BASE OF THE BRAIN. 517 



The convolution of the corpus callosum (gyrus fornicatus] is always well 

 marked. It lies parallel with the upper surface of the corpus callosum, com- 

 mencing on the under surface of the brain in front of the anterior perforated 

 space ; it winds round the curved border of the corpus callosum, and passes 

 along its upper surface as far as its posterior extremity, where it is connected 

 with the convolutions of the posterior lobe ; it then curves downwards and 

 forwards, embracing the cerebral peduncle, passes into the middle lobe, forming 

 the hippocampus major, and terminates just behind the point from whence it 

 arose. 



The supra-orbital convolution on the under surface of the anterior lobe is well 

 marked. 



The convolution of the longitudinal fissure bounds the margin of the fissure 

 on the upper surface of the hemisphere. It commences on the under surface of 

 the brain, at the anterior perforated space, passes forwards along the inner margin 

 of the anterior lobe, being here divided by a deep sulcus, in which the olfactory 

 nerve is received ; it then curves over the anterior and upper surface of the hemi- 

 sphere, along the margin of the longitudinal fissure, to its posterior extremity, 

 where it curves forwards along the under surface of the hemisphere as far as the 

 middle lobe. 



The convolutions on the outer convex surface of the hemisphere, the general 

 direction of which is more or less oblique, are the largest and the most complicated 

 convolutions of the brain, frequently becoming branched like the letter Yin their 

 course upwards and backwards towards the longitudinal fissure : these convolutions 

 attain their greatest development in man, and are especially characteristic of the 

 human brain. They are seldom symmetrical on the two sides. 



UNDER SURFACE OR BASE. 



The under surface of each hemisphere presents a subdivision, as already 

 mentioned, into three lobes, named, from their position, anterior, middle, and 

 posterior. 



The anterior lobe, of a triangular form, with its apex backwards, is somewhat 

 concave, and rests upon the convex surface of the roof of the orbit, being separated 

 from the middle lobe by the fissure of Sylvius. The middle lobe, which is more 

 prominent, is received into the middle fossa of the base of the skull. The posterior 

 lobe rests upon the tentorium, its extent forwards being limited by the anterior 

 margin of the cerebellum. 



The various objects exposed to view on the under surface of the cerebrum in 

 the middle line are here arranged in the order in which they are met with from 

 before backwards. 



Longitudinal fissure. Tuber cinereum. 



Corpus callosum and its peduncles. Infundibulum. 



Lamina cinerea. Pituitary body. 



Olfactory nerve. Corpora albicantia. 



Fissure of Sylvius. Posterior perforated space. 



Anterior perforated space. Crura cerebri. 

 Optic commissure. 



The longitudinal fissure partially separates the two hemispheres from one 

 another ; it divides the two anterior lobes in front, and, on raising the cerebellum 

 and pons, it will be seen completely separating the two posterior lobes, the inter- 

 mediate portion of the fissure being arrested by the great transverse band of white 

 matter, the corpus callosum. Of these two portions of the longitudinal fissure, 

 that which separates the posterior lobes is the longest. In the fissure between 

 the two anterior lobes the anterior cerebral arteries may be seen ascending to the 

 corpus callosum ; and at the back part of this portion of the fissure, the anterior 

 curved portion of the corpus callosum descends to the base of the brain. 



