108 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



lies mesial to the Gasserian ganglion and lateral to the otic vesicle. The pattern 

 of the deep and the superficial vessels in plate 4, figure 1, deserves careful study. 

 The place of origin of the roots of the Gasserian ganglion is marked by a plexus 

 of the deep vessels which are growing around it, leaving a non-vascular area 

 where the nerves emerge. The deep plexus is also forming a dorsal arch around 

 the otic vesicle which now lies between the deep and the superficial vessels. The 

 pattern of the vessels also indicates the position of the acoustic complex of ganglia 

 and the giosso-pharyngeal ganglion, both of which lie between the deep capillaries 

 and the superficial veins, one cephalic to the otic vesicle and the other just caudal 

 to it. It is clear that the relations of the primary head-vein to the Gasserian 

 ganglion and to the acoustic complex are the same in the pig as in the chick, and 

 are due to the fact that this vessel forms while the ganglia are attached to the 

 skin in their respective placodes. The primary head-vein develops mesial to the 

 placode of the Gasserian ganglion, but curves dorsalward opposite the acoustic 

 ganglia and opposite the ganglion of the glosso-pharyngeus. Sections of an 

 embryo slightly older than that of plate 4, figure 1, cut so that a long stretch of the 

 primitive head-vein is included in one section, show that the lateral border of the 

 acoustic ganglion is in a straight line with the mesial border of the Gasserian gang- 

 lion, so that the superficial vein takes the shortest course in passing mesial to the 

 ganglion of the trigeminus and lateral to the ganglia of the acoustic complex. 



The relations of the branches of the vena capitis prima are very important 

 at the stage of plate 4, figure 1. The branches from the aortic arches are not 

 injected, nor are the primitive maxillary veins. The lateral veins from the cere- 

 brum have hardly begun. The superficial veins opposite the midbrain have a 

 very characteristic pattern; they are, as it were, creeping along on the deep plexus 

 toward the mid-dorsal line. It will be noted that the deep plexus itself has not 

 yet reached the mid-dorsal line at this stage, but it is in advance of the superficial 

 veins. This gradual extension of the branches of the vena capitis prima to the 

 mid-dorsal line characterizes the branches of this vein over the entire brain. 

 When the superficial veins meet in the mid-dorsal line they will give rise to all of 

 the sinuses and veins of this line, as has been shown by Mall and Streeter. 



Opposite the hindbrain the branches of the vena capitis prima have the 

 same fundamental relation to the deep plexus. It is true that caudal to the otic 

 capsule there are a few veins from the deep plexus draining into the ventral border 

 of the vena capitis prima, which may be forerunners of the small ventral veins 

 of the medulla in the adult, but almost all of the veins of the hindbrain drain 

 into the dorsal border of the primary head-vein. These veins have the same 

 characteristics as the rest of the neural veins; that is, they gradually creep dor- 

 salward on the deep plexus. Over the hindbrain, however, the pattern of the 

 veins is not as simple as over the midbrain, because here they are profoundly 

 affected by the ganglia of the hindbrain and by the otic capsule. 



It has been shown that the deep plexus makes an arch of capillaries around 

 the roots of the nerves, as seen around the root of the trigeminus in plate 4, 

 figure 1. The superficial veins also curve around the roots of the nerves. Their 



