30 THE DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM 



that it consists of a single large channel from the point of entry of the superior 

 petrosal sinus to the jugular fossa (in other words, the sigmoid portion) and has the 

 same tributaries and the same general relations that are found in the adult. The 

 remainder or proximal portion of the transverse sinus is less well established, and 

 the large capillary meshwork found along its dorsal margin shows that the blood- 

 channels here are still in the formative stage and must still be spoken of as the 

 temporary tentorial plexus. The main channel is forming along the anterior margin 

 of this plexus, into which the inferior cerebral vein empties. It can be seen how 

 this portion of the transverse sinus undergoes spontaneous migration backward 

 in adjustment to the growth of the hemisphere and thus comes to assume a more 

 and more horizontal course. This change in direction, together with an increase 

 in length and diameter of the main channel at the expense of the formative mesh- 

 work, remains to be completed before the adult condition can be considered as 

 estabhshed. The variations found in the adult in the region of the confluens sinuum 

 can be readily understood as variations in channel selection through this tentorial 

 meshwork. 



In the region of the forebrain a fold of dura is interposed between the two 

 hemispheres and is compressed into a flattened sheet which is to constitute the fak 

 cerebri. This and the vascular meshwork belonging to it are directly continuous 

 with the tentorium. Like the tentorium, it passes through a prolonged adjust- 

 mental period. In embryos 50 mm. long two of its permanent channels, which are 

 to belong to the dural sinus system, can be readily recognized; these are the superior 

 sagittal sinus and the straight sinus. In figure 6 the superior sagittal sinus is 

 quite irregular in outline, which is a result of shrinkage of the sjiecimen. In the 

 normal state, as seen in other embryos, it passes evenly along the margin of the 

 cerebrum. Certain details regarding the vessels belonging to the falx cerebri and 

 the drainage of the chorioidal masses will now be taken u]) in connection with the 

 formation of the superior sagittal sinus. 



DEVELOPMENT OF SINUS SAGITTALIS SUPERIOR. 



Under the description of embryos 21 mm. long mention was made of the 

 formation of a plexus sagittalis as a subdivision of the anterior dural plexus. At 

 that stage the plexus is clearly differentiated from the remainder of the anterior 

 dural plexus, as can be seen in the dorsal view of an embryo of about that age shown 

 in figure 8 and in the reconstruction shown in figure 26. Earlier than this, in em- 

 bryos about 14 mm. long (fig. 7), the plexus can be recognized, though here it is 

 not so clearly separated from the general plexus. In such embryos it can be seen 

 that the larger tributaries of the anterior and middle dural plexuses stop short of 

 the median line, with the exception of anteriorly, where they merge into a longi- 

 tudinal plexus that dips in between the developing hemispheres. It is in the meshes 

 of this plexus that we find the beginning of the superior sagittal sinus; and the 

 principal steps in its transformation can be seen by comparing figures 7, 8, and 9. 

 Sketches like these necessarily have to be simplified, and on examining them it 

 should be remembered that only the larger channels are shown and that in between 



