THE CEREBRUM 417 



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

 area of the hemisphere which lies on a deeper plane than the 

 remainder of the surface of the hemisphere; it is hidden 

 from view by the adjacent margins of the frontal, parietal, and 

 temporal lobes, which overlap it and constitute the opercula 

 of the fossa lateralis. The fossa lateralis is the depression, at 

 the bottom of the lateral fissure, in which the insula lies. 

 Round the anterior, superior, and posterior borders of the 

 insula runs a sulcus, called the sulcus circularis. It separates 

 the insula from the adjacent parts of the hemisphere. At the 

 apex or lowest part of the insula there is a rounded fold of 

 the brain substance ; it is directly connected with the lateral 

 of the two striae which extend from the posterior end of the 

 olfactory tract and with the piriform area of the hippocampal 

 gyrus, and it forms part of the olfactory area of the hemi- 

 sphere. The region in which it is situated is known as the 

 limen insulce. 



The surface of the insula is divided into an anterior or 

 frontal portion and a posterior or parietal portion by the 

 sulcus centralis insulcz, which is in a plane parallel with the 

 plane of the central sulcus on the supero-lateral surface of 

 the hemisphere. On both portions of the insula there are 

 two or more gyri. 



At this stage, the dissector should study the portion of the 

 middle cerebral artery which was left in situ when the mem- 

 branes were removed from the hemisphere (p. 387). He 

 will find that it passes along the stem of the lateral fissure, 

 crosses the limen insulae, and breaks up on the surface of the 

 insula into the terminal branches which were noted on the 

 supero-lateral surface of the hemisphere (p. 388). 



When the positions of the terminal branches of the 

 middle cerebral artery have been studied, the vessels and 

 the surrounding membranes may be removed. 



Lobus Olfactorius. Each olfactory lobe consists of several 

 parts; they are: (i) the olfactory bulb; (2) the olfactory 

 tract ; (3) the olfactory striae, medial and lateral; and (4) the 

 olfactory trigone. The olfactory bulb^ which is the most 

 anterior part of the olfactory region of the brain, lies on 

 the lower surface of the frontal lobe, in the anterior part 

 of the olfactory sulcus. On its lower surface it receives the 

 olfactory nerves, which arise in the olfactory mucous mem- 

 brane of the nose and terminate in the olfactory bulb. They 



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