120 



ANATOMY OF THE CENTRAL NERVOUS SYSTEM. 



Since the optic tract enters the frontal end oi the midbrain, one 

 will meet it only in sections which are cut far forward. Such a section (as 

 shown in Fig. TO) encroaches upon the interbrain. 



The Tectum mesencephali is, in all lower vertebrates, brought into 

 intimate relations with the great nuclei of the interbrain (thalamen- 

 cephalon) through a large bundle: the Tradus tedo-thalamicus. The 

 bundle is so large that it will doubtless be found in mammals also. It is 

 lost between the layers of the roof. The cerebral connections probably exist 

 in reptiles; in birds and mammals, however, its existence is certain. 



Important in the investigation of the brain of lower vertebrates is the 

 large-celled roof-nudeus {Dadikern). The nucleus in question lies close on 



Fig. 71. — Sagittal section of a lizard-brain, showing the position of the 

 nuc. lat. mesencephali; also showing well the different bundles of the Opticus 

 and the course of the fibers from the deep medullary stratum into the fillet. 



either side of the median line, but does not occupy the whole length of the 

 roof. It is not present in mammals, but one meets there a group of quite 

 similar cells, which degeneration-experiments have shown to belong to the 

 fiber-system of the Trigeminus: Radix mesencephalica Nervi V It has 

 not been demonstrated, however, that the origin of the midbrain-root of the 

 Fifth nerve is a homologue of the DacKkern in question. 



The basis of the midbrain is distinguished by the numerous already 

 described decussations which lie in it, but the fact that several important 

 bundles pass into it from the interbrain, and finally by the presence of 

 several separate nuclei. 



