176 GEORGE L. "STREETER 



ment. In the replacement channel there is the formation of a 

 new channel and the obliteration of an old one, as in migration. 

 It however differs frona migration in that it is not a gradual and 

 progressive change in position, but an abrupt and immediately 

 complete one. Furthermore, the new channel lacks the mor- 

 phological characteristics of the old one. This process is illus- 

 trated by the transverse sinus which replaces part of the primary 

 head vein, as will be presently described. 



Adjustment of the dural channels occur early in the region of 

 the middle ear. In the same way that the facial nerve is bent 

 out of its original course by the development of the membranous 

 labyrinth and the middle ear, so the dural veins are definitely 

 influenced by the same structures. Ow^ng to the growth of these 

 the course of the primary head vein, ventro-lateral to the otic cap- 

 sule, becomes an unfavorable one and this segment of it becomes 

 obliterated. To make the necessary adjustment two things hap- 

 pen (figs. 11, 12 and 13). First, an anastomosis is established 

 above the otic capsule through which the middle plexus drains 

 caudally into the posterior plexus. Secondly, the anterior plexus, 

 which originally drained into the primary head vein, fuses with 

 the middle plexus and drains caudally through this and through 

 the newly established channel, dorsal to the otic capsule. This 

 makes a complete trunk for the drainage of the head which is 

 everywhere dorsal to the primarj^ head vein as far as the jugular 

 foramen, where it is continuous with the internal jugular vein. 

 Of the primary head vein there is left, in addition to the cardinal 

 portion of it or internal jugular vein, only that part in the region of 

 the trigeminal nerve. This may now be called the 'cavernous 

 sinus.' Into it drain the veins from the orbit and it, in turn, 

 drains upward through the original trunk of the middle plexus, 

 which is now the 'superior petrosal sinus,' into the newly estab- 

 lished dorsal channel. By comparing with later stages (figs. 14- 

 17) it will be seen at once that this dorsal channel is the 'trans- 

 verse sinus' of which that part between the superior petrosal 

 sinus and the jugular foramen forms its 'sigmoid portion.' Thus 

 in the 21 mm. embryo the dural channels in the region of the 

 temporal bone have acquired essentially all their permanent con- 



