MIDBRAIN AND THALAMUS OF NECTURUS 283 



tions coming in either by the lemniscus from the bulbar and 

 spinal centers or by the pedunculus cerebri from the higher re- 

 gions of the thalamus and -cerebral hemispheres. 



As we have seen, the dorso-medial part of the tectum only 

 is reached by the optic tracts and this part may, therefore, 

 be termed tectum opticum and considered homologous with 

 the mammalian colliculus superior. Most of the optic fibers 

 end by free arborizations in the rostral end of this region (figs. 

 36, 37) ; whether any of them reach as far as the caudal end 

 of the tectum in Necturus is not clear — apparently not. With- 

 in the tectufn opticum there is an area of neuropil in the stratum 

 album where the most numerous optic synapses are found. 

 This area is rather sharply separate from the larger and denser 

 area of neuropil l^ing farther ventrally which receives the lem- 

 niscus terminals only and which, accordingly, corresponds with 

 the colliculus inferior of mammals (figs. 9 to 14, 36, 37). 



While the terminals of the optic tract seem to be confined to 

 the dorsal part of the tectum and chiefly to its rostral end, 

 the terminals of the acoustico-lateral and spinal lemniscus 

 tracts, on the other hand, reach all parts of the tectum and are 

 not limited to the ventral and caudal parts (the colliculus in- 

 ferior). Some fibers of both of these tracts are clearly seen to 

 spread throughout the optic region also, at a deeper level than 

 the optic termini (fig. 24). 



No definite regions of the tectum can be assigned as the 

 terminal nuclei of the acoustico-lateral and spinal lemnisci, 

 though probably the tectal distribution of these tracts is not 

 exactly coextensive. The tractus bulbo-tectalis (trigeminal 

 lemniscus?), on the other hand, is definitely distributed to the 

 nucleus posterior tecti (figs. 52, 53) and the ascending second- 

 ary visceral tract is even more sharply limited to the superior 

 secondary visceral nucleus (figs. 51, 52). 



The nucleus posterior tecti has just been mentioned as a 

 special area at the caudal end of the tectum which contains in 

 both Necturus and Amblystoma a specially dense group of 

 neurons with thick contorted dendrites. The mesencephalic 

 ventricle is here dilated to form the recessus posterior mes- 



