ii6 



NERVOUS SYSTEM OF VERTEBRATES. 



ganoids and amphibians a bundle of very coarse fibers enters the- 

 brain with the sensory trigeminal root and ends in the tectum 

 mesencephali. This may be called the mesencephalic root of 

 the trigeminus and corresponds in position to the bundle which 

 in man is usually described as a motor root. Thus in lower verte- 

 brates the cutaneous fibers are widely distributed through all 

 that most dorsal column of gray matter which connects the dorsal 

 horns of the cord with the cerebellum and the dorsal part of the 

 mesencephalon. Indeed, in cyclostomes there is no marked 

 distinction in structure between the different parts of this column. 



Arcuate fibers 



Fig. 58. — A, transverse section of the tuberculum acusticum of Lampetra; B, a 

 sagittal section of the cerebellum of the same animal, ac, acusticum. 



The structure and secondar}' connections of these centers in 

 cyclostomes may now be described as showing the most primitive 

 known condition of the general cutaneous centers in the brain 

 of vertebrates. The nucleus funiculi, although simple, has 

 essentially the structure above described. The tuberculum 

 acusticum differs from it in being somewhat more voluminous 

 and in possessing a larger number of cells. The neurites of the 



