MIDBRAIN AND THALAMUS OF NECTURUS 285 



view receives further support from the fact that lemniscus 

 fibers are distributed throughout the optic tectum (p. 246) 

 and from the arrangement of the tectal neurons described abo^'e. 



Bellonci ('88, p. 31) estabhshed a true principle in general 

 when he affirmed that the differentiation of the roof of the mid- 

 brain into superior and inferior colliculi arose from the fact that 

 this region receives its chief afferent fibers from two sources — the 

 thalamus and the medulla oblongata. The optic fibers coming 

 in from the thalamus led to the differentiation of the colliculus 

 superior away from the colliculus inferior. (The principle 

 which he enunciated stands in spite of the fact that in the frog 

 he incorrectly identified the nucleus magnus of Reissner ,('64), 

 or ganglion isthmi of Edinger, with the corpus posterior, or in- 

 ferior colliculus ) But the separation of these functionally 

 distinct regions from the primitive tectum mesencephali has 

 advanced only a very short way in Necturus. 



From the preceding account it is evident that the tectum of 

 Necturus lacks the well defined lamination which is so char- 

 acteristic a feature of most other vertebrates. Since all of the 

 usual functional systems except the optic are well represented 

 here, it follows that the customary type of lamination is corre- 

 lated with the optic apparatus. 



The fibers of the stratum album have the following arrange- 

 ment. At the rostral end of the midbrain the fibers of the 

 commissura posterior pass downward close to the central gray 

 for their entire length (figs. 7, 8, com.post.). Farther caudad 

 the commissura tecti occupies the same position in the mid- 

 dorsal plane, but farther ventrally these fibers are displaced 

 laterally (figs. 9 to 14). Medially of the latter fibers are those 

 of the mesencephalic V root (figs. 11 to 14, r.V.mes) and farther 

 ventrally at the boundary between the gray and white layers is 

 the tractus tecto-peduncularis profundus (figs. 9 to 14, 

 tr.t.ped.p.). The spino-tectal and spino-thalamic tracts lie 

 ventrally of the mesencephaHc V root and immediately laterally 

 of the tractus tecto-peduncularis profundus (figs. 9 to 14). 

 Farther laterally is the acoustico-lateral lemniscus (Im.) and 

 the associated neuropil of the colliculus inferior, and farther 



