22 THE DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM 



This can be seen in figure 2, which shows a graphic reconstruction of a human 

 embryo 18 mm. long (No. 144, Carnegie Collection, CR length 18 mm. in 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 midbrain and forebrain is still drained by the primary 

 head-vein. From Markowski it differs in that in our specimen the single large 

 channel passing backward from the anterior and middle dural plexuses is not yet 

 established, but instead the region occupied by the anterior and middle plexuses 

 still shows an extensive anastomosing network not differing much from the pattern 

 we have already seen in figures 1, 24, and 25. The basis for this new channel 

 from the middle to the posterior plexus, dorsal to the otic capsule, already existed 

 in slightly younger stages (fig. 25) in the form of a venous plexus extending across 

 this region. We are not to conclude, however, that this plexus was placed there 

 for this particular purpose; it is only such a plexus as tends to form everywhere 

 throughout the dural .system. 



An interesting feature in connection with the establishment of the new channel 

 just described is the fact that the trunks that originally drained the middle dural 

 plexus into the primary head-vein nearly disappear, owing to the fact that the 

 blood that they heretofore carried — i. e., from the cerebellar region and the pos- 

 terior part of the midbrain — adopts the new channel that is formed dorsal to the 

 otic capsule and is thus drained into the posterior dural plexus. As a result of this 

 the original trunks that connected the middle plexus with the primary head-vein 

 become relatively small and partially break up into a small plexus. We shall see 

 later, however, that with the next change in the head-vein a trunk will open up here 

 again as an important channel. 



In taking up the question of terminology for figure 2, it is found that most of 

 the terms used in figures 1, 24, and 25 are still applicable. There are the three dural 

 plexuses draining into the primary head-vein and also the ophthalmic and maxillary 

 veins. The anterior dural plexus, however, can be seen to be reshaping itself so 

 as to come into a more free anastomosis with the middle dural plexus. The middle 

 dural plexus, by draining as it does over the otic capsule, presents the first stage in 

 the formation of the transverse sinus — that is, the sigmoid portion of it. The 

 posterior dural plexus shows less change in its form and connections than any other 

 group of the head-veins, and this is true also in the later stages. There are some 

 minor alterations in its pattern, but otherwise it simply extends to become the 

 occipital sinus of the adult. The primary head-vein can be subdivided into the 

 trigeminal portion that is to form the cavernous sinus and the otic portion which 

 passes lateral to the otic capsule accompanying the seventh nerve, and lastly, the 

 cervical portion or internal jugular vein, the boundary of which is indicated in 

 figure 2 by the label for. jug. The otic portion already shows a diminution in 

 volume as a result of the establishment of the new drainage-channel dorsal to 

 the otic capsule. Dorsal to the otic capsule there is sufficient free space for the 

 development of a vascular channel, whereas the region ventro-lateral to it becomes 



