BASE OF THE BRAIN. 587 



of the hemisphere, 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 compli- 

 cated convolutions of the brain, frequently becoming branched like the letter 

 Y in 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. (Fig. 329.) 



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 recieved 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 and near 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 cerebel- 

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

 the intermediate portion of the fissure being filled up by the great transverse 

 band of white matter, the corpus callosum. Of these two portions of the lon- 

 gitudinal 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. 



The corpus callosum terminates at the base of the brain by a concave margin, 

 which is connected with the tuber cinereum through the intervention of a thin 

 layer of gray substance, the lamina cinerea. This may be exposed by gently 

 raising and drawing back the optic commissure. A broad white band may be 

 observed on each side, passing from the under surface of the corpus callosum 

 backwards and outwards, to the commencement of the fissure of Sylvius; 

 these bands are called the peduncles of the corpus callosum. Laterally, the 

 corpus callosum extends into the anterior lobe. 



The lamina cinerea is a thin layer of gray substance, extending backwards 

 above the optic commissure from the termination of the corpus callosum to the 

 tuber cinereum; it is continuous on either side with the gray matter of the 

 anterior perforated space, and forms the anterior part of the inferior boundary 

 of the third ventricle. 



The olfactory nerve, with its bulb, is seen on either side of the longitudinal 

 fissure, upon the under surface of each anterior lobe. 



