as 
OCCIPITAL LOBE. 523 
to the Sylvian fissure below. It is bounded in front by the fissure of Rolando and 
behind by the superior and inferior postcentral sulci. 
The superior parietal gyrus is the area of cerebral cortex which lies between the 
supero-mesial border of the hemisphere above and the ramus horizontalis below. In 
front it is bounded by the superior postcentral sulcus, whilst behind it is connected 
with the occipital lobe by the arcus parieto-occipitalis. It is continuous round the 
supero-mesial border with the preecuneus. 
The inferior parietal gyrus lies below the ramus horizontalis and the ramus occi- 
pitalis and behind the inferior precentral sulcus. It is more or less directly con- 
tinuous with the occipital lobe behind and the temporal lobe below. From before 
backwards it presents three arching convolutions, viz. the supra-marginal, the 
angular, and the postparietal. The supramarginal convolution is bent round the 
upturned end of the posterior limb of the Sylvian fissure, and stands in continuity 
behind and below this, with the first temporal gyrus. The angular convolution 
arches over the upturned end of the parallel sulcus and becomes continuous with 
the second temporal convolution. The postparietal convolution winds round the 
upturned end of the second temporal sulcus and runs into the third temporal gyrus. 
OccipiraL Lope.—The occipital lobe forms the hinder pyramidal part of the 
cerebral hemisphere ; and although very imperfectly mapped off from the temporal 
and parietal lobes, which he in front of it, it is nevertheless one of the most natural 
subdivisions of the cerebral hemisphere. It is not developed in the brain of the 
quadruped. . Man and the ape alone possess a well-marked occipital lobe, and it 
may be defined as being that part of the hemisphere which encloses the posterior 
horn of the lateral ventricle. Being pyramidal in form, it presents three surfaces 
and an apex or occipital pole. On the mesial aspect of the hemisphere it is 
separated from the parietal lobe (i.e. the precuneus) by the internal parieto-occi- 
pital fissure. On the tentorial or inferior surface it is not marked off in any way 
from the temporal lobe and the limbic lobe, which he in front of it. It is necessary, 
therefore, on this aspect to employ an arbitrary line of demarcation ; one which 
extends from the preeoccipital notch on the infero-lateral border of the hemisphere 
to the isthmus of the limbic lobe (7.e. the narrow part of the limbic lobe immedi- 
ately below the hinder end of the corpus callosum) will serve the purpose. On the 
external surface the external parieto-occipital fissure, and an arbitrary lne from 
this to the preeoccipital notch, may be regarded as separating the occipital from the 
parietal and temporal lobes. The upturned end of the second temporal sulcus may 
lie in the course of this line. 
In the ape, in which the occipital lobe is better marked than in man, there is a very definite 
boundary on both the mesial (internal parieto-occipital fissure) and the external aspect of the 
hemisphere (“‘ Affenspalte”) ; and in the human foetus there is, in most cases, a fissural infolding 
on each of the three aspects of the hemisphere marking off the occipital lobe in the most definite 
manner. The internal infolding is retained as the internal parieto-occipital fissure ; the external 
infolding, or the fissura perpendicularis externa, is the representative of the “ Affenspalte” of the 
ape, and it gradually disappears from the surface of the brain; the inferior infolding, which is 
connected with the mid-collateral fissure, is also to some extent transitory. 
On the mesial aspect of the occipital lobe we find: (1) the calcarine fissure, 
(2) the cuneus, and (3) the gyrus lingualis. 
The calearine fissure begins on the occipital pole by a bifurcated extremity, 
which les in the groove which is formed on this part of the brain by the lateral 
sinus. From this it pursues a slightly arched course forwards, and ends by cutting 
into the limbic lobe immediately below the splenium or thickened hinder margin of 
the corpus callosum. The calcarine fissure is joined by the internal parieto-occipital 
fissure at a point somewhat nearer its anterior than its posterior extremity. 
Together, the two fissures present a <-shaped figure. Between the two limbs of 
the Y is placed the cuneus. 
If the calearine and internal parieto-occipital fissures are opened up so as to expose the 
bottom in each ease, three well-marked deep or submerged annectant gyri will usually be dis- 
played. One of these, the best marked, called the gyrus cunei, marks off the parieto-occipital from 
the calearine fissure and joins the cuneus with the limbic lobe. In the chimpanzee and in the 
lower apes the gyrus cunei is on the surface, and there is no communication between the two 
fissures; in the orang, gibbon, and microcephalic idiot, it may either be submerged or on the 
