OF THE BRAIN OF THE HUMAN EMBRYO. 13 



tutes the middle portion and completes the formation of the main drainage-channel 

 of the head, the primary head-vein. In the pig, the primordial blood-channel 

 along the margin of the hindbrain, instead of enlarging as a simple temporary 

 channel as seen in the chick, becomes resolved almost at once into a prolifer- 

 ating plexus, and from its superficial loops, and perhaps also from some of the 

 loops of the adjacent branchial plexuses, there is evolved the middle portion of the 

 primary head-vein, which is completed nearly as soon as the pre-trigeminal portion 

 and before there is any considerable circulation of the blood. The caudal part of 

 the primary head-vein is made up of the anterior cardinal vein, which, as we have 

 seen, is originally continuous with the primordial channel, joining it (in the pig) 

 in front of the occipital myotomes instead of through the transverse vein of the 

 first interspace, as seen in the chick. As the more superficial loops, sprouting from 

 the primordial system, become established lateral to the otocyst in the formation 

 of the middle portion of the primary head-vein, their communication with the 

 anterior cardinal becomes larger, whereas the original communication of the ante- 

 rior cardinal with the primordial channel becomes more restricted and breaks up 

 into a plexus; thus, by the linking-ui) of these three parts, a continuous superficial 

 channel is formed which extends the whole length of the head and provides for the 

 adequate and efficient drainage of all its structures. 



It has been noted that, from the beginning, communications exist between the 

 aortic system and the more dorsally placed primordial vascular system of the head. 

 The largest and most constant are the paired trunks that connect the first aortic 

 arches with the deeper loops of the ventral part of the forebrain plexus. What 

 was originally a plexiform communication is later resolved into a single trunk 

 that eventually forms part of the internal carotid artery of its respective side. 

 Along the hindbrain region are other irregularly placed, slender communications, 

 and on reaching the myotome region there are the segmental dorsal branches 

 of the dorsal aortse, which anastomose with the proUferating plexus of the neural 

 tube. The primordial blood-channel along each side of the hindbrain proliferates 

 in the form of a plexus, and as the plexuses of the two sides spread on the surface 

 of the brain-wall they gradually establish an anastomosis along the mid-ventral 

 line. At the same time a series of the more superficial loops of the plexus, in 

 association with the communications from the dorsal aortse, become elaborated 

 into a slender bilateral longitudinal channel which is continued caudally into the 

 spinal region, and orally it connects with similar loops which are associated with 

 the embryonic internal carotid artery. There is thus established, on each side 

 along the ventral surface of the neural tube, a continuous arterial channel which is 

 connected dorsally by many loops with the neural capillary plexus and ventrally 

 by a few branches with the aortic system. From these simple channels are derived 

 later the main arteries of the brain and spinal cord. 



With the estabUshment of the primary head-vein, we may regard the first 

 type of circulation of the head as completed. It consists essentially of a series of 

 arterial feeders from the aortic system, which lose themselves in the sheet of capil- 

 laries that mvests the neural tube, which capillaries in turn are drained by many 



