286 C. JUDSON HERRICK 



forward in this level the tractus tecto-thalamicus rectus (figs. 

 5 to 8). Scattered through this level ventrally run also the 

 fibers of the tractus tecto-bulbaris (figs. 11 to 14, tr.t.h.). At 

 the extreme caudal end of the tectum the most superficial fibers 

 are those of the tractus bulbo-tectalis, together with the asso- 

 ciated neuropil of the nucleus posterior tecti and the secondary 

 visceral tract and nucleus (figs. 14, 50, 51, 52). The fibers of 

 the tractus tecto-thalamicus et hypothalamicus cruciatus run 

 slightly external to those of the acoustico-lateral lemniscus 

 and the uncrossed tractus tecto-thalamicus, but they are not 

 strictly superficial. In the rostral part of the tectum the fibers 

 of the optic tract form a thin layer close to the pial surface, 

 immediately internal to which is the neuropil of the optic tectum. 



These relations may give some clues to the sequence of evolu- 

 tion of the tectal systems, for it is a general rule (wdth, however, 

 many exceptions) that more recently developed fiber systems 

 are added superficially to the more primitive systems. 



In summary, the tectal fiber tracts are as follows. The 

 • afferent systems include the following tracts : radix mesencephal- 

 ica trigemini, tractus opticus, tractus spino-tectalis, tractus 

 bulbo-tectalis, lemniscus acustico-laterahs, tractus visceralis 

 ascendens. The efferent systems include: tractus tecto-bul- 

 baris, tractus tecto-peduncularis, tractus tecto-habenularis, 

 tractus tecto-thalamicus rectus, tractus tecto-thalamicus et 

 hypothalamicus cruciatus. The posterior commissure and com- 

 missura tecti diencephali also transmit nervous impulses from 

 the tectum to the pedunculus cerebri. 



The pars dorsalis thalami 



This is the great receptive center of the diencephalon for 

 somatic afferent impulses, which enter it by the lemniscus and 

 tecto-thalamic systems and by the optic tracts (which might 

 well be called the optic lemniscus). The lemniscus systems 

 which ascend from the spinal cord and medulla oblongata in 

 part traverse the midbrain to connect directly with the pars 

 dorsahs thalami; but the larger part of these fibers are inter- 



