DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 113 



the myotomes. Opposite the occipital myotomes the capillary plexus drains, by a 

 series of veins on the one hand into the main vein of the medulla, on the other hand 

 into the anterior cardinal vein. The history of the neural branches of this vein 

 of the medulla involves the entire subject of the circulation of the medulla. The 

 relation of the branches from the occipital myotomes involves the subject of the 

 development of the external jugular vein and its branches. The main stem of the 

 vein was shown by Mall, in 1905, to become a part of the great transverse sinus. 

 For this vein I am using the term primitive posterior cerebral vein. It might also 

 be termed the primitive vein of the medulla. 



The stage of plate 7 shows the beginning of the veins of the hindbrain. It 

 will be seen that the primitive branches of the primary head-vein draining the 

 hindbrain are greatly modified by the ganglia of the hindbrain and the otic 

 capsule. Opposite the midbrain these veins are regular and nearly equidistant; 

 opposite the hindbrain they are grouped according to the ganglia. Of these 

 veins of the hindbrain, the group caudal to the Gasserian ganglion and the stem 

 of the posterior cerebral vein bear the most important relations to the future 

 cerebral sinuses at the base of the brain. 



In this account of the early blood-vessels of the neural tube three facts have 

 been brought out which are essential to an understanding of the development of 

 the neural vessels. First, there forms a ventral neural artery, originally paired, 

 which extends along the ventral surface of the entire neural tube from the base of 

 the optic cup to the caudal end of the spinal cord, which is an anastomosis of all 

 the direct neural arteries from the aorta; second, this artery leads to a capillary 

 plexus which completely invests the neural tube and all its ganglia; third, the 

 primary veins of the neural tube are all transverse vessels superficial to this 

 primary plexus, and they gradually extend toward the mid-dorsal line and are 

 profoundly modified by the ganglia, both cerebral and spinal. All of the veins 

 of the brain drain into the primary head- vein. As has been shown by Mall and 

 Streeter, the only segment of the vena capitis prima which remains as a part of 

 the dural sinuses becomes the cavernous sinus, which is that portion of the primary 

 head- vein medial to the Gasserian ganglion. All other dural sinuses develop from 

 the branches of the vena capitis prima. 



It has been shown that the middle segment of the vena capitis prima develops in 

 the pig, as in the chick, as a chain of capillaries between the aortic arches and the 

 anterior cardinal vein; it becomes very large, because it makes a more direct outlet 

 for the primitive cerebral vein. The vena capitis prima develops from three seg- 

 ments and is the first true vein for the head; the primitive vessel of the hindbrain 

 serves temporarily as a vein for the brain and then gives rise to the capillary plexus 

 of the upper part of the hindbrain. 



