VENOUS SINUSES OF THE DURA MATER 167 



migrates backward in adjustment to the growth of the hemisphere 

 and thus comes to assume a more and more horizontal course. 

 This change in direction, together with growth in length and di- 

 ameter of the main channel at the expense of the formative mesh- 

 work, remains to be completed before the adult condition can be 

 considered as established. The variations found in the adult in 

 the region of the confluens sinuum can be readily understood as 

 variations in channel-selection through this tentorial meshwork. 

 In the region of the fore-brain a fold of dura is interposed be- 

 tween the two hemispheres and is compressed into a flattened 

 sheet which is to constitute the falx cerebri. This, and the vas- 

 cular meshwork belonging to it, is directly continuous with the 

 tentorium. Like the tentorium it passes through a prolonged ad- 

 justment period. In embryos 50 mm. long two of its permanent 

 channels, which are to belong to the dural sinus system, can be 

 readily recognized. These are the superior sagittal sinus and the 

 straight sinus. In figure 6 the superior sagittal sinus is quite 

 irregular in outline, which is possibly a result of shrinkage of the 

 specimen. Very likely in a normal state, as seen in profile, it 

 would pass evenly along the margin of the cerebrum. The de- 

 tails regarding the vessels belonging to the falx cerebri and the 

 drainage of the chorioidal masses will not be taken up in the 

 present paper, but certain features in the formation of the 

 superior sagittal sinus will now be referred to. 



SINUS SAGITTALIS SUPERIOR 



Under the description of embryos 21 mm. long, reference was 

 made to the formation of a 'plexus sagittalis' as a subdivision of 

 the anterior dural plexus. At that stage the plexus is clearly 

 differentiated from the remainder of the anterior dural plexus, as 

 can be seen in the dorsal view of an embryo of about that age 

 shown in figure 8. Earlier than this (in embryos about 14 mm. 

 long; figure 7) the plexus can be recognized, though here it is not 

 so clearly separated from the general plexus. In such embryos 

 it can be seen that the larger tributaries of the anterior and middle 

 dural plexuses stop short of the median line — with the exception of 

 anteriorly — where they merge into a longitudinal plexus that dips 



