118 



ANATOMY OF THE CENTRAL NERVOUS SYSTEM. 



in the nuclei of the posterior columns, and in the gray matter of the spinal 

 cord: Tractus tecto-spinales et tecto-bulbares. In adult animals the deep 

 medullary layer of the midbrain may be best studied where it is best 

 developed: i.e., in birds or in fishes. The teleosts present the especial ad- 

 vantage that the region in which the Stratum medullare prof, lies is rela- 

 tively simple in structure; so that the tracing of the fibers is facilitated. 

 If one becomes once familiar with the relations in teleosts he may find them 

 readily again in any of the higher vertebrates. 



In Fig. 68 one may find several of the structures above mentioned. The 

 tegmental decussation is not, however, to be readily found. In the enor- 

 mous extension of the roof, which is found in the teleosts, these fibers have 

 come into another position. They lie now directly upon the decussation 

 which the lateral position of the medullated stratum makes, and thus in- 



Optie stratum — Jji 



Ti-actiis npticiia — 



Tractus strio-thalatnieus 

 Optic root 



Fig. 69. — Transverse section through the midbrain of a toad: Buf. ciii. 



creases the commissura ansulata. In horizontal sections one may readily 

 separate the two parts of the commissure (see Fig. 91). 



The fiber-system of the medullary stratum from the roof of the mid- 

 brain fills a large part 'of the basal portion of that brain-segment, encircling 

 and traversing it. Because of the great number of transverse fibers, the base 

 of the midbrain has been called the Pars commissuralis. 



The roof of the midbrain gives origin to still another system from the 

 same layer: viz., the fibers which, taken together, are much greater than 

 parts mentioned, and which, throughout the whole roof region, pass in 

 lateral direction from right to left or vice versa. Through these fibers there 

 is formed in the median line the dorsal decussation of the midbrain. The 

 whole structure is the Lamina commissuralis mesencephali. This decussa- 

 tion is exceedingly constant, and from Petrom3'zon to man it is always 

 present. In Fig. 68 it may be seen as a shaded inner layer of the roof. It 



