THE NERVOUS SYSTEM ,397 



Olfactory Lobe. 



The olfactory lobe is rudimentary in man. It is composed of 

 two lobules, anterior and posterior (His). 



Anterior Lobule. — ^This consists of the foUowmg parts : (i) the 

 olfactory bulb ; (2) the olfactory tract, with its two roots, mesial 

 and lateral ; (3) the trigonum olfactorium ; and (4) the area of Broca, 



The olfactory bulb is the enlarged anterior extremity of the 

 olfactory tract. It is oval, and its upper surface is in contact 

 with the orbital surface of the frontal lobe, whilst its lower surface 

 rests upon one half of the cribriform plate of the ethmoid bone. 

 The lower surface receives the olfactory nerves, which arise from 

 the olfactory cells of the olfactory mucous membrane and pass 

 through the foramina of the cribriform plate. 



The olfactory tract is a white band which extends backwards 

 from the olfactory bulb, both of them occupying the olfactory sulcus 

 on the inner part of the orbital surface of the frontal lobe. Pos- 

 teriorly it di\ndes into two roots, inner and outer, which diverge and 

 enclose between them the trigonum olfactorium. 



The inner or mesial root passes inwards, in a curved manner, 

 behind the area of Broca. Some of its fibres pass into this area, 

 and others enter the anterior extremity of the caUosal gyrus, or 

 gyrus fomicatus. 



The outer or lateral root passes backwards and outwards over 

 the outer part of the posterior lobule (locus perforatus anticus), and 

 enters the anterior part of the hippocampal gyrus. 



The trigonum olfactorium is the area of grey matter which lies 

 between the diverging mesial and lateral roots of the olfactory 

 tract. It is sometimes described as the middle or grey root of the 

 olfactory tract. 



The area of Broca is situated in front of the inner root of the 

 olfactory tract. It is continuous with the commencement of the 

 callosjd gjTTis. 



Posterior Lobule. — This portion of the olfactorj' lobe is otherwise 

 known as the locus perforatus anticus. It is bounded anteriorly by 

 the fissura prima of His, which separates it from the trigonum olfac- 

 toriiun; posteriorly by the caput gyri hippocampi ; externally by 

 the lateral root of the olfactory tract ; and internally by the anterior 

 part of the optic tract and the laterail margin of the optic commis- 

 sura Its openings give passage to the central or ganghonic branches 

 of the anterior and middle cerebral arteries. 



The grey matter of the locus perforatus anticus is continuous 

 superiorly with the grey matter of the lenticular nucleus and of the 

 large anterior end of the nucleus caudatus. 



Development. — The olfactory lobe is developed as a hollow protrusion from 

 the anteroinferior part of the cerebral vesicle. This protrusion becomes 

 solid, and gives rise to the olfactory tract and olfactory bulb. 



