THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



221 



1 86). The branches of the superior cerebral vein extend over the cerebral 

 hemispheres and unite with their fellows of the opposite side to form the 

 superior sagittal sinus which lies in the medial line (Figs. 186 and 192). 

 The superior sagittal sinus is at first naturally drained by the superior cere- 

 bral veins; but later, as the cerebral hemispheres enlarge and extend farther 

 toward the mid-brain region, it is carried back and joins the middle cerebral 

 vein; still later, for the same reason, it joins the inferior cerebral vein (Fig. 

 192, A and B). During these later changes the connection between the 



MU. cereb. vein Cotfl. of sff 



OUc vesicle 



Ayt. c*rd. reiy Sufi. s*f. 

 ~0tic reside 



/Let veiy o/ 



L*t. veirj o 



WJT 



CAK si 9 us Sufrcere, 



lyf. cere 6. vei 



CoijfLof slijuses 



sinus Sub, C6re6. Vein 



\.Viih) ' ' 



ercb.vtin 



FIG. 192. Diagrams representing four stages in the development of the veins of the 

 head in human embryos. Mall. 



superior sagittal sinus and the superior cerebral vein is lost (Fig. 192). The 

 middle cerebral vein becomes the superior petrosal sinus which forms a com- 

 munication between the cavernous sinus and transverse sinus. The trans- 

 verse sinus represents the channel between the superior sagittal sinus and the 

 cranial end of the cardinal vein; or in other words, its cranial portion repre- 

 sents the connection between the superior sagittal sinus and the inferior 

 cerebral vein while its caudal portion represents the inferior cerebral vein 

 itself (Fig. 192, compare C and D). The caudal end of the superior sagittal 

 sinus becomes dilated to form the confluence of the sinuses (confluens 



