784 THE NERVOUS SYSTEM. 



between the two roots, is the trigonum olfactorium. The external district, 

 external to the external root, is very small and has no especial name. 



The posterior olfactory lobule or anterior perforated space (anterior perforated 

 lamina) is situated at the inner side of the fissure of Sylvius. It is bounded 

 in front by the fissura prima (transverse part) and the orbital convolutions 

 of the frontal lobe ; behind, by the optic tract ; externally, by the temporal 

 lobe and commencement of the fissure of Sylvius (valleeula) ; internally, it is 

 continuous with the lamina cinerea. It is crossed internally and posteriorly by 

 the posterior part of the peduncle of the corpus callosum, and externally by the 

 external olfactory root. It is of a grayish color, and corresponds to the under 

 surface of the corpus striatum (lenticular nucleus) and part of the claustrum. It 

 has received its name from being perforated by numerous minute apertures for 

 the transmission of small straight vessels into the substance of the corpus stri- 

 atum, constituting the antero-median and antero-lateral ganglionic branches of the 

 anterior and middle cerebral arteries. 



The Olfactory Roots. The external root passes outward across the anterior 

 perforated space and the fissure of Sylvius to the temporal lobe i. e. end of 

 hippocampal gyrus (possibly nucleus amygdalae also) where it meets the termi- 

 nation of the peduncle of the corpus callosum. 



The internal root passes inward and joins the lower end of the gyrus forni- 

 catus after bending around and behind the area of Broca, into which also some 

 of its fibres pass. 



The trigonum receives a few fibres directly from the end of the tract /. e. 

 between the divergence of its roots. When these fibres are well marked they 

 constitute the so-called " middle root." From the end of the tract a few fibres 

 also pass directly dorsally into the white matter of the frontal lobe, upper or 

 dorsal root (Henle). 



Each root of the olfactory tract is thus seen to be connected with an extrem- 

 ity of the limbic lobe the external with the end of the hippocampal gyrus, 

 and the internal with the beginning of the gyrus fornicatus. 



Under Surface or "Base" of the Encephalon. The various objects exposed to 

 view on the under surface of the brain, in and near the middle line, are here 

 arranged in the order in which they are met with from before backward (Fig. 

 473) : 



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



Longitudinal fissure. Under surface of frontal lobe. 

 Under surface of rostrum of corpus Olfactory bulb, 



callosum and its peduncles. Olfactory tract. 



Lamina cinerea. Olfactory roots. 



Optic commissure. Anterior perforated space. 



Pituitary body. Fissure of Sylvius. 



Infundibulum. Optic tract. 



Tuber cinereum. Crusta. 



Corpora albicantia. Under surface of temporal lobe. 



Posterior perforated space. Under surface of hemisphere of 

 Tuber annulare of pons. cerebellum. 



Medulla oblongata (ventral surface). 



The longitudinal fissure partially separates the two hemispheres from each 

 other : it divides the two frontal lobes in front, and on raising the cerebellum and 

 pons it will be seen completely separating the two occipital lobes. Of these two 

 portions of the longitudinal fissure, that which separates the occipital lobes is tlic 

 longer. The intermediate portion 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. 



Interpeduncular Space. Immediately behind the diverging optic tracts, and 



