Franklin P. Mall. 9 



The posterior communicating branch mnst be formed by shifting 

 nearly all of its branches, as shown in Fig. 4, back to the third and fourth 

 nerves to produce the posterior cerebral artery, while the cerebral hemi- 

 sphere is growing over the thalamus and mid-brain. Or by a series of 

 arches, as indicated in Fig. -i, the arteries to the more dorsal portions of 

 the thalamus and mid-brain, as well as to the structures which wander 

 into this region, are gradually transferred towards the basilar, leaving 

 the small branches of the posterior communicating to supply the imme- 

 diate neighborhood, as it does in the adult. At any rate the large 

 vessel arising from the posterior communicating in Fig. 1 arises from 

 the posterior cerebral, probably as the posterO-lateral set, in the adult. 

 The question is further complicated by variations, Avhich are quite 

 numerous, the most common variation of the posterior cerebral being 

 in its origin, which is transferred from the basilar to the internal 

 carotid." I have also foijnd all kinds of combinations of this artery 

 with neighboring arteries in the embryo pig, which I interpret in part as 

 transformation stages. Furthermore, it may be possible that shifting 

 of arteries takes place until the individual is fully grown, for Bean ' 

 has shown recently tliat the branches of the subclavian artery of the 

 infant differs from those of the adult. 



The branches from the basilar and vertebral arteries are more easily 

 followed, for in this region there is less shifting, and the landmarks 

 I)rove to be of more value. In the upper part of the mid-brain there is a 

 cluster of branches which are destined to become the superior cerebellar 

 arteries (Fig. 4). This group is reduced to a single artery in Figs. 5 

 and 6, where it is just behind the isthmus. The next group in Fig. 4 

 is the transverse pontine between the superior cerebellar and the otic 

 vesicle. Then the anterior cerebellar between the seventh and eighth 

 nerves near the otic vesicle. Finally, the group which perforates the 

 root of the twelfth nerve is destined to form the posterior inferior cere- 

 bellar. This branch is also shown in Fig. 1. Between this and the otic 

 vesicle there are a couple of branches, shown in Fig. 4, the fate of whicli 

 is uncertain. 



Veins. 



While the arteries of the brain undergo many changes in their devel- 

 opment, their history is relatively simple when compared with the gyra- 



=* According to Windle (Journal of Anatomy and Physiology, XXII, 1888), 

 this variation occurred 28 times in 400, or in 7 per cent of the cases. 

 "Bean: American Journal of Anatomy, Vol. 4. 



