6 Development of Blood-Vessels of Human Brain 



made in their walls by the excessive pressure, no matter how careful 

 we are in making the injection. 



The delicate veins and capillaries can be injected without extravasation 

 of the fluid in case it is done by injecting a small quantity of India ink 

 into the liver and allowing it to run from there to the head by gravity. 

 In this way I have often obtained beautiful specimens which are clear 

 and sharp. This pressure^, which is often not over one centimeter of water, 

 is so small that it cannot possibly be imitated with a syringe. In fact it 

 is similar to that produced by the embryo's heart, and with these normal 

 pressures no extravasation takes place. 



I may add that in all cases the embryos were placed at once in the 

 strongest alcohol in order to prepare them so that they may subsequently 

 be cleared in a one per cent solution of potassium hydrate. Specimens 

 of this sort are beautiful and instructive; a black vascular system shows 

 through a translucent embryo. These specimens proved to be a most 

 valuable control in the study of the sections of the human embryos, for 

 I had them in abundance, and I also got some ideas of the variations of 

 the blood-vessels and their general relation to the surrounding structures. 



Arteries. 



It has been shown during recent years that in the embryo a series of 

 segmental arteries arise from the aorta, which in the head-end of higher 

 vertebrates unite on their distal ends to produce the two vertebral arteries. 

 These in turn unite at the middle line to produce the basilar artery, as 

 has been shown by His in his monograph on the human embr3^o. By 

 this process of loop-throwing we have produced in the human embryo of 

 four weeks two vertebral arteries Avhich unite to form the basilar and on 

 the anterior end join with the internal carotids. So as soon as the 

 vertebral arteries unite to form the basilar we have marked off the circle 

 of Willis, and considering its relation to the neural tube we can identify 

 its branches to the brain as they arise. In the specimen four weeks old 

 (Fig. 3) the arteries are not well marked, and it is difficult to outline the 

 primary circle of Willis, let alone the branches arising from it. A 

 specimen a little older has in it all of the circle of Willis with the 

 primary arteries to the brain beautifully outlined (Fig. 4), and it is 

 possible to follow them through the capillaries over to the veins. Were 

 it not for the great number of variations found in the arrangement 

 of blood-vessels it would be easy to identify most of the arteries in this 

 specimen by considering them in relation to the cranial nerves and other 

 structures. 



