VENOUS SINUSES OF THE DURA MATER 173 



Simplified sketches of the successive stages have been as- 

 sembled on one page, as figures 10 to 17, and it is hoped that this 

 will facilitate the identification of the steps in this process. It 

 hardly needs to be pointed out that these steps overlap one an- 

 other, and also that embryos of apparently the same age exhibit 

 a considerable variation in the pattern of their venous plexuses. 



The primary arrangement for the drainage of the capillaries 

 of the head (figs. 10-11) consists of a 'primary head vein', which 

 starts in the region of the mid-brain and runs caudal ward along- 

 side of the brain tube and terminates at the duct of Cuvier. The 

 primary head vein is composite in origin. That portion of it oral 

 to the vagus nerve is an intrinsic vein of the head; the remaining 

 caudal portion is in reality a neck vein and constitutes the ante- 

 rior cardinal vein — eventually the internal jugular vein. To- 

 gether these portions form a continuous channel, the primary 

 head vein, into which the blood from the capillary sheet that im- 

 mediately invests the brain tube is drained by means of anas- 

 tomosing venous loops. These loops gradually become arranged 

 more or less in the form of three plexuses, the 'anterior dural 

 plexus,', 'middle dural plexus,' and 'posterior dural plexus.' 

 Other small tributaries empty into the primary head vein which 

 drain the structures ventral and lateral to the brain tube, such 

 as the nerve ganglion masses. The largest of these come from the 

 eye region and these eventually form the ophthalmic vein. 



While the three head plexuses are forming (figs 11-12) the 

 outlines of the dura mater and the arachnoid spaces make their 

 appearance, and first of all in the ventral parts. This results in 

 a general separation or cleavage of the more superficial primary 

 head vein and its three tributary-plexuses from the subjacent 

 vessels that arise from and drain the capillary sheet directly in- 

 vesting the brain tube. This deeper system, however, continues 

 to drain into the former at certain places, notably in the more 

 dorsal parts. The primary head vein and its three tributary 

 plexuses thus become established as a true dural system as dis- 

 tinguished from the deeper 'cerebral veins' belonging to the 

 arachnoid-pial membrane. The diploic veins are a later sub- 

 division of the dural system. The superficial veins of the head 



