SURFACES OF CEREBELLUM. 211 



merated on the upper and under surfaces of the corpus callosum, along the 

 middle line, together with the band of the convolution of the corpus cal- 

 losum : these fibres are connected with the anterior perforated spot of the 

 base of the brain. 



Structure of the optic thalamus. The thalamus is about an inch deep, 

 and the following is a summary of its structure, as displayed in the pre- 

 vious dissections. 



The upper and inner half is formed chiefly of gray matter, with which 

 the undermentioned white bands are connected : Thus through it pass the 

 fornix in front, and the posterior commissure behind ; whilst the pedun- 

 cles of the pineal body lie along the inner side, and the taenia semicircu- 

 laris along the outer. 



The lower and outer part consists mainly of white fibres directed 

 upwards, and these are derived fr >m the peduncle of the cerebrum in- 

 feriorly, and from the peduncle of the cerebellum and the corpora quadri- 

 gemina superiorly : to the hinder part of them two slips of fibres are added 

 from the corpora geniculata. 



The corpora geniculata contain gray substance inside. Into these 

 bodies fibres of the optic tract enter; and from each issues a band to join 

 the fibres of the crus cerebri. They seem to serve as accessory ganglia to 

 the peduncular fibres of the cerebrum. 



Corpus striatnm. By slicing through the corona radiata on the left 

 side, so as to bring into view the extraventricular nucleus of the corpus 

 striatum, the extent and form of that mass, and the situation of the ante- 

 rior commissure in it, will be apparent. 



Grus cerebri. By a vertical section through the left peduncle of the 

 cerebrum, the disposition and the thickness of the two layers of its longi- 

 tudinal fibres ; and the situation and extent of the locus niger between 

 them, may be perceived. 



SECTION V. 



THE CEREBELLUM. 



Dissection. The cerebellum (fig. 59, k) is to be separated from the 

 remains of the cerebrum, by carrying the knife through the optic thala- 

 mus so that the small brain, the corpora quadrigemina, the crura cerebri, 

 the pons, and the medulla oblongata, may remain united together. 



All the pia mater is to be carefully removed from the median fissure on 

 the under surface ; and the different bodies in that fissure are to be sepa- 

 rated from one another. Lastly the handle of the scalpel should be passed 

 along a sulcus at the circumference, which is continued from the crus, be- 

 tween the upper and under surfaces. 



The cerebellum, little brain (fig. 66), is flattened from above down, so 

 as to be widest from side to side, and measures about four inches across. 

 This part of the encephalon is situate in the posterior fossae of the base of 

 the skull, beneath the tentorium cerebelli. Like the cerebrum, it is in- 

 completely divided into two hemispheres ; the division being marked by a 

 wide median groove along the under surface, and by a notch at the poste- 

 rior border which receives the falx cerebelli. 



