150 THE CEREBRAL HEMISPHERES. 



(Turner) (fig. 101) arches through the parietal lobe, commencing in its anterior 

 inferior angle, where it is sometimes, though rarely, continued into the fissure of 

 Sylvius. It ascends at first parallel to the fissure of Rolando (pars ascendent 

 inferior s. postcentralis inferior], and then turns backwards horizontally to the back 

 of the lobe (pars horizontalis s. posterior), extending nearly to the termination of 

 the parieto-occipital fissure, past which it is continued (pars occipitalis, paroccipital of 

 Wilder) to join the anterior occipital. Its horizontal portion divides the parietal lobe 

 into two parts, the superior and inferior parietal lobules, and it is frequently bridged 

 across by annectent convolutions connecting those lobules. One or two well-marked 

 rami pass into the superior parietal lobule from the upper side of the pars horizon- 

 talis, and as many shallow sulci extend from its lower concave side into the inferior 

 parietal lobule. 



Above the pars ascendens inferior and often separated from it by an annectent 

 gyrus connecting the superior parietal lobule with the ascending parietal gyrus is a 

 short vertical fissure (postcentralis superior), which is termed by Cunningham the 

 pars ascendens superior, since in a large number of instances it is directly continuous 

 with the pars ascendens inferior (the bridging gyrus being then absent), and the 

 two conjoined parts then constitute a sulcus which runs parallel to the fissure of 

 Rolando, and is termed the postcentral sulcus (Ecker). 



Although the condition with a detached pars ascendens superior is to be regarded as typical 

 of this sulcus as shown both by its occurrence in most of the Quadrumana and the course of 

 its development in the human embryo, it is by no means the most common in the adult 

 human brain, the most usual condition being that here figured (fig. 101) in which there is a 

 complete postcentral sulcus from which the horizontal arm passes off backwards at an oblique 

 angle towards the occipital lobe, although generally interrupted near the anterior limit of that 

 lobe by an annectent gyrus. An arrangement of this character was met with by Cunningham 

 in 56 p. c. of his cases. In 16 p. c. a more or less complete postcentral sulcus was present, 

 and was cut off from the horizontal part of the fissure by an annectent gyrus. Occasionally 

 the postcentral sulcus communicates above, and sometimes also below, with the fissure of 

 Rolando. Three deep annectent gyri partially interrupt the horizontal part of this fissure and 

 join the superior and inferior parietal lobules (Eberstaller) ; these have been supposed to be 

 a human characteristic, but they occur in the Chimpanzee and occasionally in the Baboon 

 (Cunningham). 



In rare cases the pars ascendens inferior of this fissure is cut off from the pars horizontalis, 

 and the latter is continuous with the pars ascendens superior alone. In other and also rare 

 instances all the parts of the fissure are separated from one another by bridging convolutions, 

 so that the fissure appears as four distinct pai'ts, viz., the two portions of the postcentral 

 sulcus (lower and upper) , the pars horizontalis and the pars occipitalis. 



The occipital continuation of the intraparietal fissure is sometimes separated from the 

 rest of the fissure (paroccipital fissure of Wilder). It usually terminates posteriorly by 

 joining a sulcus which is nearly vertical to its direction, the sulcus transversus occipitalis of 

 Ecker (sulcus occipitalw anterior, fig. 101). This will be further noticed in connection with 

 the occipital lobe. In the monkey's brain the occipital termination is concealed within the 

 deep cleft (Affenspalte) formed by the occipital operculum. 



Below the postcentral sulcus and cutting into the margin of the f ronto-parietal operculum, 

 is a small oblique sulcus (*. retro-centralis transversus of Eberstaller) (fig. 101, rtc.tr.'). This 

 sometimes serves to prolong the intraparietal fissure into the fissure of Sylvius. 



The intraparietal fissure is usually developed in the foetus in four segments (Cunningham), 

 which appear in the following order, viz., inferior postcentral. horizontal, occipital, and 

 superior postcentral. It is better developed, i.e., more continuous and deeper, in apes than in 

 man ; but the superior postcentral part is either not developed or is rudimentary in them 

 (fig. 107). The so-called horizontal part is not truly horizontal, but has an upward inclination 

 which is more marked in the male human brain than in the female. 



The posterior central, postcentral, or ascending parietal convolution 

 (fig. 101, A. P.) lies behind the fissure of Rolando, between this and the postcentral 

 sulcus, and parallel to the ascending frontal convolution, with which it is usually 

 continuous below the fissure of Rolando. Above, it is connected by a broad annec- 

 tent gyrus with the superior parietal convolution, and is continued on to the mesial 

 surface by a narrow annectent which unites it with the posterior part of the marginal 

 gyrus (paracentral lobule). 



