284 C. JUDSON HERRICK 



encephali, a relation which is much more obvious in young 

 larvae of urodeles. The nucleus posterior tecti is the cUfferen- 

 tiated wall of this ventricular recess. It contains dorsally a 

 special group of neurons of the mesencephalic V nucleus, and 

 more laterally the terminal nucleus of the tractus bulbo-tectalis. 

 From the latter area an important component of the tractus 

 tecto-thalamicus et hypothalamicus cruciatus arises (p. 257), 

 and also the tractus tecto-peduncularis posterior (p. 254). 



The superior secondary \dsceral nucleus has been described 

 on page 248, where it is shdwn that it receives fibers from the 

 ascending secondary visceral tract derived from the nucleus 

 of the fasciculus solitarius. This small nucleus appears to 

 correspond with the 'Rindenknoten' of Mayser ('82) in teleosts; 

 but the commissural fibers which connect these nuclei through 

 the decussatio veli in teleosts are not apparent in Necturus. 

 The tertiary visceral tract leaving this nucleus, which I have 

 demonstrated in teleosts ('05), is but feebly developed in Nec- 

 turus and most of the fibers of the secondary tract appear to 

 pass forward beyond the level of the secondary nucleus in com- 

 pany with the less numerous tertiary fibers. Their rostral 

 terminus has not been clearly demonstrated. 



It is quite probable that the differentiation of the tectum 

 opticum from the remainder of the tectum mesencephali is 

 incidental to the partial degeneration of the optic apparatus in 

 Necturus, rather than an expression of primitive relations; for 

 Johnston ('05 a, p. 237) has commented upon the fact that such 

 a differentiation is not evident in the generalized fishes. It 

 seems more probable that the tectum mesencephali was primi- 

 tively a correlation center for impulses received from the skin, 

 the muscles, the ear, the lateral line organs, and the eye, and 

 that these special systems obtained separate localized centers 

 later in the evolutionary history, the optic system developing 

 the colliculus superior and the acoustic system the colliculus 

 inferior, the muscle sense of the head retaining its priinitive 

 relations with the mesencephalic V nucleus, and the cutaneous 

 and proprioceptive systems in general for the most part being 

 transferred to the correlation centers of the thalamus. This 



