166 GEOKGE L. STREETER 



to be confused with the developing arachnoid tissue, from which 

 it is everywhere separated by the dura. Blood vessels supply- 

 ing and draining the brain are also found in the arachnoid at this 

 time and they are quite numerous in some regions, such as the 

 region of the Sylvian fissure and along the more ventral parts 

 of the mid- and hind-brain. These cerebral vessels are every- 

 where separated and distinct from the dural blood channels, 

 with the exception of the few points where they empty into the 

 big dural channels, as occurs in the adult. The connection be- 

 tween the dural system and the cerebral system is no longer by a 

 multiple anastomosis of small vessels, but instead, by isolated 

 larger veins. 



Examination of figure 6 shows that we have here an arrange- 

 ment of the dural venous system that in most respects follows the 

 adult arrangement. The cavernous sinus has as yet a simpler 

 character than is found in the adult. It is situated median and 

 ventral to the semilunar ganglion and has the large ophthalmic 

 tributaries in front. Caudally it communicates with the main 

 blood stream by means of the superior and inferior petrosal sinuses. 

 The superior petrosal sinus is a long slender channel that passes 

 over the cochlear part of the otic capsule and empties above into 

 the transverse sinus. The inferior petrosal sinus consists of a 

 plexus of veins that passes median to the otic capsule to empty 

 at the point of origin of the internal jugular vein. 



As regards the transverse sinus, it has been pointed out that 

 the terminal or jugular portion of it is established first. In 

 figure 6 it can be seen that from the point of entry of the superior 

 petrosal sinus to the jugular fossa — in other words, the sigmoid 

 portion — it consists of a single large channel, 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 remainder of the anterior dural 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 channel 



