204 DISSECTION OF THE BRAIN. 



The tcenia semicircnlaris (fig. 63, f) is a thin and narrow white band 

 of longitudinal fibres, which lies between the corpus striatum and the optic 

 thalamus. In front this band becomes broader and joins the pillar of the 

 fornix; and behind it is continued, along with the pointed end of the cor- 

 pus striatum, into the white substance of the roof of the descending cornu 

 of the lateral ventricle. Superficial to the anterior part of the ta?nia is a 

 yellowish semi-transparent layer (lamina cornea) ; and beneath this pass 

 some small veins from the corpus striatum, in their course to the veins of 

 Galen. 



The optic thalamus is only partly laid bare in this stage of the dissec- 

 tion, and its examination may be omitted till the third ventricle has been 

 learnt. 



The hippocampus minor (calcar avis) resembles a cock's spur (fig. 

 63, i), as it lies in the posterior cornu of the ventricle. It is pointed at 

 its posterior extremity, and is covered on the free surface by a medullary 

 layer continuous with the corpus callosurn. When it is cut across a gray 

 stratum will be found beneath the white; and the eminence itself will be 

 seen to be produced by the extension inwards of the calcarine sulcus at 

 the inner surface of the hemisphere (p. 198). 



The hippocampus major (fig. 63, /) is the curved projection in the 

 floor of the descending cornu of the lateral ventricle. Convex on the 

 surface that looks to the cavity, this body is curved in the same direction 

 as the cornu, and has its concavity turned inwards. The anterior ex- 

 tremity is the largest, and presents two or three indentations, which give 

 it the appearance of the foot of a feline animal ; it is named pes hippo- 

 campi. 



All along the inner or concave margin is the small band or tsenia (f?) 

 that is prolonged from the fornix; it ends below by joining the small 

 recurved part of the uncinate convolution. 



Dissection. To examine more fully the hippocampus, the parts of the 

 corpus callosum and fornix, which remain in the middle line, should be 

 divided longitudinally, and the posterior part of the right hemisphere 

 should be drawn away from the rest of the brain. When the pia mater 

 has been removed from the inner side of the hippocampus, and this pro- 

 jection has been cut across, its structure will be manifest. 



The hippocampus is covered on the ventricular surface by a medullary 

 layer, with which the taenia or the band of the fornix blends. On its 

 opposite surface is the hollow of the dentate sulcus on the exterior of the 

 brain, which is filled with gray substance. Along the free margin of the 

 hippocampus the gray matter projects in the form of a notched ridge, the 

 lamina dentata: this is external to the cavity of the ventricle, beneath 

 the taenia, and has been named the dentate convolution (p. 199). 



Transverse fissure of the cerebrum. By drawing the separated right 

 hemisphere away from the crus cerebri and the optic thalamus, and re- 

 placing it, the dissector will comprehend the position, and the boundaries 

 of jthe great cleft at the posterior part of the brain. 



This fissure lies beneath the fornix, and opens into the lateral ventricle, 

 on each side, along the edge of the fornix, from the foramen of Monro to 

 the extremity of the descending cornu. The part of the slit entering the 

 lateral ventricle is bounded by the edge of the fornix on the one side, and 

 by the optic thalamus and crus cerebri on the other. A piece of pia mater 

 projects into the transverse fissure, forming a horizontal central piece, 

 velum interpositum, beneath the fornix (fig. 64, g ), and thinner lateral 



