THE NERVOUS SYSTEM 



i393 



convolutions. The occipito-temporal gyrus is situated bet^veen the 

 occipito-temporal sulcus and the collateral fissure, and extends 

 from the occipital pole to the temporal pole. External to the 

 occipito-temporal sulcus there is the narrow inferior or tentorial 

 surface of the third temporal gyrus. 



Island of Reil, Central Lobe, or Insula. — ^This lobe is situated 

 deeply within the fissure of Syhnus, and is concealed from view by 

 the opercular gyri, to be presently 

 described. It is triangular, the 

 apex being directed downwards 

 towards the vallecula Sylvii and 

 locus perforatus anticus. The cir- 

 cular or limiting sulcus being here 

 absent, the grey matter of the 

 apex is continuous with that of the 

 locus perforatus anticus, this point 

 being called the linwn insula. 

 Elsewhere the island of Reil is sur- 

 rounded by the circular or limiting 

 sulcus, which has been already 

 described. The insula presents 

 several sulci, which diverge as 

 they pcLss from the apical region to 

 the base, and these map it out 

 into gyri. One of these sulci is 

 known as the sulcus centralis 

 Insulse. It extends from the apex 

 to the base in an upward and back- 

 ward direction, almost in line %vith 

 the fissure of Rolando, and it 

 divides the insula into two lobules, 

 precentral and postcentral. 



The precentral lobule is com- 

 posed of three short gyri, called 

 the gyri breves, which converge as 

 they descend from the base, but 

 they do not reach the apex or pole 

 of the precentral lobule. The 

 postcentral lobule is formed by 

 the gyrus longus, which is usually 

 broken up into two gyri towards 

 the base of the insula. 



The direct internal or mesial rela- 

 tion of the insula is the claustrum, internal to which there are, in 

 succession, the external capsule, the nucleus lenticularis, the internal 

 capsule, and the nucleus caudatus. 



The insula and the corpus striatum represent the stem of the cerebral 

 hemisphere, whilst the remainder of the hemisphere is known as the palUum 

 or mantle. 



88 



Fig. 586. — The Inferior Surface 



OF THE Left Cerebral Hemi- 

 sphere, SHOWING THE GyRI 



AND Sulci. 



