THE CEREBRUM 479 



The hippocampal gyrus is bounded below and laterally by 

 the anterior part of the collateral fissure, and anteriorly by 

 the incisura temporalis, which separates its hooked extremity, 

 or uncus, from the temporal pole. Supero- medially it 

 is limited by the hippocampal fissure ; whilst posteriorly it 

 is divided into two parts by the anterior extremity of the 

 calcarine fissure. Of these, the upper is the isthmus, which 

 connects it with the gyrus cinguli, whilst the lower portion 

 is continuous with the gyrus lingualis. 



If the hippocampal fissure, which lies above the hippocampal gyrus, 

 is opened up, in a soft brain, the fascia dentata and the fanbria lying side 

 by side will be brought into view. In a hardened brain the examination 

 of this region should be deferred (p. 492). 



THE CORPUS CALLOSUM. 



A dissection should now be made with the view of exposing 

 the upper surface of the corpus callosum, which is the com- 

 missural band connecting the cerebral hemispheres at the 

 bottom of the longitudinal fissure. 



Dissection. With a long knife slice off the top of the right hemisphere 

 at the level of the sulcus cinguli. The white medullary centre of the 

 cerebral hemisphere, enclosed on all hands by the grey cortex, is brought 

 into view, and the appearance receives the name of centrum semi-ovale. 

 From the central white mass medullary prolongations proceed into all 

 the gyri. 



A transverse incision may next be made through the middle of the 

 gyrus cinguli ; then the anterior and posterior parts of the gyrus cinguli 

 should be torn away from the hemisphere in a lateral direction. If this 

 is done successfully the manner in which the fibres of the corpus 

 callosum enter the hemisphere will be seen. In cases where the student 

 is dissecting the brain for the second time the knife should not be used 

 at all in carrying out this dissection. The top of the hemisphere down 

 to the level of the sulcus cinguli should, in the first instance, be torn off, 

 and then the gyrus cinguli may be treated in the same way. By this 

 expedient the fibres of the corpus callosum may be traced into the gyri. 



Cingulum. If the deep surface of the gyrus cinguli, 

 which has been torn away, is examined, a large bundle of 

 longitudinally directed fibres will be noticed embedded in its 

 substance. This is the cingulum. It can be easily dislodged \ 

 a very slight degree of traction is all that is required to 

 lift it out of its bed. It begins anteriorly at the substantia 

 perforata anterior, curves round the convexity of the corpus 

 callosum, and then descends, round its posterior end, and 

 terminates in the hippocampal gyrus. The cingulum is a 



