214 THE BRAIN OF THE TIGER SALAMANDER 



larval stages, and they are expanded so as to form a noticeable ex- 

 ternal eminence, which disappears in the adult except in the mid- 

 plane. 



SENSORY ZONE 

 TECTUM OPTICUM: SUPERIOR COLLICULUS 



Nothing need be added here to the statements in chapters iv and 

 xvi and the description of the mesencephalic V nucleus in chapter x. 

 For details my paper of 1942 may be consulted. 



NUCLEUS POSTERIOR TECTi: INFERIOR COLLICULUS 



The small nucleus posterior tecti is an undifferentiated primordium 

 of the inferior colliculus, with perhaps some additions in higher ani- 

 mals from the posteroventral part of the optic tectum. 



The most characteristic cells of the posterior nucleus are small ele- 

 ments with thick bushy dendrites directed forward into the tectum 

 opticum. Slender axons arise from these dendrites and are directed 

 forward to the thalamus as an important component of the bra- 

 chium of the inferior colliculus ('42, p. 265 and figs. 51 and 79). 

 Other efferent fibers go out in large numbers in tractus tecto-thala- 

 micus et hypothalamicus cruciatus posterior (p. 297; '42, p. 221), 

 of which the strongest component decussates in the postoptic com- 

 missure and distributes to the tegmentum in tegmental fascicles (8) 

 and (6). Other shorter efferents reach all surrounding parts. The 

 chief connections of this region are shown diagrammatically in 

 figures 10-18 and 21. 



In Amblystoma there is no sharp boundary between the optic 

 tectum and the nucleus posterior. The distinguishing features of the 

 latter are the absence of optic terminals and the presence of two 

 special tracts — the primordial lateral lemniscus {tr.b.t.l.) and the 

 efferent tr. tecto-bulbaris posterior (tr.t.b.p.). The spinal lemniscus 

 and the general bulbar lemniscus terminate chiefly in the optic 

 tectum, though the spinal lemniscus has a larger proportion of ter- 

 minals in the nucleus posterior. The tecto-cerebellar tract arises 

 chiefly in the nucleus posterior. These connections suggest proprio- 

 ceptive functions (chap. xii). This suggestion is strengthened by the 

 presence of many cells of the mesencephalic V nucleus here and by 

 the undeveloped condition of the auditory apparatus. The dominance 

 of the cochlear connection is a later acquisition in land animals, with 

 resulting transformation of the nucleus posterior into the colliculus 

 inferior and differentiation of the medial geniculate body. This trans- 



