THE MIDBRAIN. 



113 



relatiye size only changes, and one who is acquainted solely with the rela- 

 tiyely small corpora quadrigemina of man is surprised when he sees the 

 immense optic lohes of a fish or a bird. But the minute structure is always 

 the same. In the dorsal layers of the hemisphere — partially divided dorsally 

 by a sagittal fissure — ^the optic nerve always ends. From the ventral layers 

 always arises a system of sensory fibers: the deep marrow, which contains, 

 among others, the already described Tractus tecto-spinales et tecto-bulbares. 



This is very beautifully seen in the sagittal section of the brain of an 

 amphibian larva, because here scarcely any fibers in the midbrain except 

 these two tracts are medullated. 



The roof of the midbrain is so large in fishes, and birds especially. 





Fig. 64. — Sagittal section of the brain of Axolotl, the amblystoma- 

 laiva of Siredon. 



because it produces such an unusually large opticus. In amphibians and 

 reptiles it is also relatively larger than in selachians and mammals. The 

 ventricle of the midbrain in the first-named animal is correspondingly large 

 (see Fig. 68), while it is reduced to a crevice — the aquseductus Sylvii — in 

 selachians and mammals. The extension of the roof in birds and in the 

 teleosts has also led lateral pendulous projection over the base of the mid- 

 brain (see Fig. 65, D). One, therefore, sees the roof-formation inclosing the 

 base externally. If one lay the brain of a bird or of a fish with the base 

 up, he will see, on either side, the optic, arising from great white promi- 

 nences, ■ which, in spite of the fact that they embrace the base, are, on 

 inspection, evidently nothing else than the strongly developed midbrain- 

 foof. 



