82 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



the entire trunk the ophthalmic veins. The emphasis on the fact that this group 

 of veins belongs in the maxilla, bringing it into line with the veins of the man- 

 dibular arch and with the veins from the rest of the aortic arches, is interesting 

 in connection with the origin of the middle segment of the primary head-vein. 



Cephalic to this plexus of maxillary veins is an extensive series of veins from 

 the marginal vein of the optic cup. The vein in the margin of the optic cup is very 

 characteristic. Above these is a smaller, but very important, group of veins 

 which drain the cerebrum proper. As can be seen in plate 6, they tap the deep 

 plexus of the cerebrum at their tips; they gradually creep dorsalward on the deep 

 plexus until they meet with those of the opposite side in the mid-dorsal line. The 

 anastomosis of these veins in the mid-dorsal line will ultimately give rise to the 

 superior sagittal sinus, as has been shown by Mall and Streeter. On account of 

 the relation of the primitive veins of the neural tube to the ultimate formation 

 of the dural sinuses, this process of the creeping of the primitive veins toward the 

 mid-dorsal line on the deep plexus is very important. 



Over the thalamus at this stage is the main root of the primitive cerebral 

 vein and one large accessory root. For the midbrain the superficial branches 

 of the primitive cerebral brain have not yet appeared, and all the blood of the 

 midbrain drains through the deep plexus toward a characteristic deep vessel 

 along the cerebellar ridge, which joins the primitive vessel of the hindbrain. In 

 plate 2, figure 2, it is clearly shown that at the time when the circulation begins 

 all the venous blood of the forebrain and midbrain must pass through the deep 

 channel of the hindbrain and the transverse vein of the first interspace in order 

 to reach the heart. This figure (plate 2, fig. 2) shows that the vessel of the hind- 

 brain is mesial in position both to the primitive cerebral vein and to the anterior 

 cardinal vein. Plate 6 shows that it is also farther dorsal than either of these 

 veins; and also, what is well known, that the primitive vessel of the hindbrain 

 is mesial to the Gasserian ganglion, the acoustic complex, the otic vesicle, and the 

 ganglion of the glosso-pharyngeus. The cephalic end of the ganglion of the vagus, 

 on the other hand, is mesial to the transverse vein of the first interspace; that is, 

 the primitive vessel of the hindbrain runs down to the region of the cephalic end 

 of the ganglion of the vagus. 



The primitive vessel of the hind-brain serves as a transitory vein for the 

 brain of the chick during the second and third days of incubation, as is very 

 evident in any living chick. On the other hand, it serves as the only channel 

 for the blood to the hindbrain, and it can receive arterial blood directly from the 

 aorta through tiny branches. These branches are so small and are so seldom 

 fully injected that it is probable that only a small amount of blood actually passes 

 through them in the riving chick into the primary channel of the hindbrain. 

 The permanent arteries for the hindbrain develop later, as will be shown. 



Plate 6 shows how the primary vascular channel of the hindbrain ceases to 

 serve as a vein for the forebrain and midbrain, and how the true head-vein, the 

 vena capitis prima, develops. The specimen here shown also indicates the fate 

 of the primary vessel of the hindbrain. The deep plexus of the cerebrum, the 



