VENOUS SINUSES OF THE DURA MATER 177 



nections, with the exception of the inferior petrosal sinus which 

 appears a httle later (fig. 15). Otherwise, there remains to com- 

 plete the adult condition only a certain amount of passive mi- 

 gration in accommodation to the changes in the adjacent parts. 



The adjustment in the dural channels rendered necessary by 

 the protracted growth of the hemispheres extend much later in 

 fetal life. A large part of this adjustment is accomplished by 

 spontaneous migration of the principal channels and for this a 

 venous plexus is essential. We thus find a continuous persistence 

 in the neighborhood of the advancing occipital pole of the hemi- 

 spheres of the transitory anterior dural plexus from which are 

 evolved all the veins of the falx cerebri and of the tentorium 

 cerebelli. 



An anterior subdivision of the plexus extends forward in the 

 median hne as the 'plexus sagittalis' which is interposed as a ver- 

 tical curtain between the hemispheres. Among its dorsal meshes 

 is developed an asymmetrical longitudinal channel which we know 

 as the 'superior sagittal sinus.' In its early stages this channel 

 is made up of several collateral anastomosing veins. The even- 

 tual single channel is formed in the anterior portions by the se- 

 lection and enlargement of the most favorable vein with a cor- 

 responding disappearance of the others. In the posterior portions 

 there is apparently some coalescence of adjacent veins. The an- 

 terior part of the sinus is completed first. As the hemispheres 

 extend backward the sinus correspondingly elongates itself by 

 incorporating the more caudal loops of the plexus. Transverse 

 sections through this part of the sinus in older fetuses thus usu- 

 ally reveal incomplete coalescence of the separate loops. The 

 sagittal plexus very early exhibits a tendency to drain more to 

 one side of the head than to the other and usually toward the 

 right side. As the superior sagittal sinus becomes established we 

 thus find it continuous caudalward usually with the ventral main 

 channel of the right anterior plexus which eventually forms part 

 of the right transverse sinus. The ' straight sinus' is formed in the 

 ventral part of the sagittal plexus and its caudal adjustment is 

 essentially like that of the superior sagittal sinus. It may drain 

 chiefly toward the right or left anterior plexus or equally toward 

 both. 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 18, NO. 2 



