160 GEORGE L. STREETER 



has little connection with the cavernous sinus and morphologically 

 represents a metencephalic vein. Regarding the eventual fate of 

 the vena prootica he has apparently made no observations, though 

 he pictures it as a large channel in an embryo 46.5 mm. long. 

 From the specimens I have examined I cannot confirm Markow- 

 ski's description of the superior petrosal sinus and I feel convinced 

 that his vena prootica and the superior petrosal sinus are one and 

 the same thing, and that which he regards as the superior petrosal 

 sinus is, instead, one of its tributaries. Mall ('05, p. 17) also 

 described the superior petrosal sinus as the adult form of the 

 'vena cerebralis media,' which, it will be remembered, is the same 

 as the trunk of our middle dural plexus. 



With the alterations in the primary head vein, the anterior, 

 middle and posterior dural plexuses are drained by means of the 

 new dorsal channel, which empties through the jugular foramen 

 into the internal jugular vein. This channel can at once be recog- 

 nized as the transverse sinus and the sigmoid portion of it pre- 

 sents relations that are much the same as is found in the adult. 

 The three dural plexuses are still of the embryonic type. The 

 posterior plexus is practically the same as was seen in 18 mm. 

 embryos. Only its coarser meshes are shown in figure 4. A finer 

 plexus extends from this toward the median line and the region of 

 the tentorium. 



The whole dural area lying between the cerebral hemispheres 

 and the margin of the cerebellum constitutes the tentorium 

 cerebelli. It is very broad dorsally and is more constricted 

 ventrally, thus in profile it is wedge-shaped. In the loose tissue 

 composing it are found the meshes of the dural plexus. As this 

 region becomes more compressed, consequent upon the growth of 

 the cerebrum and cerebellum, there is a continual adjustment of 

 the contained venous channels and repeated alterations in the pat- 

 tern of the meshes. In general we find the larger channels radiat- 

 ing upward toward the mid-brain region, and as we approach the 

 median line the plexus becomes finer and there is an intimate an- 

 astomosis with the subjacent plexus belonging to the brain wall. 



On comparing embryos 21 mm. long with those 18 mm. long 

 there are seen two characteristic changes that occur in the pattern 



