24 THE DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM 



the anterior part of the brain, passing through this extracranial course to empty 

 into the internal jugular vein. In the stage we are studying the drainage of the 

 cavernous sinus is upward over the semilunar ganglion into what may now be 

 recognized as the transverse sinus. This communication is through a short channel 

 that approximately represents the original trunk of the middle dural plexus and 

 constitutes the superior petrosal sinus. This channel is designated by Markowski 

 (1911) as the vena prontica, and he gives a different origin for the superior petrosal 

 sinus. According to him (p. 599), it takes its origin from a small cerebral vein 

 derived from the basal surface of the hindbrain which empties into the vena prootica. 

 In the further development, the oj^ening of the vein migrates by anastomosis along 

 the vena prootica toward the sinus transversus and empties either into that sinus 

 or into the vena prootica near it. According to Markowski, the superior petrosal 

 sinus has little connection with the cavernous sinus and morphologically represents 

 a metencephalic vein. Regarding the eventual fate of the vena prootica, he has 

 apparently made no observations, though he ]iictures it as a large channel in an 

 embryo 46.5 mm. long. From the si^ecimens I have examined I can not confirm 

 Markowski's description of the superior petrosal sinus, and I feel convinced that his 

 vena prootica and the superior petrosal sinus are one and the same thing, and that 

 which he regards as the superior petrosal sinus is, instead, one of its tributaries. 

 Mall (1905, p. 17) also described the superior petrosal sinus as the adult form of the 

 "vena cerebralis media," which, it will be remembered, is the same as the trunk 

 of our middle dural plexus. 



With the alterations in the primary head-vein the anterior, middle, and pos- 

 terior dural plexuses are drained by means of the new dorsal channel which empties 

 through the jugular foramen into the internal jugular vein. This channel can be 

 at once recognized as the transverse sinus, and the sigmoid portion of it presents 

 relations that are much the same as are found in the adult. The three dural plexuses 

 are still of the embryonic type. The posterior or occipital plexus is practically 

 the same as was seen in 18 mm. embryos. Only its coarser meshes are shown in 

 figure 3. It is more completely shown in figure 26. It will be noted that it is 

 rather of a different character from the now combined anterior and middle dural 

 plexuses and (as we shall later see) it is to take less part in the further meta- 

 morphosis of these vessels. 



The whole dural area lying lietween the cerebral hemispheres and the margin 

 of the cerebellum constitutes the tentorium cerebelli. It is very broad dorsally 

 and is more constricted ventrally; thus in profile it is wedge-shaped. In the loose 

 tissue composing it are found the meshes of the dural plexus, the combined anterior 

 and middle plexuses. As this region becomes more compressed, consequent upon 

 the growth of the cerebrum and cerebellum, there is a continual adjustment of the 

 contained venous channels with repeated alterations in the pattern of the meshes. 

 In general we find the larger channels radiating upward toward the midbrain region, 

 and as we approach the median line the plexus becomes finer and there is an inti- 

 mate anastomosis with the subjacent plexus belonging to the brain-wall, 



