156 



GEORGE L. STREETER 



SIN TRANSVERSUS 



PLEXUS MEDIALIS 



PLEXUS POST 



Fig. 3 Profile reconstruction of the veins of the dura mater in a human 

 embryo 18 mm. long (No. 144, Carnegie Collection). The 'x' indicates the origi- 

 nal trunk of the middle dural plexus; it corresponds to the superior petrosal 

 sinus. The 'xx' marks the upper end of the internal jugular vein. Enlarged 

 about 12 diameters. 



There are the three dural plexuses draining into the primary head 

 vein and also the opthalmic veins. The anterior dural plexus, 

 however, can be seen to be reshaping itself so as to come into 

 closer anastomosis with the middle dural plexus. The middle 

 dural plexus by draining, as it does, over the otic capsule presents 

 the first stage in the formation of the transverse sinus, that is, 

 the sigmoid portion of it. The posterior dural plexus shows less 

 change in its form and connections than any other group of the 

 head veins and this is true also in the later stages. There are 

 some minor alterations in its pattern, but otherwise it simply 



