PARIETAL LOBE. 521 
lobe is continuous inferiorly with that of the temporal lobe, and an arbitrary line 
drawn backwards on the surface of the brain, in continuation of the horizontal 
part of the posterior limb of the Sylvian fissure, is taken as its inferior limit. 
Posteriorly, 1t is separated from the occipital lobe at the supero-mesial border of 
the hemisphere by the external parieto-occipital fissure. Below this fissure the 
surface of the parietal lobe is continuous with that of the occipital lobe; and an 
arbitrary line drawn across the outer surface of the hemisphere, from the extremity 
of the external parieto-occipital fissure to an indentation on the infero-lateral border 
of the hemisphere, termed the preoccipital notch, may be regarded as furnishing a 
posterior limitation. 
The preeoccipital nate th is, as a rule, only visible in brains that have been hardened in situ. It 
is placed on the infero-lateral border of the hemisphere, about an inch and a half in front of the 
occipital pole, and is produced by a vertical fold or wrinkle of the dura mater on the deep aspect 
of the parieto-mastoid suture, and immediately above the highest part of the lateral blood-sinus. 
There is great variability in the degree to which this fold of dura mater projects in different 
individuals. In the child it is always very salient, and often produces a deep cleft in the brain, 
but as the full size of the cranium is gradually attained it becomes much less projecting. In the 
young skull two or three such folds in this locality are usually apparent. Related to the pree- 
occipital notch there are likewise some cerebral veins which turn round the infero-lateral margin 
of the hemisphere to join the lateral blood-sinus. 
On the mesial surface of the hemisphere the parietal lobe is represented by 
the precuneus or quadrate lobule. This district, which is somewhat quadrilateral in 
form, lies between the upturned end of the calloso-marginal sulcus and the internal 
parieto-occipital fissure. It is imperfectly separated below from the limbic lobe by 
a somewhat variable furrow called the post-limbic sulcus. 
The gyri and sulci, on the outer surface of the parietal lobe, are the following :— 
‘Sulcus -postcentralis 
Suleus intrapari- a Inferior. (Gyrus ascendens parietalis or gyrus 
etalis (of Tur-- Sulcus postcentralis : postcentralis. 
ner). superior. Gyrus parietalis superior. 
Ramus horizontalis. Gane Gyrus supra- 
Sulei < Ramus occipitalis. ee marginalis. 
Upturned ends of— Gyrus angul- 
(a) Sylvian fissure. Gyrus parietalis inferior J set 
(b) Parallel fissure. . Jorn post- 
(c) Second tempo- parietalis. 
ral fissure. 
The intraparietal sulcus (of Turner) is a composite furrow, and is built up 
of four originally distinct factors. Two of these, termed the sulcus postcentralis 
inferior and the sulcus postcentralis superior, take a more or less oblique course 
across the hemisphere, and are most frequently united into one continuous fissure. 
The other two factors are placed horizontally, one behind the other, and are termed 
the ramus horizontalis and the ramus occipitals. 
The sulcus postcentralis inferior lies behind the lower part of the fissure of 
Rolando, whilst the suleus postcentralis superior occupies a similar position in 
relation to the upper part of that fissure. When confluent they form a long, con- 
tinuous fissure, which stretches across the hemisphere behind the fissure of Rolando 
and parallel to it. 
The ramus horizontalis is continuous, as a rule, with the upper end of the sulcus 
postcentralis inferior, and extends backwards, with a slight inclination upwards 
between the superior parietal gyrus, which les above it, and the inferior parietal 
gyrus, which is placed below it. With the two confluent postcentral sulci it 
presents a figure like the letter 4 placed on its side. 
The ramus occipitalis is a curved sulcus which bounds externally the arcus 
parieto-occipitalis, or, in other words, the arching convolution which surrounds the 
external parieto-occipital fissure. The ramus occipitalis hes behind the ramus 
horizontalis, and is generally linked on to it; less frequently it is separate. The 
posterior end of the ramus occipitalis enters the occipital lobe, and, behind the arcus 
parieto-occipitalis, bifurcates into two widely-spread-out branches. These form a 
short transverse fissure in the occipital lobe, termed the sulcus occipitalis transversus. 
