OF THE BRAIN OF THE HUMAN EMBRYO. 37 



Otherwise there remains to complete the aduU condition only a certain amount of 

 passive migration 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 reason a venous i)lexus is essential. We thus find in the neighborhood 

 of the advancing occipital pole of the hemispheres a continuous persistence of the 

 transitory or embryonic 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, being interposed as a vertical 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 eventual single channel is formed 

 in the anterior portions by the selection and enlargement of the most favorable 

 vein with a corresponding disappearance of the others. In the posterior portions 

 there is apparently some coalescence of adjacent veins. The anterior part of the 

 sinus is completed first. As the hemispheres extend backward the sinus corre- 

 spondingly elongates itself by incorporating the more caudal loops of the plexus. 

 Transverse sections through this part of the sinus in older fetuses thus usually 

 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 that caudalward it is usually continuous with the ventral main channel of 

 the right anterior plexus (or tentorial plexus as it is better called in the late stages), 

 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 plexus or equally toward both. 



In embryos between 35 and 50 mm. long (figs. 18 and 19) we can recognize a 

 main channel of the tentorial plexus that, is to become the transverse sinus. 

 If we disregard the sigmoid portion of it, it forms a fairly straight line with the 

 internal jugular vein. In the interval between the 50 mm. embryo and the adult 

 the transverse sinus bends backward until it comes to lie at an angle of 90° with 

 the internal jugular. This marked change in ijosition is accomplished in large 

 part by spontaneous migration, by the repeated shifting l)ack of the main blood- 

 current into more caudal loops of the plexus, with subsequent dwindling of the 

 discarded anterior loops. As the sinus becomes more definitely established the 

 tentorial plexus becomes relatively smaller (fig. 20) and the final change in position 

 is completed by passive migration, that is, actual traction on the vein-wall by its 

 environment. In this change in position of the transverse sinus the superior 

 sagittal sinus and the straight sinus participate and we find in the adult, at the point 

 where they meet, an anastomosis, the conflucns sinuiini which is usually ]ilexi- 

 form in character and represents the last trace of the embryonic tentorial plexus. 



