MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



313 



According to the view advanced in this paper the structure of the parietal 

 region of the occipi to-frontal lobe is as follows : 



One fissure, the interparietal, is recognized separating it into two convolutions, 

 a gyrus parietalis superior, P\ and a gyrus parietalis inferior, P 2 P2 1 . 



Morphologically, however, I regard it differently. We have seen, in speaking of 

 the fissura centralis, c c, reasons for believing that this fissure represents the posterior 

 portion of a superior occipito-frontal fissure developed downward. If, therefore, 

 we imagine or consider the central fissure pushed upward and backward into its 

 primitive morphological position we will find that the convolution A C. or anterior 



central convolution, will take a position above the 

 superior parietal convolution. The diagrams, figs. 

 13, 14, 15, 16" and 17, will make my meaning clear. 

 Figure 13 represents the condition of the fissures 

 and convolutions in the brain of Man and the higher 

 monkeys, and fig. 11 the arrangement as found in 

 the Lemurs ; which, through the brain of Chiromys, 

 chh-omys. fig. 15, is plainly related to the type of fissuration 



as found in the Carnivora, Plate XLV, figs. 9-22. We have already seen that 

 we may trace the steps in the Lemurs by means of which the central fissure 

 gradually develops from a downward prolongation of the posterior portion of a 

 superior occipito-frontal fissure, OF 1 , figs. 13 and 14, Cheiromys. Considering the 

 fissure, c c, as restored gradually to its position as found in the Lemurs, we have the 

 relations represented in fig. 17, and by comparing this arrangement with an interme- 



Fig. 16. 



Fig. 17. 



Gibbon. 



Lemuk. 



diate form like the Gibbon, fig. 16, the various transitions from a Lemur type of brain 

 to that of Man will be readily understood and it will be perceived that the primi- 

 tive type of structure of the lateral surface of the occipito-frontal lobe is similar to 

 that of the occipito-temporal, consisting of two longitudinally-running occipito-frontal 

 fissures, o. f. 1 and o. f. 2 , a superior and an inferior, separating three primitive occipito- 

 frontal convolutions, corresponding to three similarly related convolutions found 

 in the occipito-temporal lobe as indicated in fig. 17. The anterior central con- 



