BRAIN OF ONE OF THE SALAMANDERS I05 



Metencephalon. — The cerebellum is an inconspicuous part of the 

 brain. It curves around the posterior surface of the mesencephalon, 

 and is composed chiefly of white matter. It comprises the fibers of 

 two commissures. A single layer of cells surrounds the white matter. 

 Its appearance on the dorsal surface of the brain is shoAArn in Fig. 17, 

 and its appearance on transection in Figs. 10 and 16. 



Myelencephalon. — This part of the brain is of the usual Amphibian 

 type. The fourth ventricle is covered by a choroid plexus. The 

 cranial nerves from the fifth to the tenth arise from this segment. The 

 gradual transformation from medulla to spinal cord takes place in the 



usual way. 



Fiber Tracts 



In the Amphibian brain, several well-defined fiber tracts are clearly 

 recognizable. Some of these can be pretty satisfactorily homologized 

 with structures in higher vertebrates while the homologies of others are 

 doubtful. Regarding the names applied to some of these there is also 

 no very great agreement. The terms used in the following description, 

 are, so far as could be determined, those most generally used. 



Posterior longitudinal fasciculus. — The fibers of this tract can be 

 distinguished as a well-marked bundle in the ventral part of the medulla 

 (Fig. 8). Thence they pass forward into the mesencephalon, where they 

 cannot longer be distinguished as forming a definite tract. 



Basal prosencephalic tract. — The fibers designated by this name form 

 a tract which has been described under various names. Kingsbury 

 ('95) uses the term adopted here. This was used before by Edinger. 

 In sagittal sections it appears to be a forward continuation of the poste- 

 rior longitudinal fasciculus, though the fibers are probably not contin- 

 uous. In cross-section it appears as a very definite bundle on either 

 side of the median Hne (Figs. 5 and 6). Beginning well back in the 

 mesencephalon, it can be traced forward into the ventral part of the 

 hemispheres. The fibers of this bundle are mostly non-meduUated. 



Anterior commissure. — This commissure is composed chiefly of non- 

 meduUated fibers, and by the methods used, it could not be traced very 

 satisfactorily. The main bundle on either side of the median hne 

 inchnes sHghtly downward and then passes forward just below the basal 



