112 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



of the Gasserian ganglion in any injected specimen up to the stage measuring 

 20 mm., when the transformation of the veins into the dural sinuses is well 

 advanced, as can be seen in Streeter's figure 3 (Amer. Journ. of Anat., 1915, 

 XVIII, page 156). The superficial vessels around the ganglia of the eighth nerve 

 are still in the form of capillaries in plate 7. Opposite the otic vesicle the deep 

 plexus has completely covered the surface of the hindbrain; there are a few super- 

 ficial veins across the lateral surface of the vesicle, which are shown cut off in the 

 drawing close to the primary head-vein. Two of these transverse veins make a 

 border for the otic vesicle exactly as do those above and below the Gasserian 

 ganglion. In other words, the veins of the hindbrain can be most simply de- 

 scribed as a series of transverse vessels, some of which are forced to curve by the 

 Gasserian ganglion and the otic vesicle. Opposite the ganglion of the glosso- 

 pharyngeal nerve is a series of transverse veins draining into the primary head-vein. 



The veins opposite the vagus nerve are also very interesting. It is clear 

 that the largest vein of the medulla at this stage is one which in a general way 

 follows the roots of the vagus nerve. This vein was called the posterior cerebral 

 vein by Mall. In general, the place where it joins the vena capitis prima marks 

 the cephalic end of the anterior cardinal vein; it may be a single vein at its roots 

 or a group of veins. In the pig the vagus nerve curves around its cephalic border, 

 passing in the angle between this vein and the primary head-vein. Some of the 

 injections show the nerve passing through a venous loop in this angle. Stracher 

 describes the vagus nerve just caudal to the vein in the chick. The relations of 

 the vagus nerve to the primary head-vein formed the basis of Kastchenko's original 

 study of the primitive veins of the head. 



As will be seen in plate 7, the main vein of the medulla primarily follows the 

 course of the roots of the vagus nerve. It arches caudalward along the dorso- 

 lateral surface of the medulla in the line of the spinal accessory nerve and roots 

 of the vagus. The line of the vein on the medulla can be well seen by following 

 the vagus roots in Streeter's plate 11 (Amer. Jour, of Anat., 1905, IV). 



While it is clear that this vein and its tributaries originally follow the path 

 of the vagus nerve, if its development is followed it will be seen that it becomes 

 a very important vein of the embryo, not even limited to the drainage of the 

 neural tube. At the stage of plate 7 it anastomoses with the lateral venous 

 plexus of the lower medulla, and the first and second occipital veins are correspond- 

 ingly small. Subsequently it gives rise to an extensive group of dorsal branches 

 that grow over the caudal part of the roof of the fourth ventricle and largely drain 

 the developing choroid plexus. The posterior cerebral vein next develops an 

 exceedingly interesting relation to the vascular system of the occipital myotomes. 

 This relation was illustrated in two figures from injected embryo pigs in my article 

 on the origin and development of the lymphatic system (1913, figs. 4 and 5). 

 Opposite the entire zone of the myotomes a plexus of capillaries develops, forming 

 the third vascular sheet of this region. Primarily there is a plexus of capillaries 

 on the surface of the neural tube; secondly, a more lateral plexus of capillaries and 

 veins especially related to the ganglia; thirdly, this sheet of capillaries lateral to 



