PROSENCEPHALON OF MAMMALS. 



127 



Base of the brain, Orang-utan. xxix\ 



fig. 116. Secondary fissures subdivide the orbital as well as the 

 frontal and falcial surfaces of the anterior lobe : the surface resting 

 on the orbital plate of the frontal o 



bone, in the Orang's brain, fig. 

 112, shows the following con- 

 volutions : — ' postorbital,' o, mid- 

 orbital, 0', entorbital, 0", ect- 

 orbital, 0'', and antorbital, 0*. 

 That which lies external to the 

 rhinal fissure or depression is not 

 subdivided into ectorhinal and 

 entorbital folds as in Man, fig. 

 120, d, 0". Similar secondary 

 chinks furrow the occipital lobe, 

 on the tentorial surface of which 

 the tentorial fold, fig. Ill, r, 

 the entotentorial, r', and ecto- 

 tentorial, r" , are now defined by 

 the fissures, 18, is', is'\ These 

 folds are more or less continuous with the basirhinal, b, and sub- 

 sylvian, f, tracts. The increasing number of secondary fissures 

 and the greater depth and more winding course of the pri- 

 mary ones mainly characterise the brain in the Orang (vol. ii. 

 fig. 148) and Chimpanzee, fig. 114. The tract between the 

 interhemispheral and supersylvian fissures is subdivided into 

 medial, /, medilateral, m, and supersylvian, g, folds, fig. 116, 

 Chimpanzee : we have evidently here the corresponding parts of 

 the hemispheres that form these folds, or parts of them, in 

 Carnivora. 



D. Archencephala. — The same principle carried abruptly to an 

 extremely greater degree, as in figs. 115, 116, Homo, associated (as 

 compared with Gorilla, e. g.,) with a greater proportional bulk of 

 the brain to the body, and with a still greater proportional size of 

 the cerebrum to the rest of the brain, characterise the Archencepha- 

 lous subclass, from the lowest varieties (Australian, Boschisman, 

 Hottentot) to the highest. These proportions have thoroughly 

 stood the severest tests, as where the diminutive female in such 

 varieties has been selected to exemplify the brain-characters, 

 with a view of reducing the chasm between the gyr- and arch- 

 encephalous brains to a minimum. 



Before entering into the details of the complex convolutional 

 surface of the human cerebrum, I may premise some recapitu- 

 latory remarks. 



