DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 87 



It must be made very clear indeed that the longitudinal artery seen along 

 the hindbrain in plate 6 is a neural artery and is not the vertebral artery. This 

 is a specimen of the third day of incubation and the artery shown in this specimen 

 forms along the neural tube at a stage when the occipital arteries supply only the 

 neural tube. On the fifth day of incubation, on the other hand, these same arteries 

 also supply the corresponding myotomes with vessels, and there then forms a 

 second longitudinal anastomosis on either side along the upper segmental arteries 

 which is nearer the aorta than the neural . vessel. These second longitudinal 

 vessels become the vertebral arteries. These arteries form at the stage of the fifth 

 day of incubation in the chick and are present in a pig measuring 15 mm., a very 

 much older stage than the one shown in plate 7, which measured 6.5 mm. The 

 vertebral arteries form as the heart is shifting farther caudal ward; and indeed 

 it is clear that the basilar and anterior spinal arteries together, as well as the 

 vertebral arteries, provide for the arterial supply of the hindbrain when the 

 shifting relations in the neck interfere with the direct arteries from the aorta. 

 The fundamental relations of the neural arteries to the plexus on the surface of 

 the neural tube has now become clear. This plexus is fed with arterial blood 

 from bilateral longitudinal arteries which are along the ventro-lateral border of 

 the plexus and eventually come to lie for the most part in the mid- ventral line. 

 Over the surface of the subthalamus the vessels remain bilateral. 



It is now necessary to consider how the neural plexus becomes related to the 

 veins. In the study of the development of the veins of the brain as distinct from 

 those of the spinal cord, it is of primary importance to study how the deep plexus 

 of vessels becomes related to branches of the primary head- vein. This point I 

 have worked out more in detail in the pig and shall therefore take up its consider- 

 ation later. The fundamental points are, however, (1) that the branches of the 

 primary head-vein opposite the forebrain and midbrain are transverse veins 

 superficial to the deep plexus which constantly tap the deep plexus at their tips 

 and grow toward the mid-dorsal line; (2) that the transverse veins of the hind- 

 brain are profoundly influenced by the presence of the ganglia of the hindbrain 

 and by the otic vesicle. The sensory ganglia become as completely surrounded 

 by a capillary plexus as the neural tube itself, and each of these plexuses gives rise 

 to a vein or group of veins. Moreover, the same is true for the spinal ganglia. 



In this account of the origin of the neural vessels great stress has been laid 

 on the development of the vessels of the hindbrain, on account of the peculiar 

 relations of the primitive vessel of the hindbrain to the drainage of the forebrain. 

 In the course of the development of the vessels of the hindbrain the direction of 

 the circulation of the blood is ultimately exactly at right angles to its original 

 course. This change takes place, (1) by the completion of the true head-vein, by 

 which the pial vessel is relieved of a great volume of venous blood from the brain; 

 (2) by the development of a new longitudinal arterial channel, by which it can 

 receive a much greater arterial supply. By these changes the blood over the 

 hindbrain soon runs from the ventral toward the dorsal border, at right angles 

 to its original course from the cephalic to the caudal border. 



