254 THE ANATOMY OF THE HORSE. 



The Gyrus Fornicatus (Fig. 34). — This is comparable to a lobe, 1 rather than to a 

 convolution. It is disposed in a great curve, or arch, from which it is named. It begins 

 at the forepart of the under surface of the hemisphere, in front of the lamina cinerea, 

 and here it is narrow and pointed. It bends round the anterior extremity {genu) of the 

 corpus callosum, acquiring at its point of reflection a great increase in thickness. It 

 passes backwards above the corpus callosum, and below the antero-marginal and postero- 

 marginal convolutions. From the former body it is separated by the fissure of the corpus 

 callosum, while the calloso-marginal fissure {great limbic fissure of Broca) separates it 

 from the marginal convolutions above. In this part of its course the gyrus is distinctly 

 divided into two tiers by a fissure that traverses it in its length. Posteriorly this fissure 

 becomes very shallow, and the gyrus, losing its double character, turns round the 

 posterior end {splenium) of the corpus callosum and reaches the tentorial surface of the 

 hemisphere. At this point it becomes slightly constricted ; and, after being connected 

 with the convolutions of the postero-superior and postero-inferior lobes, it is directed 

 forwards at the base of the brain (Plate 33, Hipp, con.), between the crus cerebri and the 

 outer olfactory tract. Finally, it terminates in the nipple-like eminence already noticed 

 (Unc. con.). 



The whole of this great convolution corresponds very closely to the gyrus fornicatus 

 of human anatomy. Thus, the part which turns round the genu and rests on the upper 

 surface of the corpus callosum is the callosal convolution ; the part from the splenium 

 to the side of the cerebral crus is the hippocampal convolution; and the nipple-like 

 eminence is the uncinate co?ivolution. 



The hippocampal part of the gyrus fornicatus has a small process which projects 

 forwards under the splenium ; and as the convolution curves forwards to emerge at the 

 side of the crus it rests on the optic thalamus. By carefully raising the convolution 

 from the thalamus, there will be brought into view a fissure on the under aspect of the 

 former. This is the hippocampal fissure, and it projects the convolution into the lateral 

 ventricle as the hippocampus. Beyond this fissure the edge of the hippocampus is seen, 

 margined by a thin-edged white band — the taenia hippocampi. The hippocampus and 

 its taenia here form the upper boundary of the great transverse fissure of the brain, by 

 which the pia mater of the hemisphere projects towards the interior of the cerebrum as 

 the velum interpositum. 



Directions. — With a large, thin-bladed, sharp knife a horizontal slice 

 should be removed from the top of one or both cerebral hemispheres, 

 down to the level of the corpus callosum. 



The hemisphere will now be seen to contain both grey and white 

 matter. In the centre of the hemisphere the white matter forms a large 

 mass connected with that of the opposite side by the corpus callosum. 

 At the surface the mass sends a white core into each convolution. The 

 great sheet of grey matter on the surface of the hemisphere invests 

 the white core in each convolution, and also extends across the bottom 

 of each fissure. 



The Corpus Callosum (Plate 35 and Fig. 34) is a great commissure 

 of nerve fibres connecting the right and left hemispheres. It termin- 

 ates behind in a thickened margin — the splenium ; and in front it is 

 abruptly bent downwards and backwards, the bend being named the 



1 Paul Broca {Anatomie comparee des circonvolutions ccrebralcs) considers that this 

 part of the hemisphere represents not merely a lobe, but several lobes — that it is, in 

 fact, the equivalent of all the rest of the cerebral cortex. He accordingly divides 

 the surface of the hemisphere primarily into two great divisions — the great limbic lobe 

 (gyrus fornicatus) and the convolutionary mass. 



