304 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



the other is exceedingly variable, depending on the relative development, width 

 and outline of the anterior central convolution. 



The parietal portion lies directly back of the fissura centralis, extends 

 backward and is separated in the Simiadaa from the occipital lobe by the lateral 

 portion of the upper branch of the primary occipital arch O 1 , Plate XXXIX, 

 fig. 4, the so-called fissura occipitalis perpendicularis externa. In the brain of 

 Man this precise and definite boundary is not present and this portion of the 

 cerebral surface undergoes considerable evolution, changing to a marked degree 

 the aspect of this region. We will leave the consideration of this portion of 

 our subject, however, until we discuss the convolutions of the occipital lobe and 

 its relations to the pits de passage or annectant convolutions. Below and an- 

 teriorly it is separated from the occipito-temporal lobe by the posterior extremity 

 of the Sylvian fissure, its lowest convolution, P 2, P 2\ becoming continuous with 

 the superior occipito-temporal, to form an arch surrounding this extremity. On 

 the mesial surface, as we have already seen, the parietal region is continuous 

 with the frontal without any signs of separation, whilst posteriorly its boundary 

 is very distinct, it being either completely separated from the occipital lobe by 

 the internal portion of the occipital arch, O 1 , the fissura parieto-occipitalis, as in Man, 

 or joining it by means of a small convolution, the gyrus cunei or inferior internal 

 pli de passage of the Simiadae. 



FISSURA CENTRALIS. 



The central fissure or fissure of Rolando, begins at the upper border of the 

 hemisphere and runs obliquely downward and forward, terminating generally 

 some distance above the position of the Sylvian, as in PI. XLI, fig. 1. Turner gives 

 an instance 1 in which he found it terminating in the Sylvian. Ecker, in speaking 

 of this observation, states that he has never so found it ; whilst Bischoff, owing 

 to his peculiar morphological views regarding the arched windings of the central 

 fissure, says it cannot take place. Benedikt claims that in thirty-eight cerebral 

 hemispheres he found this complete union eighteen times. These results are 

 startling when it is considered that prior to the observation of Benedikt very few 

 instances of confluence of these two important fissures had been reported ; among 

 others one by McDonald and Mills, and two by Parker. In Zernoff's collection of 

 one hundred brains, referred to by Benedikt, it occurred in but one instance. 

 Since the publication of Benedikt's work Osier has reported that out of sixty-three 

 hemispheres, from thirty-four individuals, he found the central fissure communi- 

 cating with the fissure of Sylvius three times completely and seven times incom- 

 pletely. I have met with this condition frequently, especially as found by Bene- 

 dikt in the brains of confirmed criminals. It may be seen in the brain of a mur- 

 derer, existing in both hemispheres, PL XLIII, figs. 1 and 2 ; also in the brain 

 of a mulatto, PL XLIV, fig. 1, and in the left hemisphere of the brain of a china- 



1 Edinburgh Med. Jour., 1866. 



