254 NERVOUS SYSTEM OF VERTEBRATES. 



the medial lemniscus is the anterior corpora quadrigemina and 

 certain nuclei in the thalamus. The separation of the lemniscus 

 system into two parts is the result of a differentiation in the mesen- 

 cephalon and diencephalon the main features of which can be 

 outlined, but the details of which offer most interesting subjects 

 for investigation. 



The mesencephalon in primitive vertebrates was a segment of 

 simple tubular form comprising two primary neuromeres similar 

 to the cerebellar neuromere or even to a segment of the spinal 

 cord. In typical fishes the roof of the mesencephalon is greatly 

 enlarged and is as highly specialized as the cerebellum. This is 

 due to the entrance into the mesencephalic roof of secondary 

 tracts (i) from the general and special cutaneous centers of the 

 medulla oblongata and spinal cord, and (2) from the retina. 

 These tracts (tractus bulbo-tectalis and tractus opticus) have 

 been described and their morphology discussed in previous 

 chapters. In lowly fishes the centers for these tracts are not 

 isolated but are more or less confused or intermingled in the brain 

 roof. Indeed the differentiation of special centers within an indiffer- 

 ent region is in progress in existing fishes. 



In typical fishes the midbrain roof is an arch whose bases rest 

 on the ventro-lateral walls of the brain and whose keystone is 

 formed by a thin lamina (roof plate of His) traversed by the dorsal 

 decussation. Enlargement of the centers in the roof is secured 

 by the widening of the arch and expansion of the ventricle. The 

 thickness of the walls varies much less in different fishes than the 

 extent of the roof. In most fishes there is formed in this way a dor- 

 sal expansion of the iter known as the optic ventricle or optocoele. 

 In Fig. 127 are shown outline sections through the tectum of a 

 cyclostome, a selachian, a bony fish, an amphibian and a mammal 

 to illustrate the changes of form which this region undergoes. The 

 first important fact is that the expansion of the optic ventricle takes 

 place by bulging the side walls of the arch without separating its 

 bases. From this it results that the ventro-lateral part of the 

 tectum remains beneath the side of the expanded optic ventricle 

 and is overhung by the bulging dorso-lateral portion of the tectum. 

 The portion of the original roof which now lies beneath the optic 



