MIDBRAIN AND THALAMUS OF NECTURUS 229 



functionally determined, as will appear from the following con- 

 sideration of the related fiber tracts. 



The diencephalon of Necturus includes the following parts, 

 whose boundaries are clearly defined by the ventricular sulci 

 (figs. 63, 64): (1) the epithalamus, including the epiphysis, 

 the habenula and dorsal sac in front of the recessus pinealis and 

 attachment of the epiphyseal stalk, and the pars intercalaris 

 lying between the epiphysis and the posterior commissure; 

 (2) the pars dorsalis thalami, bounded dorsally by the sulcus 

 subhabenularis or sulcus dorsalis thalami or both and ventrally 

 by the sulcus medius thalami; (3) the pars ventralis thalami, 

 bounded dorsally by the sulcus medius thalami and ventrally 

 by the sulcus ventralis thalami; (4) the hypothalamus, bounded 

 above by the pars ventralis thalami (see further on p. 231). 

 The hypothalamus is continued directly forward across the 

 chiasma ridge into the enormous preoptic nucleus, which is 

 regarded as belonging in the telencephalon. 



The details of the pattern of the ventricular sculpturing de- 

 scribed above are somewhat variable in different specimens 

 of Necturus and still more so when different species of urodeles 

 are compared; but in a general way the pattern is characteristic 

 of all Amphibia. In these animals the habenula lies far for- 

 ward and it is large and somewhat lobulated. The sulcus dor- 

 salis may be a single groove separating the epithalamus from 

 the pars dorsalis thalami, or, as in Necturus and Amblystoma, 

 it may be separated into two parts, (1) a sulcus subhabenularis 

 follo'^ing the ventral and caudal borders of the habenula and 

 behind the latter curving dorsalward to enter the recessus pine- 

 alis, and (2) a partially or completely detached sulcus dorsalis 

 extending backward from the recessus pinealis under the pars 

 intercalaris diencephali. The details of the arrangement of 

 these sulci are variable and probably of no great morphological 

 significance, much depending upon the relative size and posi- 

 tion of the habenular and posthabenular portions of the epi- 

 thalamus. Kupffer many years ago ('93, p. 61) commented upon 

 the great extent of the posthabenular region (pars intercalaris, 

 or Schalthirn) in x4.mphibia as compared with other vertebrates 



