DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 119 



I think that it is important to emphasize the extent of the development of 

 the blood-vessels both of the membranes and of the embryo at the time when 

 the circulation begins. This has been done for the chick, and it would be of 

 great value to obtain the same observations for the mammal. 



This study gives a more complete account of the primitive vessel of the hind- 

 brain than is to be found in the literature. I have followed its origin, its rela- 

 tions, and its fate. The fate of this vessel is a very important point. This 

 primitive vessel of the hindbrain differentiates early, opposite the first part of 

 the neural tube to develop. It has been shown why it remains so long a single 

 channel, namely, because it serves temporarily as a vein for the forebrain and 

 midbrain before it takes the characteristic form of a plexus like the other early 

 vessels on the surface of the neural tube. As the vena capitis prima becomes 

 complete, so that the blood of the forebrain and midbrain is shunted out of the 

 primitive channel of the hindbrain, this channel receives new arterial connections 

 and breaks down into the very important capillary plexus of the rhombencephalon. 



It has been shown that the first true vein of the head, the vena capitis prima, 

 as contrasted with veins which drain only the brain, develops in three segments. 

 The anterior segment is a purely cerebral vein which drains the forebrain and 

 midbrain and originally empties into the primitive vessel of the hindbrain; the 

 posterior segment is the anterior cardinal vein; the middle segment develops last, 

 as a capillary chain between the capillaries of the maxillary, the mandibular and 

 the other visceral arches, and the anterior cardinal vein. This middle segment 

 anastomoses with the primitive cerebral vein from the forebrain and midbrain 

 and forms a much more direct and favorable channel for draining the brain, and 

 so rapidly supplants the more indirect channel along the hindbrain. It drains 

 the other structures of the head in addition to the neural tube. The embryonic 

 vein extending from the region of the thalamus to the duct of Cuvier is the first 

 true vein of the head, in the sense of draining the entire head, that is, the brain 

 and the visceral arches, and may thus be termed the vena capitis prima. 



In connection with the vascular system of the nervous system, it has been 

 shown that the early pattern of the blood-vessels is very uniform for the entire 

 tube. There is a capillary plexus which completely invests the tube and all of 

 its ganglia. It is fed by bilateral longitudinal arteries, which form as an anasto- 

 mosis between all of the neural arteries from the aorta and extends from the carotid 

 arteries at the base of the optic stalk to the tip of the spinal cord. The bilateral 

 character of these arteries persists only around the sub thalamus, where the circle 

 of Willis is formed; elsewhere the two arteries become a single ventral artery the 

 basilar artery and its primary continuation, the anterior spinal artery. I have 

 thus brought out the origin and the significance of the basilar and anterior spinal 

 arteries and have shown that they precede the vertebral arteries. 



The first neural veins are all transverse superficial vessels, which tap the 

 deep plexus and gradually extend dorsalward on the deep plexus. They are 

 profoundly modified by the eye, the ear, and by all the sensory ganglia. Opposite 

 the brain they all drain into the primary head-vein; all the rest of the neural veins 



