12 C. W. M. Poynter 



has studied the arteries of the base of the brain both in the embryo 

 and adult and comes to the conckision that the posterior cerebral 

 arteries are branches of the internal carotid, and that in a post- 

 embryonal period an anastomotic connection is established with the 

 basilar which later enlarges as the proximal portion of the cerebral 

 trunk contracts into a posterior communicating. In 1905 she 

 showed that the anterior cerebrals are primarily parallel vessels 

 without connections, later they form a system or plexus of con- 

 nections out of which the adult condition is developed. Vastorini- 

 Cresi (1905) thought that there was a fusion between the two 

 embryonal vessels. My own investigations convince me that De- 

 Vriese has correctly interpreted the formation of these vessels. In 

 many cases the plexiform condition persists throughout life. 



From a study of the basilar in the dog DeVriese concluded that 

 it is primarily double and represents the primary caudal terminal 

 branches of the internal carotid which later fuse, after establishing 

 communications with the vertebrales. Cases in which there are 

 two basilars, each communicating with the carotid of its side, can 

 be explained by this order of development. Cases are reported by 

 Decker (1886), Macalister (1888), Cavalari (1906) ; partial fusion 

 by Serres (1830), Otto (1838), Davey (1839) 2 per cent., Mal- 

 gaigne (1842), Quain (1844), Barkow (1869), Randall (1888), 

 Cavatorti (1907), Blackburn (1907). 



The connection between the carotid and basilar may drop out, 

 leaving the circle incomplete, Barclay (1812), Davey (1839), Hyrtl 

 (1842), Quain (1844), Barkow (1869), Muller (1871), Webber 

 (1882), Hochstetter (1885), Incoronato (1872), Flemming 

 (1894), Berry & Andefson (1909), Poynter. 



The posterior cerebrales may be unequal or double, Barkow 

 (1869), Gordon-Shaw (1909). 



As already pointed out at one stage of the development, the 

 anterior cerebrales present a plexiform condition, and, as in other 

 parts of the body, this is fruitful of many variations. In the 

 brains I have examined there has been the widest range of pattern 

 from two well-developed and separate arteries to a complex plexus 

 throughout the entire region. Various authors have commented on 

 anomalies, but the range of " normal " seems so wide that it is 

 hardlv worth while to review the literature. 



