Notes upon the Brain of the Alligator. 147 



In Figure 6, which passes nearly through the middle of one of 

 the hemispheres, the large size of the axial lobe is obvious, the 

 lateral band of olfactory fibres appears at (0) and, near it, the 

 crowded cells of the deeper parts of the crura olfactoria. The 

 taenia thalami connecting the posterior part of the mantle with 

 thalamus and superior commissure are only slightly apparent. The 

 thalamus is not as well separated from the axial lobe of cerebrum 

 anteriorly as nearer the median line. Observe the sudden deflec- 

 tion of the peduncular fibres as they enter the mesencephalon. 

 The transversely cut fibres of the posterior commissure are seen. 

 Back of the chiasm and dorsad to it are the deep-colored fibres 

 whose course can be best followed in transverse sections. The 

 ventricles of the optic lobes are large and partly closed by the for- 

 ward projection of the colliculi, which are composed of densely 

 cellular gray matter, and are separated from the ventricle by a defi- 

 nite stratum of the same nature as the inner portion of the tectum 

 opticum. Back of the optic lobes are two prominences upon 

 peduncles passing to the medulla, which the section discloses as 

 well-defined nuclei intercepting most of the fibres from the inner 

 stratum of the optic lobes. These nuclei are very large and have 

 cells of the flask variety. These prominences {corpora posteriord) 

 are homologous with the posterior corpora quadrigemina of Mam- 

 malia. It will be interesting to compare the relations of the parts 

 seen in longitudinal section of the mesencephalon of the alligator 

 with the same parts in other vertebrates. The colliculi (col.) of the 

 optic lobes are proportionally as well developed as in the frog. 

 The corpus posterius, of Bellonci (c. p.), is large, and obviously 

 receives the great bulk of the fibres from the inner stratum of tec- 

 tum opticum or colliculi. There seem likewise to be fibres from the 

 sensory root of the fifth nerve entering it. Its relations are remark- 

 ably similar to those in birds. The embryo of the fowl at fourteen 

 days has much such a prominence as appears in the alligator. The 

 lizard {Podanis) has less resemblance in this respect than the aves. 

 The alligator brain very fully substantiates Bellonci's statement that 

 the corpora posteriora are homologous with the corpora quadrigem- 

 ina posterior of mammals. He says: "The corpus posterius, 

 which is so closely connected with the corpus opticum, is, so far as 

 structure and development are concerned, more closely related 

 with the hind-brain than with the mid-brain. The nucleus of the 

 corpus posterius is developed in the lateral transition region between 



