If^fil] BRAIN OF ATKLES PANISCUS. 25/ 



proportional width of the uncinate gyrus, contained between the cal- 

 carine and dentate sulci above, and the collateral sulcus below, is 

 marked in Ateles. The transverse sections (fig. I. A', B', &c.) are no 

 less strictly comparable to those yielded by the human brain, the 

 chief differences being that, throughout the greater part of its length, 

 the calcarine sulcus possesses the bifurcated outer extremity which 

 its posterior part only presents in Man ; and that the collateral sulcus 

 is smaller and further out in proportion, and hence the uncinate 

 gyrus is larger. 



As to the hippocampus minor, the transverse sections (fig. 1) 

 clearly show how much larger it is, proportionally, in Ateles than in 

 Man ; while the horizontal section (PI. XXIX. fig. 5) exhibits its 

 exact correspondence with the definition quoted above — viz. "an 

 elevation of the inner wall of the posterior cornu of the lateral ven- 

 tricle, which is convex outwards and forwards; " and, as might be 

 expected from the transverse section, it shows the larger proportional 

 size and greater outward convexity of the Monkey's hippocampus 

 minor. 



The eminentia coUateralis, on the other hand, is far less developed 

 in Ateles than in the particular human brain whence the sections 

 are taken ; but it is quite distinctly visible at the junction of the 

 posterior and descending cornua. The floors of both these cornua, 

 however, are so narrow, that the eminentia can hardly be said to be 

 continued into them, as it sometimes is into the posterior cornu, 

 and almost always is into the descending cornu, in the human brain. 

 Thus, in exact contradiction of what has been affirmed, it is the hip- 

 pocampus minor which is developed, and the continuation of the 

 eminentia coUateralis backwards which is not developed in the 

 Monkey. 



The sulci and gyri of the outer surface of the cerebral hemispheres 

 present in Ateles paniscus the same essential arrangement as in the 

 Ateles belzebuth, described and figured by M. Gratiolet. Dividing- 

 the hemisphere into five lobes (frontal, parietal, median, temporal 

 and occipital) the median (insula — Island of Reil) hidden between the 

 lips of the Sylvian fissure, is a mere smooth convex projection, wider 

 above than below, or having somewhat the shape of a triangle, with 

 its apex downwards and forwards, and wholly devoid of sulci. The 

 small frontal lobe is divided by the horizontal sulci into the three 

 infero-frontal, medio-frontal, and supero-frontal gyri. The antero- 

 parietal sulcus is placed very far forward, at the commencement of 

 the Sylvian fissure, joins the supero-frontal sulcus, and then sends 

 a branch backwards. The postero-parietal sulcus (scissure de Ro- 

 lando) is situated so far back that the antero-parietal gyrus (F pli 

 ascendant, Gratiolet) is exceedingly thick, and it passes backwards, 

 as well as upwards, towards the inner and upper margin of the hemi- 

 sphere, close to which it terminates. The postero-parieatl gyrus 

 (2' pli ascendant) widens superiorly, in consequence of the backward 

 incUnation of the upper part of the Sylvian fissure, to form the 

 postero-parietal lobule (lobule du deuxieme pli ascendant), which 



Proc. Zool. Soc— 1861, No. XVII. 



