234 D. DAVIDSON BLACK 



ventrad and cephalad for a considerable distance and may be 

 traced into the mesial thalamic nucleus (X, figs. 42 to 47). 



Just cephalad to the posterior commissure the pineal recess 

 may be distinguished, and in front of this a few fine medullated 

 fibers cross in the thin roof and constitute the habenular com- 

 missure (C.hh., fig. 44). 



Immediately cephalad to the habenular commissure, the tae- 

 nia thalami (T.th.III, fig. 45) is to be seen and two small areas 

 of choroid plexus {Pl.Ch.III, figs. 44 to 45) project into the 

 small ventricular cavity (Vent. Ill, figs. 44 to 46). Separating 

 these two plexuses is a slightly thickened area in the roof repre- 

 senting the attachment of the anterior limb (dorsal) of the pineal 

 stalk. 



In front of this the thin roof of the ventricular cavity has 

 been torn away for a short distance and its relations cannot be 

 accurately followed. However, the roof appears again in the 

 sections somewhat more cephalad and is now seen to be thickened 

 and lacking in choroidal invaginations. Traced forward from 

 this region, the ventricle is seen to end blindly in the dorsal part 

 of the thalamic mass. 



It was noted in the gross description that the thalamic mass 

 projected into the cavity of the primary forebrain vesicle and 

 was covered with ependyma. From a study of the sections in 

 this region it now appears that the line of attachment of this 

 ependyma, or rather the line of its reflexion from the surface of 

 the thalamus, approximately coincides with the line of demarca- 

 tion between the dorsal nuclear mass and the ventral fibrillar 

 area. Thus the greater part of the thalamic mass is extraven- 

 tricular and is covered by a thick layer of pia and fibrous tissue. 

 The taenia thalami (T.th.III), noted just in front of the pineal 

 region, is not continuous with the taenia (T.th. I, figs. 45 to 

 47) over the anterior part of the thalamus to which the thin roof 

 of the forebrain vesicle is attached. These relations are best 

 seen in the diagram of a mesial sagittal section of the brain 

 (fig. 11). 



It is thus seen that there is a discontinuity in the ventricular 

 system of the brain and that this interruption occurs in the region 



