MIDBRAIN AND THALAMUS OF NECTURUS 281 



The tectum mesencephali 



In ordinary histological preparations it is evident that the 

 neurons are crowded in a thicker and more dense layer in the 

 stratum griseum of the tectum than in that of the cerebral 

 peduncle (figs. 9 to 14). The boundary between these regions 

 in the lateral wall is obscurely indicated by this difference in 

 structure, especially as seen in Golgi preparations (figs. 9 to 14, 

 32, 33). 



The stratum griseum of the tectum is obscurely three layered. 

 The deepest layer is the stratum ependymale, though clearly 

 the bodies of some of the neurons are found here also. An 

 ependymal element from the caudal part of the tectum is seen 

 in figure 34. These cells are usually clearly distinguished in 

 Golgi preparations from the neurons by their delicately plumose 

 appearance. 



The stratum ependymale at the rostral end of the tectum is 

 greatly thickened to form a peculiar massive structure below 

 the posterior commissure which has been commented on by 

 many authors and which is named by Dendy ('10) and Dendy 

 and Nicholls ('10) the subcommissural organ (figs. 5, 6, 7, o.sc). 

 The origin of Reissner's fiber from this organ is seen in figure 

 62 (f.R.), as found in a sagittal Weigert series. At the caudal 

 end of the tectum the roof of the recessus posterior mesencephali 

 immediately in front of the velum medullare anterius shows a 

 similar, though less pronounced, thickening of the ependyma, 

 associated with which is a dorsal medial collection of neurons 

 of the mesencephalic V nucleus. 



Most of the neurons of the tectum are found in the two outer 

 layers of the stratum griseum, which are incompletely separated 

 by a thin irregular band of neuropil (figs. 9 to 14). I have not 

 been able to demonstrate any constant difference between the 

 neurons of these layers, though further study may reveal it. 

 In this band of neuropil are numerous very fine unmyelinated 

 fibers extending tangentially between the tectum and the cere- 

 bral peduncle. The character of these fibers has not been de- 

 termined; probably they are mostly dendrites. At the outer 



