522 ‘THE NERVOUS SYSTEM. 4 
; 
In the human brain the intraparietal sulcus is usually developed in four separate pieces, 
corresponding to the four portions of the fissure which have been described as being present in 
the adult brain. The sulcus postcentralis inferior appears first (somewhere about the end of the 
sixth month), then the ramus occipitals and the ramus horizontalis; last of all the sulcus post- 
centralis superior comes into view. The further development of the sulcus consists in the 
running together of these early pieces. This takes place in different ways, and not infrequently 
union fails at one or more points, and thus a great variety of combinations may be noted 
in different individuals ; indeed, it may be said that every possible kind of combination may be 
met with. The most common form which the fissure assumes, however, is that in which all its 
factors have become confluent and one continuous furrow results. When such an intraparietal 
furrow is widely opened up, certain deep annectant gyri, which cross the bottom of the sulcus, 
External parieto-occipital fissure 
Ascending parietal gyrus 
to} c 
Transverse 
—_,— occipital 
Deep { fissure 
gyri | L 
Termination of Sylvian fissure 
Fic. 888.—THE INTRAPARIETAL SULCUS FULLY OPENED UP, so as to show its several parts and the 
deep annectant gyri intervening between them. 
p!. Sulcus postcentralis inferior. p®. Ramus horizontalis. 
p’. Sulcus postcentralis superior. p*. Ramus occipitalis. 
come into view. These interrupt the sulcus at the points of union between its several pieces and 
indicate its original multiple formation. 
There is reason to believe that three of the elements of the human intraparietal sulcus, viz. 
the sulcus postcentralis inferior, the ramus horizontalis, and the ramus occipitalis, are disrupted 
portions of the primitive single continuous fissure which is seen in certain of the lower apes 
(Cebus), whilst one, the sulcus postcentralis superior, is a superadded element. 
There is a strong analogy between the postcentral sulcus, the fissure of Rolando, and the 
precentral sulcus. They form a group of radial sulci on the outer surface of the foetal cerebrum 
above the Sylvian region. The fissure of Rolando makes its appearance first, then the praecentral 
sulcus, and, lastly, the postcentral sulcus. Each assumes shape in the first instance in two pieces, 
viz. an upper and lower. The two pieces of the fissure of Rolando join early, and only in very 
rare instances remain separate ; the two pieces of the postcentral furrow usually join, but in 19 
per cent of cerebral hemispheres they remain separate ; the two pieces of the praecentral furrow, 
as a rule, remain separate and distinct. 
The sulcus transversus occipitalis, or bifurcated extremity of the ramus occipitalis, is very 
generally believed to be the representative in the human brain of the conspicuous “ Affenspalte ” 
in the cerebrum of the ape. It is very doubtful if this is the case; and it is also doubtful if 
there is, as a rule, in the adult human -brain any representative of this simian fissure. The 
“ Affenspalte” is a complete fissure, and every observer is agreed that in the human fetal brain 
its equivalent is a deep infolding of the hemisphere wall which takes place in this locality, and 
which receives the name of the fissura perpendicularis externa. In the course of development 
this foetal fissure disappears; it is a transitory fissure and becomes completely wiped out from 
the hemisphere surface. The only connexion which exists between it and the suleus occipitalis 
transversus consists in the fact that the latter, at a later period, becomes developed on the ground 
occupied by the transitory fissura perpendicularis externa. 
The upturned ends of the fissure of Sylvius, of the first temporal or parallel 
sulcus, and of the second temporal sulcus ascend for a short distance, one behind 
the other, into the inferior parietal gyrus. 
The intraparietal sulcus maps out three districts or gyri on the external surface 
of the parietal lobe, viz. the ascending parietal convolution, the superior parietal 
gyrus, and the inferior parietal gyrus. 
The posterior central convolution (ascending parietal convolution) is a long gyrus 
which extends obliquely across the hemisphere from the supero-mesial border above 
