498 'Journal of Cornparative Neurology and Psychology. 



artery may be absent, extremely small, double, or treble, or it may 

 be formed of two vessels which join before junction with the 

 anterior cerebral of the opposite side then forming a Y-shaped 

 vessel. When double the anterior is usually the larger trunk. 

 In a number of cases a median anterior cerebral artery arises from 

 this vessel. This trunk is sometimes as large as or even larger 

 than the true lateral trunks, and in some cases it curves around 

 the genu, extends backward along the callosum, and finally divides 

 into two branches which supply the two medial surfaces of the 

 quadrate lobules. In such cases the true anterior cerebral arteries 

 are distributed to the anterior portion of the medial surfaces, and to 

 the orbital gyri. 



In normal conditions and in cases of complete fusion of the 

 anterior cerebrals, absence of the anterior communicating artery 

 is of slight importance; in conditions of thrombosis of the carotid 

 its absence may be of serious import. 



In my list this vessel was absent in only two cases; it was 

 unusually small in five; large in two; double in thirteen cases; 

 formed a Y-shaped vessel in one; and gave off a large median 

 anterior cerebral in two instances.^ In quite a large number of 

 cases this vessel was markedly modified by changes in the anterior 

 cerebral trunks as mentioned above; it was also absent or greatly 

 modified in cases of fusion of the anterior cerebrals, as in Nos. 

 2073, 2077, 2099, 2141 and 2169. 



In Windle's cases this vessel was double in fourteen; triple in 

 three; formed a Y in six cases; in six instances it was absent in 

 fusion of the anterior cerebrals; in two it was present under like 

 conditions, as in No. 2139; in two cases he found the vessel absent 

 on account of there being but one anterior cerebral artery; and 

 in one case in complete fusion of these arteries. There were nine 

 instances of a median anterior cerebral artery. 



Posterior Communicating Arteries. — The commonest anoma- 

 lous condition of the posterior communicating arteries is enlarge- 

 ment of one or both of these vessels. In almost every case enlarge- 



* It is probable that had a more careful search been made this artery would have been more frequently 

 found. Since the above was written an excellent example of this vessel has been found. The artery 

 took its origin from the anterior communicating, and proceeded as a single trunk with a few small 

 branches to the callosum, to about the anterior border of the precuneus, when it divided into two equal 

 sized trunks going to supply both of the quadrate lobules and adjoining portions of the paracentral 

 lobules. The true anterior cerebral vessels were a little smaller than usual and were distributed to the 

 lower and anterior medial surfaces of the hemispheres. 



