526 THE NERVOUS SYSTEM. % 
fissure. Its hinder end turns upwards into the parietal lobe and is surrounded by 
the angular gyrus. The second temporal sulcus is placed midway between the 
parallel sulcus and the infero-lateral border of the hemisphere. It is very rare to 
find it in the form of a continuous cleft; usually it is broken up into several isolated 
pieces, one behind the other. Its hinder part, which turns upwards into the parietal 
lobe, hes close to the artificial line of demarcation between the occipital and parietal 
lobes, and is surrounded by the postparietal gyrus. 
sy the two temporal sulci the outer surface of the temporal lobe is mapped out 
into three tiers of horizontal convolutions, which are termed the first, second, and 
third temporal gyri. 
On the tentorial surface of the temporal lobe there is one fissure, termed the 
occipito-temporal sulcus. The oceipito-temporal sulcus lies to the outer side of the 
collateral fissure and close to the infero-lateral margin of the hemisphere. It runs 
in an antero-posterior direction, and is not confined to the temporal lobe, but 
extends backwards towards the occipital pole. It is usually broken up into two 
or more separate pieces. 
The occipito-temporal convolution is situated between the collateral fissure and 
the occipito-temporal sulcus. It extends from the occipital pole behind to the 
temporal pole in front. 
The narrow strip of surface on the outer side of the occipito-temporal sulcus is 
continuous, round the infero-lateral margin of the hemisphere, with the third 
temporal convolution on the outer surface of the cerebrum, and may be reckoned as 
a part of it. 
The three temporal convolutions and the occipito-temporal convolutions run 
into each other at the temporal pole. 
ISLAND OF REIL on [NSULA——The insula is a triangular and somewhat bulging 
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. It is 
circumscribed by a limiting sulcus (sulcus circularis), already described, and its 
dependent apical part, which is directed downwards, is in close relation to the 
anterior perforated spot and the Sylvian vallecula. Here the limiting sulcus is 
absent and the gray matter on the surface of the insula passes continuously into 
the anterior perforated spot. The place of transition is called the limen insule. 
The insula is divided into several diverging convolutions by a series of radiating 
sulci. Of the latter, one, which presents the same direction and lies in the same 
plane as the fissure of Rolando, receives the name of the sulcus centralis insule 
(Fig. 382, C, p. 515). It divides the insula into an anterior frontal part and a 
posterior parveto-limbie part. 
Limpic Lope.—This lobe is seen on the mesial surface of the hemisphere in the 
form of an elongated ring-like convolution, the extremities of which approach 
closely to each other at the locus perforatus anticus. These extremities are con- 
nected by the roots of the olfactory tract, and in this manner the limbic ring may 
be considered to be closed. The upper part of the limbic lobe is placed in intimate 
relation to the extremities and upper surface of the corpus callosum, and receives 
the name of callosal convolution or gyrus fornicatus. The lower portion of the 
lobe is termed the hippocampal convolution and forms the inner part of the 
tentorial surface of the hemisphere. The continuity between the hippocampal 
gyrus and the callosal convolution is established below the hinder end of the corpus 
callosum by a narrow portion of the limbic lobe, called the isthmus. From this 
point the hippocampal convolution extends forwards towards the temporal pole. 
Finally, on the side of the crus cerebri, the hippocampal gyrus is folded back on 
itself and ends in a recurved hook-like extremity, called the uncus. The uncus does 
not reach so far forwards as the temporal pole. 
The callosal convolution begins below the anterior end of the corpus callosum at 
the locus perforatus anticus, and, winding round the genu of the corpus callosum, it 
is continued backwards on its upper surface to its hinder thickened extremity or 
splenium. Curving round this, it becomes greatly narrowed through the calcarine 
fissure cutting into it. This narrow part is termed the isthmus, and constitutes the 
link of connexion between the callosal gyrus and the hippocampal gyrus. The 
