OF THE BRAIN OF THE HUMAN EMBRYO. 



21 



growth that rapidly follows were it not that a new provision is established to take 

 care of this, as we shall now see. 



ADJUSTMENTS OF BLOOD-CHANNELS DUE TO GROWTH AND CHANGE IN 

 FORM OF THE BRAIN AND EAR 



Before the cleavage between the dural and cerebral vascular systems is com- 

 pleted, certain alterations in their pattern, associated with the changes in their 

 environrnent, rapidly follow one another. If one examines a number of series of 

 about the same age as that just described, and a little older, it is seen that the 

 primary head-vein maintains the same general course and relations, but the pat- 

 tern of the dural plexuses is constantly changing, which in the end results in 

 a change in the primary head-vein itself. In embryos about 18 mm. long an 



PLEXUS TENTORII 



SIN. TRANSVERSUS 



PLEXUS 

 SAGITTALIS 



V OPTHAL 



N. TRIGEMINUS 



SIN. CAVERNOSUS 



Figure 3. 



rofile reconstruction of the dural venous system in a human embryo 21 mm. long (Carnegie Collec- 

 tion, No. 460). The sinus transversus is now clearly established and there is left of the primary 

 head-vein only that portion which persists as the sinus cavernosus. There is a remnant of it-s otic 

 portion (marked x) that originallj- connected the cavernous region with the internal jugular vein. 

 A more complete reconstruction of the blood-vessels of the right side of this same embryo is shown 

 on plate 4. Enlarged about 11 diameters. 



important change occurs by which the blood from the middle dural plexus, which 

 heretofore had drained into the primary head-vein, in the interval between the 

 trigeminal and the acustico-facial ganglia, now drains caudalward into the posterior 

 dural plexus through anastomosing loops that exist between these two plexuses, 

 passing dorsalward to the otic capsule and just lateral to the endolymphatic sac. 



