THE BRAIN AND ITS MEMBRANES. 



783 



The olfactory tract , when traced backward, divides into two slips or roots, 

 external and internal, at its base. The so-called middle or gray root is the tri- 

 (fn,t>ii/i olfii'-toriniii. which is enclosed by the two roots. Traced forward, these two 

 roots unite and form the tract, which is a flat band, narrower in front than behind, 

 and of a somewhat prismoid form on section. It is soft in texture and contains 

 gray matter (neuroglia) in its substance. As it passes forward it is contained in 

 a deep sulcus, the olfactory sulcus, which subdivides the internal orbital convolu- 

 tion, lying on the under surface of the frontal lobe on each side of the longi- 

 tudinal fissure, and is retained in position by the membrane (pia mater), which 



Frontal lobe. 



Temporal lobe. 



rncus of hippo- 

 campal gyrus. 



l_Part of fourth 

 temporal gyms. 



Occipital 

 lobe. 



FIG. 473. Base of the brain. 



covers it. On reaching the cribriform plate of the ethmoid bone it expands into 

 the i jl factory bulb. 



The trigonum olfactorium and the area of Broca constitute one and the same 

 area of cortical gray matter, bounded internally and posteriorly by a fissure 

 (fasura prima). which separates it from the anterior part of the peduncle of the 

 corpus callosum on its inner aspect, and from the anterior perforated space pos- 

 teriorly. Externally, this area passes into continuity with the cortical gray 

 matter of the internal orbital gyrus. This area is divided into three districts by 

 the passage across it, from before backward, of the two roots of the olfactory tract. 

 The internal district, lying between the internal root and anterior part of pedun- 

 cle of corpus callosum (fissura prima intervening) is the area of Broca, continu- 

 ous with the beginning of the gyrus fornicatus. The middle district, included 



