircularis and 

 pineal stria, and then curves downwards and backwards in front of 

 the optic thalamus, through the grey matter of the third ventricle, 

 to the corpus albicans. Here it makes a turn like half of the 

 figure 8, furnishing a white envelope to the grey matter of that 

 body. Finally it ascends to the fore part of the optic thalamus, in 

 the anterior nucleus of which its fibres end. The ascending band 

 from the corpus albicans into the optic thalamus is commonly 

 named the bundle of Vicq d'Azir. 



The bodies lying behind the third ventricle, viz., the corpora 

 quadrigemina, the pineal body, and the posterior commissure may 

 be next examined. 



Dissection. All the pia mater should be carefully removed from 

 the surface of the quadrigeminal bodies, especially on the right 

 side, on which they are to be seen. The posterior part of the 

 hemisphere of the same side may be taken away if this has not 

 been done already. 



The constricted portion of the brain between the optic thalami 

 above and the pons and cerebellum below is known as the isthmus 

 cerebri or mesencephalon, and occupies, when the brain is in the 

 skull, the aperture of the tentoriimi cerebelli. The dorsal part of 

 the mesencephalon is formed by a layer which is marked on 

 the surface by four eminences the corpora quadrigemina, and is 



