HYLOBATES HOOLOCK, THE GIBBON 413 



The water displacement ol the brain is: 



Forebrain 53 c.c. 



Midbrain 2 c.c. 



Hindbrain 10 c.c. 



Total displacement 65 c.c. 



The lorebram indices, computed on the basis ot weight and vohime, 

 give the gibbon a forebrain index of 81 per cent by weight and 82 per cent 

 by volume, thus placing it in the group characterized by manual differentia- 

 tion, although it occupies a rather low position in this group. 



Surface Appe.ar.ance of the Brain in Hylobates Hoolock 



From some of the more important superficial features on the external 

 surface of the cerebral hemisphere, it is evident that the brain of the gibbon 

 in many respects is little more advanced in its development than that of the 

 baboon or macacus. The two great concavities on the basilar surface, i.e., 

 the orbital concavity and the cerebellar concavity, are equally as pronounced 

 as in the other intermediate primates. This fact in itself calls attention to 

 the degree of limitation imposed upon the frontal and occipital lobes in 

 their expansion. For it is almost in direct proportion as these basal concavities 

 become less pronounced, that the two important territories of the cortex, 

 the frontal and the occipital lobes, show progressive expansion. The olfac- 

 tory bulb and tract are detachable as far back as the trigonum olfactorium. 

 A rudimentary olfactory sulcus is present and a correspondingly rudimen- 

 tary gyrus rectus. The interorbital keel is prominent. As in the case of the 

 baboon and the macacus, the boundaries of the superior longitudinal fissure 

 tend to diverge as they approach the occipital pole. This divergence permits 

 of a marked widening of the fissure which is in the interest of accommodating 

 the superior vermis of the cerebellum whose appearance on the tentorial 



