110 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



The arterial plexus over the subthalamus leads into a finely meshed plexus 

 which covers the entire cerebrum except a small area in the mid-dorsal line near 

 the thalamus. This plexus is not shown in the drawing, but it has the same 

 character as the plexus over the midbrain. The cerebral plexus completely 

 surrounds the optic stalk; in this plexus the only vessel larger than the rest is the 

 cerebral artery, which is seen dorsal to the eye in plate 7. The longitudinal neural 

 artery along the ventral border of the midbrain and the hindbrain gives off a 

 series of nearly equal, regular, small arteries which lead into the capillary plexus 

 on either side of the neural tube. 



The capillary plexus on the neural tube is very characteristic. As has been 

 said, it is finely meshed over the cerebrum, the thalamus, and the midbrain; it 

 is more coarsely meshed over the hindbrain, where the plexus has developed 

 later, especially around the roof of the fourth ventricle, which has not yet been 

 invaded by the vessels. The plexus on the hindbrain in plate 7 demonstrates 

 the fate of the primitive vessel of the hindbrain, the beginning of this plexus as 

 coming from the primitive vessel of the hindbrain having been seen in the living 

 chick. The primitive vessel of the hindbrain disappears only in giving rise to 

 the capillary plexus of the hindbrain. If the pattern of the neural plexus in 

 plate 7 is observed carefully it will be seen that there is just a suggestion of trans- 

 verse lines in the plexus, indicating that the direction of the flow of the blood is 

 from the ventral to the dorsal border of the neural tube. In this plexus will 

 ultimately come transverse arteries. Opposite the first somite will be noticed the 

 beginning of three layers of vessels, a deep layer of very fine capillaries, a second 

 layer of larger vessels also shown in gray, and a third layer of more superficial 

 veins. This is the very beginning of the next stage in the development of the 

 neural vessels. 



The most important point about the form of the deep plexus on the neural 

 tube is the way it conforms exactly to the neural tube and its nerves. Over the 

 midbrain the plexus is very uniform, but over the hindbrain the character of the 

 plexus indicates very clearly the position of the nerves. At the stage of plate 7 

 there are bare spots, that is, places with no blood-vessels, on the hindbrain 

 corresponding to each nerve root; in later stages the vessels penetrate between 

 the small bundles of the fibers of each root and then an injection of the deep plexus 

 does not show the position of the nerves so clearly. As seen in plate 7, the posi- 

 tions of the roots of the trigeminus nerve and of the acoustic group of nerves are 

 very clear. The otic capsule now lies just lateral to the deep capillary plexus, and 

 thus its position is indicated only by the superficial veins. Opposite the ganglion 

 of the glosso-pharyngeus is a bare spot in the deep plexus, which is nearly hidden 

 by a very extensive group of superficial veins. The position of the roots of the 

 vagus and the spinal accessory roots along the line of the posterior cerebral vein 

 is very important. It is clear that the deep plexus outlines this long line of nerve 

 roots, and the same is true along the more ventral line of the medulla, where the 

 pattern of the vessels indicates the position of the roots of the hypoglossal nerves. 



