HEMISPHERES OF THE BRAIN. 655 



the callosal convolution ; in otlier words, the inner root is continuous with one 

 extremity and the outer root with the other extremity of the limbic lobe. 



(3) The trigonum olfactorium is situated between the diverging roots of the 

 olfactory tract, and is sometimes described at the middle or gray root of the tract. 

 It is part of an area of gray matter, which forms the base of the anterior olfactory 

 lobule ; another portion of it is termed (4) the area of Broca ; and a third portion, 

 of no special significance, is situated external to the outer root of the olfactory 

 tract. This area of gray matter is bounded internally and posteriorly by a fissure 

 (fissura priiH) which separates it from the peduncle of the corpus callosum and 

 from the posterior olfactory lobule. The area of Broca is continuous with the 

 gyrus fornicatus. 



The posterior olfactory lobule or anterior perforated space is marked off from 

 the anterior lobule by the fissura prima, and is situated at the commencement of 

 the fissure of Sylvius. Internally, it is bounded by the peduncle of the corpus 

 callosum, and is continuous with the lamina cinerea. Posteriorly it is bounded 

 by the optic tract, and it is partially concealed by the temporal lobe which overlaps 

 it. It has received the name of anterior perforated space from its being perforated 

 by numerous openings, which transmit blood-vessels to the interior of the brain, 

 and it corresponds to the under surface of the lenticular nucleus and part of the 

 claustrum. 



Under Surface or Base of the Encephalon (Fig. 350). Having considered the 

 surface of the hemispheres, the student should direct his attention to the base of 

 the brain, before commencing the study of the component parts which make up 

 the two hemispheres. 



The base of the brain presents for examination the under surfaces of the 

 frontal and temporal lobes ; the structures contained in the interpeduncular space, 

 with the crura cercbri or cerebral peduncles ; the under surfaces of the pons 

 Varolii, cerebellum, and medulla oblongata ; and the superficial origins of the 

 cranial nerves. 



The various objects exposed to view (with the exception of the origins of the 

 cranial nerves, which will be considered in another section) in the middle line and 

 on either side of the middle line, are here arranged in the order they are met with 

 from before backward. 



In the Middle Line. On Each Side of the Middle Line. 



Longitudinal fissure. Frontal lobe. 



Rostrum and peduncles of Olfactory lobe. 



corpus callosum. Fissure of Sylvius. 



Lamina cinerea. Optic tracts. 



Optic commissure. Crus cerebri. 



Tuber cinereum. Temporal lobe. 



Infundibulum. Hemisphere of cerebellum. 



Pituitary body. 

 Corpora albicantia. 

 Posterior perforated space. 

 Pons Varolii. 

 Medulla oblongata. 



The longitudinal fissure partially separates the two hemispheres from each 

 other. It divides completely the anterior portions of the two frontal lobes : and 

 on raising the cerebellum and pons, it will be seen to separate completely the two 

 occipital lobes: of these two portions of the longitudinal fissure, that which 

 separates the occipital lobes is the longer. The intermediate part of the fissure 

 is filled up by the great transverse band of white matter, the corpus callosum. 

 In the fissure between the two frontal lobes the anterior cerebral arteries ascend 

 on the corpus callosum. 



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



