CORPUS CALLOSUM. 199 



hippocampus major in the descending cornu of the Lateral ventricle. 

 Upwards it is limited by the corpus callosum (Gal] and downwards it 

 intervenes between the hook and the body of the uncinate convolution. 



In the dentate sulctis is the gray substance of the hemisphere, which 

 presents a notched border at the inner edge of the temporo-sphenoidal 

 lobe ; this has been called the dentate convolution, and will be better seen 

 in a subsequent stage of the dissection of the brain. 



Structure of the convolutions. Each convolution is continuous with the 

 interior of the brain on the one side (base) ; and is free on the surface of 

 the brain on the' other side, where it presents a summit and lateral parts. 

 On a cross section it will be seen to consist externally of gray cerebral 

 substance as a cortical layer, which is continued from one eminence to 

 another over the surface of the hemisphere ; and internally it is composed 

 of white brain substance the medullary part, which is derived from the 

 fibrous mass in the interior. The cortical layer is composed of two, or in 

 some parts of three strata, which are separated by their intervening paler 

 layers ; and an outer white stratum, which covers the surface, is most 

 marked over the internal and lower portions of the uncinate convolution. 



INTERIOR OF THE CEREBRUM. The cerebrum consists on each side of 

 a dilated part or hemisphere, and of a stalk or peduncle. In the interior 

 is a large central space, which is subdivided into smaller hollows or ven- 

 tricles by the before-mentioned connecting pieces. And the whole, 

 except the peduncle, is surrounded by a convoluted crust. 



In conducting the dissection of the cerebrum, the student will learn the 

 form and situation of the several constituent parts, and the connections 

 between these by means of fibres. 



Dissection. Supposing both hemispheres entire, the left is to be cut off 

 to the level of the convolution of the corpus callosum. When this has 

 been done, the surface displays a white central mass of an oval shape 

 (centrum ovale minus), which sends processes into the several convolu- 

 tions. In a fresh brain this surface would be studded with drops of blood 

 escaping from the divided vessels. 



Next, the convolution of the corpus callosum is to be divided about the 

 middle, and the two pieces are to be thrown backwards and forwards. 

 Under it lies a thin narrow band, the covered band of Reil, which bends 

 down before and behind the corpus callosum. 



The same steps of the dissection are to be taken on the opposite side ; 

 and the tops of the hemispheres being removed to the level of the corpus 

 callosum, the transverse fibres of that body are to be defined as they radi- 

 ate to the convolutions. 



Now a much larger white surface comes into view (larger ovan centre), 

 which has been named centrum ovale, Vieussens ; and the white ma>< in 

 each hemisphere is seen to be continuous, across the middle line, through 

 the corpus callosum. 



The corpus callosum reaches from the one-half of the cerebrum to the 

 other, and forms the roof of a space (lateral ventricle) in each hemisphere. 

 Between the halves of the brain, where it occupies the longitudinal fissure, 

 it is of small extent, being about four inches in length, and somewhat 

 arched from before backwards. It is narrower in front than behind, and 

 extends nearer to the anterior than the posterior part of the cerebrum. 



In front the corpus callosum is bent to the base of the brain (fig. ) ( .^ w), 

 as before described (p. 192); and behind it ends in a thick roll, which is 

 connected with the subjacent fornix. 



