VENOUS SINUSES OF THE DURA MATER 171 



paper is not concerned. The caudalward growth, however, of the 

 superior sagittal sinus in adjustment to the corresponding growth 

 of the hemispheres is of interest in our general problem. By 

 comparing figures 7, 8 and 9 it can at once be seen that this caudal 

 development is accomplished at the expense of the meshes of the 

 anterior dural plexus, in which process the transverse and straight 

 sinuses also take part. These channels gradually obtain a more 

 caudal course by what we have already described as ' spontaneous 

 migration.' The channel repeatedly shifts into a more caudal 

 loop of the plexus, the new loop enlarging and the old loop 

 dwindling. The veins marked X in figures 8 and 9 may thus be 

 interpreted as discarded channels. The eventual 'confluens 

 sinuum' (torcular herophili) represents the point at which this 

 caudal development reaches its completion. It usually re- 

 tains a trace of the plexiform character that is found throughout 

 the embryonic stages. 



It is interesting to note that the asymmetry of the superior 

 sagittal sinus expresses itself in the embryo, as well as in the 

 adult, by a tendency to drain more to one side of the head than to 

 the other. This becomes estabUshed by the time the embryo is 

 20 mm. long. The drainage is preponderantly toward the right 

 side. It happens that in figure 9 the main drainage was in 

 reality toward the left side. In reproducing the sketch the 

 figure was reversed, right for left, in order to facilitate its com- 

 parison with figures 7 and 8. In table 1 is given a list of embryos 

 which were examined as to this point and it will be seen that of 

 eighteen specimens all but two drained predominantly toward the 

 right side, that is, about 89 per cent. In order that account 

 should be 'taken of the artificial element introduced in those speci- 

 mens where the vascular system had been injected with coloring 

 matter, such specimens are indicated in the table by an asterisk. 

 No explanation has thus far been reached to explain this interest- 

 ing asymmetry. The drainage of the straight sinus could not 

 be determined as well in the younger stages and there were not 

 enough of the older stages upon which to base an average. If we 

 may judge from the adult, it would generally drain to the opposite 

 side, that is, the left. 



