98 UNIVERSITY OF COLORADO STUDIES 



Yet these structures are the parts which in man and other mammals 

 comprise by far the greater part of the brain. The mesencephalon^ 

 which in this brain is quite as conspicuous as the cerebral hemispheres, 

 is the part of the brain which, in aU vertebrates below the mammals, 

 forms the optic lobes. In the mammals it forms four inconspicuous 

 bodies known as the corpora quadrigemina, hidden away beneath the 

 other parts of the brain. The cerebellum of higher forms, containing 

 the well-known arbor vitae, or "tree of life" is represented in this brain 

 by a small band of fibers crossing from one side of the brain to the other. 

 It appears in Fig. 17 just back of the mesencephalon. The medulla 

 oblongata, also clearly shown in this figure, shows on the other hand 

 no very marked external differences from that of higher forms. This 

 structure is much more constant throughout the vertebrate series than 

 are the other parts of the brain. It is also of interest to note that the 

 ten pairs of cranial nerves as shown here, both in their distribution and 

 place of origin, correspond quite closely to the first ten of the twelve 

 pairs of cranial nerves found in the human brain. 



A study of the drawings made from sections, will give a good idea 

 of the size and arrangement of the ventricles of the brain which also 

 occur here in simple relations. It will be noticed also, that the gray 

 matter, represented by the shaded portions of the drawings, is arranged 

 around the ventricles, and that there is no layer of gray matter placed 

 at the surface to form a cortex as in higher forms. 



Methods and Terminology 



On account of the small size of Plethedon, it was necessary to use 

 microscopic methods in the study of this brain. Entire decalcified 

 heads were embedded in celloidin and sections cut and mounted in 

 series. Various stains were used. Borax-carmine and Delafield's 

 haematoxylin each showed some features clearly. Weigert's haema- 

 toxylin (Pal's method) gave the best results for the peripheral distribu- 

 tion of nerves, and iron-alum haematoxylin for internal fiber tracts. 



In this paper, so far as applicable, the Basel Anatomical Nomen- 

 clature [BNA] has been used. In some places, where the homology 

 of the structure seemed doubtful, it seemed better to use some other 



