VENOUS SINUSES OF THE DURA MATER 155 



3. Human embryos 18 mm. long 



On examining different series of about the 14 mm. period of 

 development and a little older one can see that the primary head 

 vein maintains the same course and relations, but the pattern of 

 the dural plexuses is constantly changing. In embryos about 

 18 mm. long an important change occurs by which the blood from 

 the region of the middle dural plexus drains caudalward into the 

 posterior dural plexus through an anastomosing channel that be- 

 comes established between these two plexuses, passing dorsal to 

 the otic capsule and just lateral to the endolymphatic sac. . This 

 can be seen in figure 3, which shows a graphic reconstruction of 

 a human embryo, 18 mm. long (No. 144, Carnegie Collection, 

 crown-rump length 18 mm. formalin; 14 mm. on slide). This is 

 the same embryo shown in Mall's figure 11 and is about the same 

 age as the embryo pictured in figure 2 of Markowski. In some 

 respects the reconstruction referred to differs from both of these. 

 From Mall it differs in that the greater part of the mid- and fore- 

 brain is still drained by the primary head vein. From Markow- 

 ski it differs-in that there is not yet a single large channel passing 

 backward from the anterior and middle dural plexuses, but in- 

 stead this region still shows an extensive anastomosing network not 

 differing much from the pattern we have already seen in figure 2. 



An interesting feature in connection with the dural plexuses 

 presents itself in that the trunk that originally drained the 

 middle dural plexus into the primary head vein nearly disappears, 

 owing to the fact that the blood that it heretofore carried (i.e., 

 from the cerebellar region and the posterior part of the mid-brain) 

 adopts the new channel that is established dorsal to the otic cap- 

 sule and is thus drained into the posterior dural plexus. As a 

 result of this the original trunk that connected the middle plexus 

 with the primary head vein becomes relatively small and partially 

 breaks up into a small plexus. We shall see later, however, that 

 with the next change in the head vein this trunk will open up 

 again as an important channel. 



In taking up the question of terminology for figure 3, it is 

 found that most of the terms used in figure 2 are still applicable. 



