VENOUS SINUSES OF THE DURA MATER 151 



taries from the deeper veins, arising from and draining the capil- 

 lary sheet that immediately surrounds the brain tube. This 

 deeper system, however, continues to drain into the former at 

 certain restricted places. We can thus distinguish between veins 

 of the dura mater and the cerebral veins. It is the former that 

 are chiefly concerned in the formation of the venous sinuses and 

 with which we are chiefly interested in the present study. 



2. Human embryos I4 mm. long 



In figure 2 is shown an embryo in which the veins of the dura 

 mater are already separated to a considerable extent from the 

 cerebral veins. The veins of the head of this embryo (No. 940, 

 13.8 mm. long, Carnegie Collection) were distended with a natural 

 blood injection and at the same time the surrounding tissues were 

 quite transparent, so it was possible from a surface examination 

 to determine their arrangement with considerable detail. The 

 specimen was photographed and a print made, which was then 

 elaborated with the details that could be seen in the specimen 

 with the aid of a binocular microscope. Comparisons were also 

 made with serial sections of other embryos of about the same 

 age and in which the blood vessels had been injected with a 

 colored mass. In the collection No. 544 is a particularly good 

 series of that kind, showing about this same arrangement of the 

 drainage of the head. Mall ('05) has pictured about the same 

 stage in his figure 9. This stage is also pictured by Markow- 

 ski ('11) in his figure 1. In the main points all three figures cor- 

 respond rather closely. A large venous channel is formed in the 

 region lateral to the diencephalon and passes backward, median 

 to the trigeminal nerve and lateral to the otic capsule, through 

 the region of the future middle ear, where it bends sharply down- 

 ward in the neck region finally to empty into the duct of Cuvier. 

 All the veins of the cranial region drain into this main channel. 

 This constitutes the primary head vein, with which we are already 

 familiar. It was described by different writers as the 'anterior 

 cardinal vein' until Grosser ('07) showed that only the caudal 

 portion of it — the part that is found in the region of the somites 



