HEAD AND NECK OF THE HORSE 187 



the optic tract and cerebral peduncle, and medial to the posterior part 

 of the rhinal fissure. In front of its most prominent part is a steep 

 declivity that may be mistaken for the anterior limit of the lobe. It is 

 stated, however, that a small continuation reaches to the olfactory 

 trigone. Posteriorly the piriform lobe is continuous with the hippo- 

 campal gyrus. 



The term substantia perforata anterior is applied to an area in front 

 of the most prominent part of the pirifoi-m lobe, in which there are 

 numerous small openings for the admission of vessels to the deeper part 

 of the brain. 



The subcallosal gyrus (gyrus subcallosus) is a vague area of the 

 cerebral cortex ventral to the rostrum of the corpus callosum ; and the 

 supracallosal gyrus (indusium griseum) consists of a very thin layer of 

 grey matter covering the dorsal surface of the corpus callosum and 

 containing the longitudinal strias of this commissure. 



The fascia clentata is a narrow and slightly crenated ridge separated 

 from the hippocampal gyrus by the hippocampal fissure. 



The cingulum ^ is contained in the gyrus fornicatus, and consists of 

 association fibres that begin at the anterior perforated substance, run 

 round the genu and over the trunk of the corpus callosum, curve round 

 the splenium, and end in the hippocampal gyrus and piriform lobe. 



Dissection. — Part of one cerebral hemisphere should be removed down 

 to the level of the corpus callosum. This must be done by making a 

 succession of slices in planes parallel to the commissure. The specimen 

 in which the two hemispheres have been separated by a longitudinal 

 incision is of service as a guide to the direction and depth to which the 

 slices may be removed. As soon as the corpus callosum is plainly visible, 

 it is well to remove the remains of the convolution immediately dorsal to 

 it by tearing rather than by cutting. This is necessary because the lateral 

 ventricle, a cavity roofed over by the corpus callosum, projects above the 

 level of the medial part of the commissure. There is, therefore, danger 

 of opening the ventricle prematurely if the slicing process is continued. 



When sufficient of one hemisphere has been removed, the other should 

 be treated in tlie same way. 



During this dissection some features of the inner structure of the 

 cerebrum will be revealed. The depth of some of the sulci will be 

 demonstrated, and the fact that tlie convolutions are composed of an 

 outer rind of grey matter enclosing a core of white matter will be 

 disclosed. In the deeper sections the white cores of the convolutions 

 become confluent ; and finally a large island of white matter, connected 

 by the corpus callosum with a similar island in the other hemisphere, 

 occupies the central part of the section of each hemisphere. 



The study of the corpus callosum should he carried out by an 

 examination of the hemi-sected brain concurrently with that of the 

 specimen just prepared. 



Corpus callosum. — The corpus callosum"^ is a broad, transverse 



1 Cingxdum [L.], a girdle, belt. 



- Corpus [L.], body. Callosus [L.], thick, hard, solid. 



