OF THE BRAIN OF THE HUMAN EMBRYO. 15 



three groups: (1) an anterior group from the forebrain and midbrain, leading into 

 the pre-trigeminal portion of the primary head- vein; (2) a middle group from the 

 cerebellar region, emptying into its otic or middle segment, that portion between 

 the trigeminal and glossopharyngeal nerves; and (3) a posterior or occipital group 

 which accompanies the vagus rootlets and empties near the junction of its middle 

 and cardinal portions. This posterior group usually empties by a common trunk. 

 This trunk corresponds to the original communication between the anterior cardinal 

 vein and the primordial blood-channel of the hindbrain, which now has proliferated 

 into the meshwork forming the capillary sheet that invests the brain-wall. Before 

 the completion of the middle or otic portion of the primary head-vein, this trunk 

 from the occipital group of tributaries was the only communication between the 

 anterior cardinal and the vessels of the more oral region of the head. Especial 

 attention is directed to these three tributary groups, as their arrangement is sig- 

 nificant for the later stages, as will presently be seen. In addition to these tribu- 

 taries from the brain, the primary head-vein receives ventral tributaries from 

 the eye region, from the nerve-ganglion masses, and from the region of the first and 

 second gill-arches, which communicate with the plexuses derived from the pro- 

 liferating elements of the first two aortic arches. 



The arterial supply to the head region at this time is primarily through the 

 internal carotid arteries, which form relatively slender though direct channels. 

 On each side they are made up of the trunk that connected the primary head- 

 plexus with the first aortic arch and the portion of the dorsal aorta corresponding 

 to the first two arches. It will be noted that the first and second vascular arches 

 are more or less incomplete, having broken up into irregular plexuses ramifying in 

 the tissues of their respective gill-bars. A part of the plexus of the second arch 

 apparently becomes incorporated in the external carotid artery, although the trunk 

 of this artery is probably represented by the short stem seen in figures 22 and 23, 

 projecting from the oral border of the third vascular arch. The capillaries and 

 venous drainage plexuses of the first and second arches are shown only at their 

 point of entrance into the primary head-vein above. The internal carotid artery 

 extends forward to the root of the optic stalk, where it bifurcates into its terminal 

 branches, which soon become lost by anastomosis with the capillary sheet of the 

 brain-wall. The continuation of this channel backward along the ventral wall 

 of the brain could not be satisfactorilj^ modeled, though this channel and its anas- 

 tomosis with the basilar and vertebral arteries doubtless aheady exist. It is 

 shown in the reconstruction by Ingalls (1907) in a 4.9 mm. embryo. 



The cut ends of communications such as originally connected the dorsal aorta 

 with the primordial vascular system of the brain are indicated. As long as they 

 persist, these must be regarded as arterial feeders through the basilar arteries to 

 the capillary sheet of the brain. One of these may doubtless be looked upon as 

 the stapedial artery (for example, the one between the first and second arches). 

 The vertebral arteries apparently were established in this specimen, but they could 

 not be satisfactorily outhned and so were omitted. They must, however, be 

 included as part of the source of blood-supply. With this manner of blood-supply, 



