418 THE BRAIN 



are about twenty in number. The olfactory bulb is continuous, 

 posteriorly, with the olfactory tract, a triangular prismatic band 

 which runs backwards, in the olfactory sulcus, to the anterior 

 border of the anterior perforated substance, where it ends in 

 a pyramidal elevation, the trigonum olfactorium. From the 

 lateral angle-of the olfactory trigone the stria olfactoria lateralis 

 passes, backwards and laterally, along the lateral margin of 

 the anterior perforated substance and across the limeh insulae 

 to the piriform area of the hippocampal gyrus (Fig. 162). The 

 dissector should understand that under cover of the lateral 

 olfactory stria there is a layer of grey matter which represents 

 the anterior part of the piriform area. 



Dissection. To display the course of the lateral olfactory 

 stria it will be necessary to raise the temporal pole and, pos- 

 sibly, it may be necessary to cut away the tip of the temporal 

 lobe ; but that must be done on one side only. . 



From the medial angle of the olfactory trigone the medial 

 olfactory stria passes round the posterior end of the gyrus 

 rectus to the medial aspect of the hemisphere, towards the 

 subcallosal gyrus. The subcallosal gyrus is situated on the 

 under surface of the anterior part of the corpus callosum, and 

 it must be looked for at a later stage of the dissection (Figs. 



Dissection. A dissection should now be made with the 

 object of displaying the upper surface of the corpus callosum. 

 For that purpose the upper portion of the hemisphere, on one 

 side, must be removed, and when that is done it will be possible 

 to study the gyri, fissures, and sulci on the medial surface of the 

 opposite hemisphere. 



With a long knife slice off the upper part of the right hemi- 

 sphere down to the level of the sulcus cinguli on the medial surface 

 (see Fig. 161). The white medullary centre of the hemisphere, 

 enclosed within the grey cortex, which is brought into view 

 when the section is made, is termed the centrum semi-ovale. 

 From the centrum semi-ovale prolongations of the white matter 

 pass into all the surrounding gyri (Fig. 161). 



A transverse incision must now be made through the centre 

 of the gyrus cinguli, which forms the medial boundary of 

 the semi - oval centre ; then the anterior and posterior parts 

 of the gyrus cinguli must be torn away from the hemisphere 

 in a lateral direction. If that is done successfully the manner 

 in which the fibres of the corpus callosum enter the hemisphere 

 will be demonstrated (Fig. 161). 



If the student is dissecting the brain for the second time he 

 should not use the knife at all in carrying out this dissection. 

 The upper part of the hemisphere to the level of the gyrus cinguli 

 should be torn off and then the gyrus cinguli may be treated in 



