DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. Ill 



Along the spinal cord the pattern of the capillary plexus shows the position of the 

 ventral nerve roots in the same manner. 



The veins which form the branches of the vena capitis prima must now be 

 followed. The veins from the visceral arches, still largely in the form of capillaries, 

 are completely injected in the specimen, but are indicated in the drawing only at 

 the point where they join the middle segment of the primitive head- vein. In 

 plate 7 the cerebral and the cardinal segments of the vena capitis prima are shown 

 in plastic form, but the middle segment is shown merely in outline in order to 

 make more plain the relations of the neural artery beneath. Beginning with the 

 maxillary vein, the entire maxilla is filled with a capillary plexus which leads to 

 the maxillary vein. This capillary plexus anastomoses with the plexus of the 

 mandibular arch. Besides these capillaries the vein receives a large group of tiny 

 superficial veins which arise in the deep plexus that covers the entire olfactory 

 area of the cerebrum, together with primitive ophthalmic veins which arise in the 

 marginal vein of the optic cup, as in plate 6. One of these subophthalmic veins 

 runs in the groove of the optic stalk. These cerebral veins from the rhinenceph- 

 alon and from the inferior part of the eye are very important in the early drainage 

 of the brain, but it is well known that the main permanent ophthalmic veins develop 

 dorsal to the eye. 



In the zone dorsal to the eye at the stage of plate 7 is a group of tiny super- 

 ficial veins opposite the cerebrum which are like the small veins over the mid- 

 brain. They were omitted in plate 7, but are adequately shown for the chick in 

 plate 6, and they are alike in both forms. These are the primitive cerebral veins. 

 Over the midbrain the veins are characteristic. It is plain that they are lifted 

 off from the surface of the neural tube, that they are all superficial to the deep 

 plexus; they spread out like a fan from the primary head-vein and clearly extend 

 along the deep plexus, which they tap at their tips, and approach the mid-dorsal 

 line. 



Opposite the hindbrain the veins are exceedingly interesting; they follow 

 exactly the same general course of development as the rest of the neural veins; 

 that is, they lie superficial to the deep plexus, are transverse to the long axis of 

 the neural tube, and gradually extend toward the mid-dorsal line, constantly 

 tapping the deep plexus at their tips. On the other hand, they are profoundly 

 modified in their development of the ganglia of the hindbrain and by the otic 

 vesicle, so that their pattern is much more complex than the pattern of those 

 opposite the midbrain. 



The vessels around the ganglion of the trigeminus deserve careful study. 

 At this stage the entire lateral surface of the Gasserian ganglion is covered by a 

 capillary plexus which was omitted in the drawing. This capillary plexus extends 

 along the second and third divisions of the nerve and becomes continuous with 

 the capillary plexus of the maxillary and mandibular processes. Besides this 

 sheet of capillaries which covers the lateral surface of the ganglion, there are two 

 transverse veins above and below the ganglion which outline the root of the 

 trigeminus nerve. These veins are very characteristic, and mark the position 



