8 Development of Blood-Vessels of Human Brain 



until their walls are fairly well developed are the arteries well fixed. 

 The arrangement shown in Fig. 4 is again present in embryo No. 74, 

 Fig. 5, and here the choroid plexus is well developed. In an older em- 

 bryo, Fig. 8, the anterior choroidal artery is well marked, and it arises 

 farther back, — from the posterior communicating artery. For this rea- 

 son I often thought that the more dorsal artery in Fig. 4 represented 

 the choroidal, and only in a much later stage than that shown in Fig. 8 

 is the anterior choroidal shifted from the posterior communicating to 

 the carotid. To test this question further I examined numerous trans- 

 parent pigs of corresponding stages in which the arteries only had been 

 injected, and in all cases the anterior choroidal arose in common with 

 the middle cerebral, and after this I Avas strongly inclined to consider 

 the origin of the anterior choroidal from the posterior communicating 

 in Fig. 8 as a variation. 



The posterior cerebral artery is relatively late to develop, and in early 

 embryos its place of origin is taken by a number of large branches to 

 the mid-brain. This is very marked in Figs. 1, and 4 to 8. In the 

 large embryo (Fig. 1) the vessels have been injected and in drawing it 

 the cerebrum was pulled forward to show the large artery to the mid- 

 brain. There is also a small posterior cerebral artery present showing 

 that for a long time the artery to the mid-brain is much more prominent 

 than the posterior cerebral. In the adult the posterior cerebral arteries 

 mark the terminations of the basilar and lie immediately in front of 

 the third and fourth cranial nerves. The arteries which fulfill these 

 requirements supply the dorsal portion of the mid-brain, corresponding 

 to the posterior quadrigeminal body in the adult brain. But in the 

 adult the posterior cerebral artery, in addition to its main branches to 

 the cerebrum, supplies much more than this, for it also sends branches 

 to the crus, posterior part of the thalamus walls of the third ventricle, 

 as well as elsewhere. In fact, all of the branches together arising from 

 the circle of Willis between the third and fourth nerves behind and the 

 origin of the middle cerebral in front (compare Figs. 4 and 9) must 

 become united to produce the posterior cerebral artery. The region 

 supplied by these numerous branches in the embryo is supplied by the 

 posterior cerebral in the adult, and in its development these branches 

 must be gradually drawn together into one stem to produce the final 

 condition. And this is to be expected. It is only after the t?erebruni 

 makes its appearance and reaches the great prominence it does in man 

 that the condition found in the lower vertebrates is overshadowed. At 

 a relatively late stage, later than the one shown in Fig. 1, all of these 

 arteries arise from a single trunk. In this there are still two main 

 trunks which must unite subsequently to form the posterior cerebral. 



