200 DISSECTION OF THE BRAIN. 



On the upper surface the fibres are directed from the hemispheres to 

 the middle line the middle being transverse, but those from the anterior 

 and posterior parts oblique. Along the centre is a ridge or raphe, and 

 close to it are two or more slight longitudinal white lines (nerves of 

 Lancisi). Still further out may be seen other longitudinal lines (covered 

 band), beneath the convolution of the corpus callosum, if all of them 

 have not been taken away in the removal of that convolution. The 

 longitudinal fibres in the middle line are continued downwards in front, 

 and joining the covered band or fillet are continued to the anterior per- 

 forated spot. 



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

 to bring into view the parts in contact with its under surface, a cut is to 

 be made through it on the right side about half an inch from the central 

 ridge : and this is to be extended forwards and backwards, as far as the 

 limits of the underlying ventricle. Whilst cutting through the corpus callo- 

 sum, the student may observe that a thin membraniform structure lines 

 its under surface. 



The corpus callosum is thicker at each end than at the centre, in con- 

 sequence of a greater number of fibres being collected from the cerebrum ; 

 and the posterior part is the thickest of all. Connected with its under 

 surface along the middle is the septum lucidum or partition between the 

 ventricles (fig. 63, by, and still posterior to that is the fornix. 



This body is the chief commissural part of the halves of the brain, and 

 reaches laterally even to the convolutions, but its fibres are not distinct 

 far in the hemisphere. 



Dissection. The left lateral ventricle is to be now opened in the same 

 way as the right ; and to prepare for the examination of the cavity on the 

 right side, as much of the corpus callosum as forms the roof of the space 

 is to be removed. A part of the ventricle extends down in the ternporo- 

 sphenoidal lobe towards the base of the brain ; and to open it, a cut is to 

 be carried outwards and downwards, through the substance of the hemi- 

 sphere, along the course of the hollow. (See fig. 63.) 



VENTRICLES OF THE BRAIN The ventricular spaces in the interior of 

 the cerebrum are derived from the subdivision of a large central hollow, 

 and are five in number. One (lateral) is contained in each hemisphere ; 

 and these constitute the first and second. The third occupies the middle 

 line of the brain near the under surface ; and the small fifth is included in 

 the partition between the lateral ventricles. The fourth is situate be- 

 tween the cerebellum and the posterior surface of the medulla oblongata 

 and pons. 



The lateral ventricles (fig. 63) are two in number, one in each hemi- 

 sphere ; they are separated incompletely in the middle line by a septum, 

 as they communicate by an aperture below that partition. The interior 

 is lined by a thin stratum of areolar tissue covered by nucleated epithelium 

 (the ependyma ventriculorum), with cilia at some spots. 



Each is a narrow interval, which reaches into the anterior, posterior, 

 and middle regions of the corresponding hemisphere. Its central part 

 (body) is almost straight, but the extremities (cornua) are curved. Thus 

 there are three cornua in each, which have the following disposition : 

 The anterior is directed outwards from its fellow in the other hemisphere. 

 The posterior or the digital cavity is much smaller in size, and is bent 

 inwards in the occipital lobe towards the one on the opposite side. And 

 the inferior corau, beginning opposite the posterior fold of the corpus cal- 



