DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 109 



beginning is shown in plate 4, figure 1. Here it is clear that branches of the 

 primary head- vein are tapping the deep neural plexus around the root of the 

 trigeminus. 



Lateral to the acoustic complex and to the ganglion of the glosso-pharyngeus, 

 the branches of the primary head-vein make a very extensive plexus. The super- 

 ficial venous arch around the otic vesicle is just beginning in plate 4, figure 1. 

 The veins around the trigeminus and around the otic capsule are exceedingly 

 important, because of their ultimate relations to the basal sinuses of the dura. 

 These vessels are shown in plate 4, figure 1, at the stage when they are scarcely 

 more than capillary sprouts. They will be traced farther in the next figure. 



The specimen of plate 7, from a pig which measures 6.5 mm. in oil, is given 

 to emphasize the fate of the primitive vein of the hindbrain, to bring out the 

 ventral artery that now extends the full length of the nervous system from the 

 base of the optic cup to the tip of the tail, and to show the characteristic relations 

 of the veins to the neural tube and its ganglia. 



The injection of the specimen of plate 4, figure 1, did not bring out the 

 ascending neural arteries as did a corresponding injection of the chick (plate 6), 

 but the specimen of plate 7 shows that there is now a longitudinal artery which 

 extends from the primary aortic branch to the brain opposite the subthalamus, 

 along the ventral or ventro-lateral border of the neural tube to its caudal tip. 

 This artery is an anastomosis between all of the neural arteries, both cerebral 

 and spinal. As can be seen in plate 7, the carotid artery. leads to an arterial plexus 

 which covers the lateral surface of the subthalamus and gives rise to a cerebral 

 artery passing dorsal to the eye. The plexus on the subthalamus anastomoses 

 with the plexus of the opposite side in the mid- ventral line; it is tapped by a vein 

 leading to the primary head-vein just cephalic to the maxillary vein. Opposite 

 the groove between the thalamus and the midbrain the two plexuses on either 

 side of the subthalamus gives rise to a single ventral artery which curves along 

 the ventral border of the neural tube down to the level of the third occipital inter- 

 space, where the single median artery becomes an arterial plexus. From this 

 point to the caudal end of the spinal cord there is a double line of capillaries, such 

 as was shown by Evans in his figure 440 (1912). As Evans showed, this double 

 capillary chain will give rise to the anterior spinal artery. The importance of this 

 longitudinal neural artery, which gives rise to the circle of Willis, the basilar artery, 

 and the anterior spinal artery, is obvious. The anastomosis of the arterial plexus 

 of the subthalamus and the ventral surface of the cerebrum with the corresponding 

 plexus of the other side across the mid-ventral line accounts for the anterior com- 

 municating artery of the circle of Willis. At the stage of plate 7 the longitudinal 

 neural artery is supplied by the two carotid arteries, by direct arteries opposite 

 the hindbrain, of which two are shown on the right side of plate 7, and by all 

 the intersegmental arteries on either side. This artery is not supplied as yet by 

 the vertebral arteries, which form later as an anastomosis between the upper 

 intersegmental arteries. 



