SUPERIOR PEDUNCLES OF THE CEREBELLUM AND VALVE. 455 



gard to the size and apparently also in the nature of the organs 

 seen at the base of the brain in Man and Monkeys on the one 

 hand, and that of almost all Mammals on the other. The 

 aquatic Mammals alone exhibit in this region a formation 

 resembling the more highly developed brains of Monkeys and 

 of Man. 



The variations in size are referable to the imperfect develop- 

 ment of the pons and of the pyramids of Mammals, as well as to 

 excessive development of the posterior segment of the medulla 

 oblongata, which is the continuation of the tegmentum of the 

 crus cerebri. The variations in form are due to the circum- 

 stance that in Man the basal surface of the segment of the 

 oblongata in question is marked by three longitudinal eleva- 

 tions : (1) the pyramids ; (2) the olivary*, bodies ; and (3) the 

 peduncle of the cerebellum, whilst in Mammals we meet above, 

 near the pyramids, with an organ that is apparently absent in 

 Man, the corpus trapezoides, forming a thick, plastic, trans- 

 verse band, extending into the peduncle of the cerebellum, 

 whilst, on the other hand, the corpora olivaria of the human 

 medulla oblongata appear to be deficient. 



From this point of view a hybrid kind of formation occurs 

 in the brains of Monkeys (Pavian), inasmuch as the corpus 

 trapezoides and olivary body are both present, though but 

 feebly developed. In the Lemuridse I found the corpus trape- 

 zoides alone. 



Nevertheless it is easy to demonstrate that the occurrence 

 or absence of either of these organs is only apparent, and 

 is occasioned only by variations of development. For the 

 corpus trapezoides consists of transverse fasciculi lying behind 

 the pyramids, which in Man are still covered by the longer 

 pons that he possesses, but are exposed in Mammals on account 

 of the narrowness or shortness of their pons; and the lower 

 olivary body can be shown to be present in transverse sec- 

 tions of the medulla oblongata of all Mammals, only it is 

 pressed back behind the pyramids, and does not therefore pro- 

 ject externally at their side. 



Reflecting on the circumstance that these variations of form 

 in the medulla oblongata are nothing else than an explicable 

 consequence of the comparative development of the cerebral 



