MIDBRAIN AND THALAMUS OF NECTURUS 225 



I have termed the eminentia subcerebellaris tegmenti. It lies 

 ventrally of the cerebellum and nucleus posterior tecti and is 

 directly continuous below with the general motor tegmentum 

 of the medulla oblongata. It is an important relay center 

 where many of the descending systems of the motor tegmen- 

 tum are interrupted by synapses. 



The diencephalon 



The boundary between the diencephalon and the mesen- 

 cephalon we have already discussed (p. 221), The rostral bound- 

 ary of the diencephalon may be defined, follo\\ing Johnston, 

 by a plane passing from the site of the embryonic velum trans- 

 versum to the optic chiasma. The exact location of this plane 

 in the adult brain is not easily determined. The extensive 

 unevaginated telencephalon medium of urodeles includes the 

 preoptic nucleus, anterior commissure, lamina terminalis, lamina 

 supraneuroporica (Johnston), paraphysis, and, according to 

 Johnston, the chiasma ridge. The remainder of the telencepha- 

 lon is represented in two lateral evaginations, the cerebral 

 hemispheres. I have discussed the morphological subdi\dsion 

 of the amphibian cerebral hemisphere and its relation to the 

 diencephalon in general in a previous contribution ('10), to which 

 the reader is referred for an explanation of some of the terms 

 here employed. 



The floor of the diencephalon (figs. 62, 63, 64) contains the 

 massive chiasma ridge (or a part of it), with the slender optic 

 chiasma and the very extensive system of postoptic commis- 

 sures. Farther caudad the wide infundibulum and hypophysis 

 extend far backward under the midbrain and rostral end of 

 the medulla oblongata. 



The roof of the diencephalon is membranous in front, the 

 postvelar arch of Minot (dorsal sac of others). Then follow 

 the commissura habenularum (superior commissure), recessus 

 pinealis, and pars intercalaris with its contained commissura 

 tecti diencephah (figs. 62, 63, 64). The epiphysis, as de- 

 scribed by Warren ('05), evaginates from the recessus pinealis 

 (epiphyseal arch) in early embryonic stages and its cavity early 



