VENOUS SINUSES OF THE DURA MATER 157 



extends to become the occipital sinus of the adult and at the 

 same time, together with its fellow, develops drainage channels 

 that empty into the plexus of the tentorium. The primary head 

 vein can be subdivided into: the trigeminal portion that is to 

 form the cavernous sinus; the otic portion which passes lateral 

 to the otic capsule accompanying the seventh nerve; and lastly, 

 the cervical portion or internal jugular vein, the boundary of 

 which is indicated in figure 3. The otic portion already shows 

 a diminution in volume as a result of the establishment of the 

 new drainage channel dorsal to the otic capsule. Dorsal to the 

 otic capsule there is sufficient free space for the development of 

 a vascular channel, whereas the region ventrolateral to the otic 

 capsule becomes crowded by the development of the cochlea and 

 middle ear. This constitutes a mechanical factor that doubtless 

 has a determining influence upon the change in the course of this 

 blood channel. 



4. Human embryos 20 mm. long 



In embryos about 21 mm. long the veins of the head have an 

 arrangement that is intermediate between the embryonic type and 

 the adult type. The veins in the basal portion of the skull closely 

 resemble the adult, while the dorsal veins still have many em- 

 bryonic features. In figure 4 is shown a graphic reconstruction 

 of the head of such an embryo (No. 460, 21 mm. long, Carnegie 

 Collection). The reconstruction of the veins in this case was 

 greatly facilitated by the work that had already been done on the 

 head of this embryo by Professor Lewis, who kindly put all his 

 tracings and photographs at my disposal. The outlines of the 

 central nervous system are taken directly from a model prepared 

 by him. The study of the veins was facilitated through the fact 

 that the blood vessels had been injected through the umbilical 

 vein with India ink by Professor Sabin, while the heart was 

 still beating, so that there is a beautiful injection of the entire 

 vascular system. Before the embryo was cut sketches and 

 photographs of the vessels that could be seen from the surface 

 were made by Professor Evans. There is enough material at 

 hand in relation to this specimen to make a very complete study 

 of its whole vascular system. The writer's attention, however, 



