OF THE BRAIN OF THE HUMAN EMBRYO. 23 



crowded by the develojjment of the cochlea and the structures of the middle ear 

 This constitutes a mechanical factor that doubtless has a determining influence 

 upon the change in the course of this blood-channel. 



In embryos about 20 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 those of the adult, while the dorsal veins 

 still have many embryonic features. In figures 3 and 26 are shown reconstructions 

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

 3, showing left side of head, is a profile reconstruction, and figxire 26, showing right 

 side of head, is from a wax-plate reconstruction. The reconstruction of the blood- 

 vessels in this case was greatly facilitated by the work already done on the head of 

 this embryo by Professor Lewis, who kindly put all of his tracings and photographs 

 at my disposal. The study was further facilitated through the fact that the blood- 

 vessels had been injected through the umbilical vein with India ink by Professor 

 t^-^abin 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. 

 For the sake of comparison another embryo sHghtly older (Carnegie Collection, 

 No. 632, 24 mm.) was studied, and a profile reconstruction of it is shown in figure 4. 



On examining figure 3, showing left side of the head, it is seen that the primary 

 head-vein is now separated into its adult parts. In the trigeminal-nerve region we 

 can speak of it as the cavernous sinus, receiving as tributaries the oi^hthalmic and 

 maxillary veins and a large cerebral vein draining the lateral wall of the dienceph- 

 alon. This latter vein belongs to the cerebral- vein system, eventually becoming 

 the middle cerebral vein, and runs for the greater part of its course through the 

 pia-arachnoid membranes. It penetrates the dura and runs a short dural course 

 before joining the cavernous sinus. It may be regarded as one of the diminishing 

 number of channels that drain the cerebral venous system into the dural system. 

 There are also smaller tributaries from a network in the region of the semilunar 

 ganglion. No tributaries were detected flowing into the cavernous sinus from the 

 caudal pole of the cerebral hemisphere, such as were found up to this time; all of 

 this blood now flows in the opijosite direction, caudalward through the middle 

 dural plexus and the developing transverse sinus. On the right side of this same 

 embryo (see fig. 26) a communication still exists between the cavernous sinus and 

 the anterior dural plexus, though it is thinning out. In this respect, then, figure 26 

 is just before figure 3, and a comimrison of the two shows just how this interesting 

 reversal of the blood-current takes place. 



Tracing the cavernous sinus backward, it can be seen that the interruption 

 between it and the internal jugular vein is complete, though there is still a remnant 

 of that connection which extends as a blind channel a short way along the facial 

 nerve. It is interesting to note that there is occasionally found in the adult skull 

 a persistent foramen, the foramen jugidare spurium of Luschka, which corresponds 

 to the exit of this decadent channel. The vein itself, however, has never been 

 described as persisting, although it exists normally in lower forms as a drainage for 



