424 Royal Society : — 



hial wall thus marked out was carefully cut away down to the base, 

 as were also the right halves of the three upper cervical vertebrae. 

 The dura mater being then removed, a photograph was taken, 

 in which the relative position of the different parts of the brain are 

 well seen. The posterior lobes of the cerebrum project to the extent 

 of i inch beyond the cerebellum, covering it more completely than 

 in the Orang, and nearly, if not quite, as much so as in man. The 

 upper part of the remaining portion of the calvarium was now re- 

 moved, a section made across the hemispheres at the level of the 

 lower surface of the corpus callosum, and the lateral ventricles 

 opened out on both sides. Although in general form and in the 

 arrangement of the structures composing their walls these cavities 

 present a great resemblance to those of the human brain, one re- 

 markable peculiarity immediately strikes the observer, viz. the 

 great development of the posterior cornu, with the contained hippo- 

 campus minor. It extends from the commencement of the descend- 

 ing cornu to near the apex of the well-developed posterior lobe, is 

 of considerable vertical depth, being curled round the voluminous 

 projection of the hippocampus minor, and is directed at first some- 

 what outwards and backwards, then directly backwards, and finally 

 takes a considerable sweep towards the middle line — the characteristic 

 form which has obtained for this part in man the name of " digital 

 cavity." The hippocampus minor is formed, as in the human brain, 

 by the deep involution of a layer of superficial grey cerebral matter, 

 covered internally by a layer of white substance, which is so thin 

 that the surface of this prominence had a darker look than the other 

 parts of the ventricular walls. It differs from a typical example of 

 the corresponding part in the human subject in its great relative 

 size, both as to length and as to the extent to which it projects into 

 the ventricle. The hippocampal sulcus (well marked on the inner 

 surface of the posterior lobe of all apes) is not only very deep, but 

 has concealed within it a convolution of considerable size, in the form 

 of a longitudinal eminence attached to the floor of the fissure. The 

 eminentia collatcralis is prominent. The hippocampus major is 

 smooth upon the surface. The anterior cornu is of the same form 

 and extent as in the human brain. 



In the brains of C. sabaus, C. mona, and C. ruler a similar dis- 

 position of these parts was found. In the last-named, the posterior 

 lobe of the cerebrum is even more prominent, and the hippocampus 

 minor of still greater size, as it tapers less towards its termination ; 

 in fact this eminence is here actually larger than the hippocampus 

 major, to which its true relation can be better studied in these apes 

 than in man. 



Macacus. — In a monkey of this genus (31. crythrcens) the pos- 

 terior cornu and hippocampus minor were observed to obtain almost 

 as large a development. 



Family 2. Platyrrhina. 



In Cebus apella the ventricular cavity resembles in all essential 

 particulars that of Cercopithccus. There is the same extent of pos- 



