250 ON THE BRAIN OF ATELES PANISCUS. [JunC 11, 



series of Sitnice, if the same character alone be considered, another 

 Simian, inhabiting the same geographical area, is at the top ; I refer 

 to Chrysothrix sciureus, whose posterior lobes, as I. G. St.-Hilaire 

 long ago proved*, are better developed than those of any other Mam- 

 mal, overlapping the cerebellum by one-fifth of their length. In 

 fact, if the Primates were arranged according to the development of 

 their posterior cerebral lobes, we should have some such descending 

 series as the following : — Chrysothrix, Cebus, Troglodytes, Man, 

 .... Mycetes — a series which sufficiently illustrates the classifica- 

 tory value of these structures. So much for the posterior lobe. I 

 turn now to the next point, the demonstration of the existence of 

 the posterior cornu in Ateles. 



When the lateral ventricle was exposed in the ordinary way 

 (PI. XXIX. fig. 5), a straight line passing from the extremity of the 

 anterior to that of the posterior cornu measured 2'1 inches. A di- 

 stance of 1 '3 inch separated the anterior eud of the anterior cornu from 

 the commencement of the descending cornu ; while a straight line ex- 

 tending from the commencement of the descending to the end of the 

 posterior cornu measured 075. Each lateral ventricle, measured 

 from the centre of the corpus callosum to the outer boundary, at its 

 widest point, or opposite the commencement of the descending cornu, 

 was about half an inch wide. The posterior cornu has a general di- 

 rection backwards, outwards, and then inwards; and, besides its general 

 curvature, it has a secondary inflexion, so as to be a little sinuous. 

 It is wide at its commencement, but rapidly narrows, until, where it 

 bends inwards, its walls are so close together as to give it the appear- 

 ance of a mere fissure, whose sides are apt to adhere together in such 

 a manner as seriously to interfere with the satisfactory definition of 

 the posterior limits of the cornu. In preparing the specimen, of 

 which fig. 5 is a representation, for the artist, I therefore took care 

 not to extend these limits artificially, rather preferring to leave a 

 portion of the cornu unopened, than to exaggerate its length. 



In the other brain I found the posterior cornu, on the right side 

 (dissected in the ordinary manner), to be traceable, without the least 

 difficulty, to within a very short distance of the posterior limit of the 

 hemisphere ; while in the left hemisphere, which I examined by 

 making successive vertical sections from behind forwards, the posterior 

 cornu ended at fully a quarter of an inch distance from the posterior 

 extremity of the hemisphere. Such sections are of particular value ; 

 for they show the extent of the cornu without any disturbance of its 

 natural dimensions; and a comparison of the woodcuts (fig. 1) A, 

 B, C, &c., and A', B', C, &c., which represent two series of sections 

 of corresponding regions of the Human and the Ateles' brain, will at 

 once show that the relative dimensions of the posterior cornu are 

 greater in the Monkey than in Man. I may remark that, of the 

 left hemispheres of three human brains which I have dissected for 

 comparison with Ateles, that whose sections are represented in the 

 figures had its posterior cornu far better developed than the other 



* See the ' Zoologie du Voyage de la Venus ' for an excellent figure of this 

 brain. 



