DIENCEPHALON 235 



tecto-hypothalamicus anterior (tr.t.hy.a. of the figures) comprise the 

 mixed system which in my previous papers was called "tr. tecto- 

 thalamicus et hypothalamicus cruciatus anterior" (p. 296). The 

 thickest myelinated fibers of this complex arise in the tectum and are 

 clearly followed through the postoptic commissure into the hypo- 

 thalamus. 



The dominant afferent connections of the pretectal nucleus appar- 

 ently are optic. The efferent connections to the hypothalamus and 

 peduncle provide pathways for optic control of the intrinsic muscles 

 of the eyeball, as in mammals, and also for possible conditioning or 

 other influence upon the primary motor paths of the brain stem. 



The pineal body appears in the premotile embryo as a low evagina- 

 tion from the roof of the diencephalon ('38, fig. 2). In early swimmers 

 it is a hollow vesicle with a lumen, communicating with the third 

 ventricle by a patent recessus pinealis ('38, figs. 3, 4, 18). Within 

 three days after the early swimming stage this communication is 

 closed ('38a, p. 18). The adult epiphysis is a small, flattened, simply 

 lobulated epithelial vesicle, entirely detached from the brain except 

 for a few fibers of the parietal nerve. No nervous elements other than 

 these fibers have been found in it, and their connections are still 

 unknown. 



In Necturus there are more of these fibers, and their courses are 

 more easily followed ('17, p. 236). These myelinated fibers are spread 

 between the cells of the epithelial wall of the pineal vesicle, from 

 which they pass into the brain in several small fascicles associated 

 with the habenular commissure and posteriorly of it. They do not 

 cross the mid-plane and they appear to have no functional connection 

 with the epithalamus but to pass through it and join the f. retro- 

 flexus. They can be followed ventralward nearly to the cerebral 

 peduncle, where they mingle with other myelinated fibers. The parie- 

 tal nerve of Amblystoma has a smaller number of myelinated fibers 

 than in Necturus (not more than 10), but their arrangement is similar 

 ('42, p. 205). These certainly are not aberrant fibers of either the 

 habenular commissure or the com. tecti, but their central connections 

 and functions are uncertain. They resemble those associated with the 

 IV nerve, which are distributed peripherally to the meninges and 

 chorioid plexus (p. 181). It is not improbable that all these fibers 

 have sensory functions, but for this there is no evidence. All chorioid 

 plexuses are abundantly supplied with unmyelinated nerve fibers, 

 probably of vasomotor function, but in the search for the sources of 



