154 GEORGE L, STREETER 



the dura mater and the tissues forming the membranous cranium. 

 There is therefore an advantage in adopting a terminology some- 

 thing hke that of Markowski ('11). In doing so, a distinction 

 between the lateral and mesial portions will not be made; on the 

 other hand, the three groups as given by Mall will be retained. 

 We will thus speak of the 'anterior,' 'middle' and 'posterior dural . 

 plexuses,' or more formally, 'plexus durae matris anterior,' 'plexus 

 durae matris medialis' and 'plexus durae matris posterior,' as they 

 are indicated in figure 2. In this figure only the larger channels 

 of the plexus are shown and it is to be understood that an 

 intervening smaller venous mesh connects them more or less 

 completely. 



In the ventral portion the dural plexuses are more or less com- 

 pletely separated from the deeper-lying plexus belonging to the 

 wall of the neural tube, from which are developed the cerebral 

 veins. In tracing the plexus dorsalward toward the median line 

 we find an increasing frequency of communication between the 

 two, and near the median line thej^ are so intimately connected 

 that it is impossible to distinguish between them; in other words, 

 in this region the cleavage between these two layers is not yet 

 established. It is interesting to note that there seem to be fav- 

 orable places for the larger channels to cross dorsally over the 

 median line; one of these is the caudal end of the roof of the 

 fourth ventricle, another at the junction of the mid-brain and 

 hind-brain, and a third over the diencephalon along the caudal 

 margin of the cerebral hemisphere. Where these vessels cross the 

 median line they are usually bilaterally asymmetrical but may 

 anastomose with the plexus of the opposite side. Other than at 

 these three regions, the larger channels as a rule do not reach the 

 median line. 



In addition to the three dural plexuses mentioned above, the 

 primary head vein receives a large ventral tributary that lies 

 median to the maxillarj^ division of the trigeminal nerve and also 

 other small veins in that region. These veins drain the tissues 

 that in part are to form the orbit and represent the future 

 ophthalmic veins. 



