134 Cincinnati Society of Natural History. 



optic lobes ; the lower, besides its relation to the infundibulum, 

 extends by the aqueduct of Sylvius to the fourth ventricle. 



4. The Corpora Bigemina (Mesencephalon) are relatively large, 

 greatly resembling those of birds, and are rather more than half 

 the length of the cerebrum. They are somewhat oblique, but the 

 longer axis is approximately at right angles to that of the brain. 

 The large ventricles of the optic lobes are nearly filled by large 

 semi-oval protuberances from the lateral and posterior walls. 

 They are separated from the ventricles by an inner wall continuous 

 with the inner fibre-layer of the tectum opticum. These bodies are 

 called by Rueckhard colliculi of the corpora bigemina. They are 

 the same masses called by Bellonci* inner protuberances. He 

 describes them in the amphibians as " containing nerve cells which 

 are arranged in irregularly concentric layers. Nothing indicates the 

 formation of a special, well-localized nucleus." In the birds it is 

 said to be similar. 



The considerable size of the nuclei corresponding to the poste- 

 rior lobes of the corpora quadrigemina may be noted as of espe- 

 cial interest. These are more fully described beyond. These nuclei 

 constitute the corpus poster ius. 



5. The Cerebellum (Epencephalon)is unusually large in the Croc- 

 odilia, affording an additional resemblance to higher vertebrates, 

 but it is perfectly devoid of convolutions. Its internal structure is 

 exceedingly simple. The form is nearly conical, with the apex 

 directed backward, the cross-sections thus being circular. The 

 corresponding organ in turtles, for example, is a mere flat leaf-like 

 body. Just in front of the base of the cerebellum the roots of the 

 trochlearis may be seen springing from the velum medullare ante- 

 rius. The connections with the remaining parts of the brain will 

 be discussed later. 



6. The Metencephalon is sub-rhombic and widely expanded in the 

 horizontal plane, narrowing rapidly anteriorly, especially ventrally, 

 to the union with the diencephalon. The fourth ventricle is only 

 partially covered by the cerebellum. After a removal of the mem- 

 branous coverings, a series of prominences may be seen upon the 

 base and sides of the ventricle. These mostly mark the site of 

 nuclei at the roots of the nerves V, VIII, X, etc., and are termed 

 tuberculi trigeminorum, eminentia acustica, eminentia vagalis, etc. 



••Josef BellorTci : " Ueber die centrale Endigung des Nervus opticus bei 

 den Vertebraten,*' Zeitschrift fuer Wiss. Zoologie, Bd. 47, 1888. 



