THE CEREBRUM 477 



inferior temporal sulcus is continuous, round the infero-lateral 

 margin of < the hemisphere, with the inferior temporal gyrus 

 on the lateral surface of the cerebrum, and is reckoned as a 

 part of it. 



The three temporal gyri and the fusiform gyrus run into one 

 another at the temporal pole. 



Insula (O.T. Island of Reil). The insula is a triangular 

 field of cerebral cortex which lies on a deeper plane than 

 the general surface of the hemisphere, and is hidden from 

 view by the four opercula which overlap it (p. 463). It is 

 bounded by a distinct limiting sulcus, sulcus circularis (Reil), 

 which has been described already ; and its dependent apical 

 part or pole, which looks downwards, is in close relation to 

 the stem of the lateral fissure, and to the substantia perforata 

 anterior on the base of the brain. 



The insula is divided into several diverging gyri by 

 a series of radiating sulci. Of the latter, one, which pre- 

 sents the same direction and lies in the same plane as the 

 central sulcus, receives the name of the sulcus centralis insulce. 

 It divides the insula into an anterior frontal part and a 

 posterior parietal portion. 



Olfactory Lobe. The olfactory lobe is small and rudi- 

 mentary in the human brain. It comprises (i) the olfactory 

 bulb and tract with the three striae, and (2) the trigonum 

 olfactorium. 



The olfactory tract is a narrow white prismatic band, which 

 expands anteriorly into a swollen bulbous extremity termed 

 the olfactory bulb. Both the tract and the bulb lie in the 

 olfactory sulcus on the orbital surface of the frontal lobe, 

 whilst the inferior surface of the bulb, when the brain is in 

 position, rests on the cribriform plate of the ethmoid bone 

 and receives the numerous olfactory nerves which reach it 

 through the foramina in that part of the cranial floor. 



Posteriorly, the olfactory tract divides into two or three 

 diverging striae. The medial stria (O.T. mesial roof] curves 

 abruptly medialwards, and may be followed into the extremity 

 of the gyrus cinguli and the subcallosal gyrus. The lateral 

 stria (O.T. lateral root) runs postero - laterally over the 

 lateral part of the substantia perforata anterior, and gradually 

 disappears from view. In animals, in which the olfactory 

 apparatus is better developed than in man, it may be 

 traced into the uncinate extremity of the hippocampal 



