FLOOR OF LATERAL VENTRICLE. 203 



cesses or crura, which will be afterwards followed to the corpora albicantia 

 and the optic thalami (p. 208). 



If the forrnix be cut across near its front, the foramen of Monro will be 

 opened, and the descending anterior pillars will be seen (fig. 64). When 

 the posterior part is raised, it will be found to be supported on a process 

 of the pia mater, named velum interpositum. And near its base (on the 

 under aspect) between the two offsets of the toenia? hippocampi, is a trian- 

 gular surface, which is marked by transverse lines : the part which is so 

 defined has been called the lyra (fig. 64). 



The fornix may be described as consisting of two bands, right and left, 

 which are united for a certain distance in the central part or body. Ac- 

 cording to this view each band, commencing in the optic thalamus, passes 

 over the foramen of Monro, and after forming the body of the fornix, is 

 continued as a distinct, piece to the uncinate convolution. 



The foramen of Monro is the interval beneath the anterior part of the 

 fornix, which opens on each side by a slit between the edge of the fornix 

 and the .optic thalamus. In it the plexus choroides lies; and through it 

 the lateral ventricles communicate with one another and with the third 

 ventricle. 



FLOOR OF THE LATERAL VENTRICLE The student may leave un- 

 touched, for the present, the membrane on which the fornix rests ; and 

 proceed to examine, on the right side, the different bodies which have 

 been enumerated as constituting the floor of the lateral ventricle. 



The corpus striatum (superior ganglion of the cerebrum) (fig. 63, e) is 

 the large gray body in the front of the lateral ventricle. It is placed op- 

 posite the island of Reil in the fissure of Sylvius; and it has received its 

 name from the striated appearance of a vertical section. 



Dissection. To see the composition of the corpus striatum, the student 

 should slice off obliquely the upper and outer part until certain white 

 fibres crossing it obliquely from within outwards are readied. The knife 

 should then be carried through this layer of white fibres until another 

 mass of gray substance, similar to the first, is arrived at. 



The striate body is a pyriform mass of gray matter of considerable 

 thickness, which is surrounded by the white substance of the hemisphere, 

 except where it projects into the lateral ventricle. Its position is oblique 

 with respect to the middle line of the brain, for the anterior part is near 

 the septum of the ventricles, whilst the posterior is external to the optic 

 thalamus. By means of the incision in the corpus striatum, white fibres 

 can be seen to be directed through it in such a way as to divide the gray 

 matter into two parts, one being situate in the ventricle (intra-ventricular) 

 a^ove the white fibres, and the other outside the ventricular space (extra- 

 ventricular), below these fibres. 



The intra-ventricular piece (nucleus caudatus) is shaped like a kite, 

 and projects into the floor of the ventricle. The end, directed forwards, 

 is large and rounded; whilst the opposite end is thin and pointed, and is 

 continued backwards, outside the optic thalamus, to the roof of the de- 

 scending cornu of the lateral ventricle. Numerous veins cover this part 

 of the corpus striatum. 



The extra-ventricular part (nucleus lenticularis) will be better seen, 

 afterwards, by sections made from the outer side or from below. It is 

 oval in form, but does not reach so far back as the other, and is bounded 

 interiorly by a white capsule ; through it the anterior commissure of the 

 brain passes very obliquely, as a subsequent dissection will show. 



