DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 83 



thalamus, and the midbrain has now made an almost complete covering for that 

 part of the neural tube. Over the midbrain the plexus is farthest developed 

 and has anastomosed across the mid-dorsal line with the plexus of the opposite 

 side; over the thalamus and the cerebrum the deep plexus has almost reached 

 the mid-line. The primary artery of the brain which supplies this extensive 

 plexus divides into two branches first, into a large arterial plexus which curves 

 around the dorsal margin of the optic stalk and leads to the plexus around the eye 

 and to the plexus of the cerebrum, and partly supplies the plexus of the thalamus; 

 second, into an artery which curves along the ventro-lateral border of the thalamus 

 and the midbrain, and is approaching the hindbrain. This artery will soon meet 

 the ascending artery seen along the ventro-lateral border of the rhornbencephalon. 



Opposite the hindbrain the development of the vessels, both the arteries 

 and the veins, is most interesting. As is shown in plate 6, there is now a most 

 important new vein. This is as yet a tiny, irregular vessel, hardly larger than a 

 capillary, which connects the veins of the maxillary, the mandibular, and the 

 second aortic arch with the anterior cardinal vein. The primitive vessel of the 

 hindbrain is a vein for the brain only; this new capillary develops out of the 

 capillaries of the visceral arches and by means of the relation of the maxillary 

 veins to the primitive cerebral vein it receives the blood of the primitive cerebral 

 vein and hence it becomes a true head-vein. We shall call this new vein, which 

 is usually called the vena capitis lateralis, the middle segment of the vena capitis 

 prima (v. cap. p. 2], and will say that as soon as this anastomosis between the 

 primitive maxillary veins and the anterior cardinal veins takes place we can 

 speak of a primary head-vein which extends from the region of the thalamus to 

 the duct of Cuvier and drains the structures of the head, namely, the brain and 

 the tissues of the visceral arches. 



The specimen in the drawing of plate 6 is not shown from an exactly lateral 

 aspect, but is tilted slightly to show the ventro-lateral surface of the hindbrain; 

 but even with this tilting it is clear that the general position of the superficial 

 vessel is such that it can become a direct line between the primitive cerebral vein 

 and the anterior cardinal vein. This direct line is very plain in plate 2, figure 2. 

 In other words, it is a more favorable vessel for the drainage of the large primitive 

 cerebral vein than is the primitive vessel along the hindbrain. 



The exact course of this tiny chain of new capillaries is most interesting, 

 because it conforms so closely to the structures that are present before it develops. 

 In this connection the relation of this new capillary to the Gasserian ganglion is 

 important to note, because it has been so little understood. As is well known, 

 the ganglion arises from the wall of the pons at the point shown in plate 6, grows 

 v; ntralward, and becomes adherent to the skin, making the placode of the tri- 

 eeminus. If sections from specimens at the stage of plate 6 are studied, it will 

 be seen that it is this attachment of the Gasserian ganglion to the skin, occurring 

 at the stage when the tiny capillaries that give rise to this superficial vein begin, 

 that renders it impossible for the new capillaries to pass lateral to the ganglion; 

 hence they grow mesial to it. The primitive vessel of the hindbrain is mesial 



