36 DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM 



While the three head-plexuses are spreading ui)ward (figs. 14 and 15), the out- 

 lines of the dura mater and the arachnoid spaces make their apjiearance, and first of 

 all in the ventral parts. This results in a general separation or cleavage of the 

 more superficial i:)rimary head-vein and its three tributary ple.xuses from the sub- 

 jacent vessels that arise from and drain the capillary sheet directly investing the 

 brain-tube. This deeper system, however, continues to drain into the former at 

 certain places, notably in the more dorsal parts. The primary head- vein and its 

 three tributary plexuses thus become established as a true dural system as dis- 

 tinguished from the deeper "cerebral veins" belonging to the arachnoid-pial 

 membrane. The di])loic veins are a later sul^idivision of the dural system. The 

 superficial vessels of the head belonging to the integument and soft parts are sep- 

 arated ofT in the more ventral regions and from there sjjread ujiward over the head 

 independently of the dural system. We then have for the head three separate 

 systems: (1) the superficial layer belonging to the integument and soft parts; 

 (2) the middle layer belonging to the dura and dijiloe; (3) the deeji layer of cerebral 

 ve.ssels belonging to the brain. It is the middle layer, or dviral system, that is 

 exclusivelj' concerned in the formation of the dural sinuses and whose changes in 

 form and position we are now following. 



In the region of the cartilaginous capsule of the labyrinth adaptive changes 

 in the dural channels occur early (figs. 14, 15, and 16). Owing to the marked 

 elaboration of these structures in this region, the course of the primary head-vein, 

 ventro-lateral to the otic capsule, becomes an vmfavorable one. If it persisted it 

 would be tortuous and remote from the area drained; instead, this part of it becomes 

 obliterated, and during this obliterating process an adjustment is made in two ways 

 (figs. 14, 15, and 16): first, a channel is established in the venous plexus above 

 the otic capsule, and through this the middle dural plexus thereafter drains caudally 

 into the loops of the posterior dural plexus; second, the anterior dural plexus, 

 which originally drained into the primary head-vein, completely reverses its direc- 

 tion of flow and drains through anastomosing loops into the middle dural plexus and 

 through the newly established channel dorsal to the otic capsule. 



In this way a complete trunk for the drainage of the head becomes established 

 which is everywhere dorsal to the primary head-vein as far as the jugular foramen, 

 where it is continuous with the internal jugular vein. Of the i)rimary 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 spoken of as the 

 "cavernous sinus." Into it drain a vein from the base of the brain and the veins 

 from the orbital and maxillary regions; whereas it, in turn, drains ui)ward through 

 the original trunk of the middle plexus, which is now the superior petrosal sinus, 

 into the newly established dorsal channel. By comparing with later stages (figs. 

 17 to 21) it will be seen at once that this dorsal channel is the transverse 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 chaimels in the region 

 of the temjioral V)one have acciuired essentially all tlieir permanent connections, with 

 the exception of the inferior petrosal sinus, which appears a little later (fig. 19). 



