PARIETO-OCCIPITAL AND COLLATERAL FISSURES. 517 
When the fissure of Rolando is widely opened up, so that its bottom and its opposed sides may 
be fully inspected, it will be seen that, in the neighbourhood of the superior genu, the two bound- 
ing gyri are dovetailed into each other by a number of interlocking gyri, which do not appear on 
the surface. Further, two of these, placed on opposite sides of the fissure, are frequently joined 
across the bottom of the sulcus in the form of a sunken bridge of connexion, which constitutes 
what is termed a deep annectant gyrus. The continuity of the fissure is thus, to some extent, 
interrupted. This condition is rendered interesting when considered in connexion with the 
development of the sulcus. The deep interlocking gyri indicate a great exuberance of cortical 
growth in this situation in the early stages of the development of the fissure ; and the presence of 
the deep annectant gyrus is explained by the fact that the fissure of Rolando generally develops in 
two pieces, which run into each other to form the continuous sulcus of the adult, viz. a part cor- 
responding to the lower two-thirds, and an upper part, which represents the upper third and 
which appears at a slightly later date. In certain very rare cases the fissure of Rolando is found to 
remain double throughout life, through a failure of its two pieces to unite. In such cases the deep 
annectant gyrus, which is frequently seen at 
the bottom of the furrow, remains on the 
surface. Heschl, who examined 2174 cerebral 
hemispheres, only found this anomaly six 
times; Eberstaller met with it twice im 
200 brains. 
Parieto - occipital Fissure. — A 
small part of this fissure appears on 
the outer face of the cerebral hemi- 
sphere. For the most part it is situ- 
ated on the internal surface. It is 
customary, therefore, to describe an 
external parieto-occipital and an in- 
ternal parieto-occipital fissure. It Fic. 385.—Lerr CEREBRAL HEMISPHERE, from a fcetus in 
must be clearly understood, however, the early part of the seventh month of development. 
that they are directly continuous with — p.c.s. Suleus precentralis superior. 
each other round the supero-mesial per Sulcus precentralis inferior. 
borderof the hemisphere. The parieto- "Hower part of Relandig Assutcs 
Ryde : : 1s Upper part of Rolandic fissure. 
occipital fissure intervenes between — »), Inferior postcentral sues | 
the parietal and occipital lobes. p®. Ramus horizontalis -Intraparietal fissure. 
: : i: re e cipitalis 
The external  parieto - occipital P ‘Ramus occipitalis J 
: e.p. External perpendicular fissure. 
fissure cuts the supero-mesial border 41, Parallel suleus. 
of the hemisphere in a transverse %. Sylvian fossa. 
: : : rn op .p So Tads za 
direction at a distance of from one [-P: Fronto-parietal wall. 
: : A F. Frontal wall. 
and a halt to two inches in front of 0. Orbital wall. 
the occipital pole. It is, as a rule, 
not more than about a half an inch long, and it is brought to an abrupt termina- 
tion by an arching convolution, which winds round its extremity and receives the 
name of arcus parieto-occipitalis. 
The internal parieto-occipital fissure is carried downwards on the inner surface of 
the hemisphere in a nearly vertical direction as a conspicuous and deep cleft. A 
short distance behind the hinder end of the corpus callosum its lower end runs into 
the calearine fissure. 
The parieto-occipital fissure is developed, as a rule, after the manner of a complete fissure. In 
the foetal brain it forms a very evident infolding of the cerebral wall. In the adult brain, how- 
ever, it does not form any eminence on the inner wall of the ventricle, because it does not extend 
downwards as far as the cavity. The wall of the ventricle during the growth of the hemisphere 
has thickened to such an extent that the part corresponding to the fissure has become solid. 
Collateral Fissure (fissura collateralis)—The collateral sulcus is a strongly- 
marked fissure on the tentorial face of the cerebral hemisphere. It begins near the 
occipital pole and extends forwards towards the temporal pole. In its posterior 
part it is placed below, and parallel to, the calcarine fissure, whilst in front it is 
separated from the hippocampal or dentate fissure by the hippocampal gyrus, which 
is the innermost convolution on the tentorial surface of the hemisphere. 
In front of the anterior end of the collateral fissure a shallow sulcus turns 
round the anterior end of the temporal lobe, so as to intervene between the temporal 
pole and the uncinate or hook-like extremity of the hippocampal convolution. 
This is the incisura temporalis, and it may be regarded as a forward prolongation 
