174 GEORGE L. STREETER 



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are separated off in the more ventral regions and from there 

 spread upward over the head independently of the dural system. 

 We then have for the head three separate venous systems: (1) 

 the superficial layer belonging to the integument and soft parts ; 

 (2) the middle layer belonging to the dura and diploe; and (3) 

 the deep layer of cerebral veins belonging to the brain. It is 

 the middle layer, or dural system, that is exclusively concerned 

 in the formation of the dural sinuses, and whose changes in form 

 and position will now receive our attention. 



The changes in form of the dural veins that occur after the 

 establishment of the primary arrangement for the drainage of the 

 head (figs. 12-17) are largely due to the mechanical factors in- 

 volved in the changes of their environment, the two most con- 

 spicuous elements being the changes in the region of the carti- 

 laginous capsule of the labyrinth and the changes involved in the 

 growth and alteration in form of the brain. The changes thus 

 produced include the reduction of the plexuses into simple chan- 

 nels, and the total obliteration or change in position of the 

 channels themselves. Under this latter phenomenon we recog- 

 nize a passive migration, where there is a change in the position 

 of the vein wall itself, due to some flexion — or traction — force 

 acting upon it. We also recognize a spontaneous migration where 

 there is a change in position of the blood stream only, where by a 

 circumfluent anastomosis the blood stream develops a new chan- 

 nel in the adjacent loops of the plexus with a corresponding 

 dwindhng of the previously used channel. An anastomosing 

 plexus is essential in spontaneous migration but is not essential 

 in passive migration. Both of these types of migration are to be 

 distinguished from the formation of replacement channels which is 

 another way, deserving mention, in which the venous channels 

 are changed in position and direction in this process of adjust- 



Figs. 10-17 Simplified profile drawings of the dural veins showing the prin- 

 cipal stages in their development in human embryos from 4 mm. to birth. It is 

 of particular interest to notice their adaptation to the growth and changes in 

 form of the central nervous system. Figure 10, Embryo No. 588, 4 mm.; figure 

 11, Embryo No. 940, 14 mm.; figure 12, Embryo No. 144, 18 mm.; figure 13, Em- 

 bryo No. 460, 21 mm.; figure 14, Embryo No. 199, 35 mm.; figure 15, embryo No. 

 96, 50 mm. crown-rump length; figure 16, Embryo No. 234 a, 80 mm. crown-rump 

 length; figure 17, adult. 



