198 A MANUAL OF ANATOMY. 



All these structures are located in the area contained 

 between the temporal lobes laterally, the pons posteriorly, 

 and the frontal lobes anteriorly. This space is called the 

 interpeduncular space. Within it the Circle of Willis has 

 already been found, page 1 6 1 . 



The lamina cinerea, optic chiasm, tuber cinereum, infundi- 

 bulum, and posterior perforated space all take part in form- 

 ing the floor of the third ventricle of the brain. 



The Superficial Origin of the Cranial Nerves. 

 I. The Olfactory. Figs. 21, 28, 29, 9, 13. 



The olfactory nerve, as described, is really not a nerve 

 but an aborted offset from the fcetal forebrain. 



Its central cavity, which is at first in communication with 

 the cavity of the forebrain, becomes obliterated as develop- 

 ment proceeds. Diag. 14. 



The nerve consists of four portions : the roots (of which 

 there are three), the tract, the bulb, and the olfactory nerves 

 proper. The three roots may be traced, one externally 

 (which passes to the amygdaloid nucleus in the anterior 

 part of the roof of the descending horn of the lateral ven- 

 tricle), a middle (which terminates by connecting with the 

 fibres of the anterior commissure within the substance of 

 the corpus striatum), and the internal (which passes to the 

 anterior part of the gyrus fornicatus). 



The tract is a slender, triangular-shaped bundle which 

 passes forward from the roots and terminates in an expanded 

 extremity the bulb. Both lie within the olfactory sulcus 

 of the frontal lobe. 



The olfactory bulb rests upon the cribriform plate of the 

 ethmoid bone, and the olfactory nerves proper pass down- 

 ward through the foramina in that plate to the mucous 

 membrane of the nose. 



