256 NERVOUS SYSTEM OF VERTEBRATES. 



Although differentiation between these centers is not complete 

 in fishes it may be said in general that the optic tract predominates 

 in the tectum opticum, while the collicular region is especially 

 related to the tractus bulbo-tectalis. That the specialization 

 of these centers is not complete is clear from the fact that hi blind 

 fishes and those from which the eye has been experimentally 

 removed, the tectum opticum is not atrophied but only certain 

 layers of its cells disappear (Ramsey). Evidently the secondary 

 cutaneous tracts enter the tectum opticum as well as the colliculus, 

 but the optic tract is more nearly confined to the tectum opticum. 

 The descending fibers from both the tectum opticum and the 

 colliculus run in the same tracts so that they are all grouped under 

 the name of tractus tecto-bulbaris. 



In the amphibia a marked change hi the form of the mesen- 

 cephalon is seen (Fig. 127 E). The form of the transverse section 

 is more nearly that of a simple tube. The wall is relatively thick 

 and there is only a slight indication of a special optic ventricle. 

 In selachians, ganoids and bony fishes there is an increasing expan- 

 sion of the tectum opticum and thickening of the massive colliculus. 

 The size of the tectum opticum is directly affected by the size 

 and importance of the eyes, while the great volume of the col- 

 liculus is due to the great development of the acustico-lateral 

 system. In amphibia the reduction of the roof is due to a 

 decrease in both optic and cutaneous tracts which end in it. 

 In most amphibia the eyes are less important than in most 

 fishes and the number of optic fibers which enter the tectum is 

 much farther reduced by the greater development of the optic 

 centers in the thalamus. On the other hand, the cutaneous 

 tracts are greatly reduced by the complete disappearance of the 

 lateral line organs in the adult. The amphibia are believed to 

 have descended from fish-like ancestors whose affinities are with 

 the lower orders of existing fishes. The amphibian brain shows 

 resemblances to that of both selachians and cyclostomes. The 

 reduction of the midbrain roof has left it in a primitive form. 

 There is no clear distinction between tectum opticum and collic- 

 ulus, but the greater thickness of the wall in the caudal part of 

 the roof indicates the beginning of formation of the posterior 



