500 'Journal of Cornparative Neurology and Psychology. 



arteries as absent in twenty-five cases; thirteen times the left, the 

 right nine times and both three times. The author states that a 

 partial anastomosis with the basilar was secured in some of these 

 by means of small twigs in the interpeduncular space. It seems 

 possible that some of these may have represented undeveloped 

 posterior cerebral and posterior communicating arteries, as in 

 No. 2177. 



Posterior Cerebral Arteries. — The most common abnormal con- 

 dition of the posterior cerebral arteries is small size of the vessels 

 at their first part before joining the posterior communicating 

 arteries. This, as mentioned above, is the condition which leads 

 to the conclusion that the vessels arise from the carotid. In some 

 cases the vessel joins the posterior communicating artery by a 

 fine practically impervious thread, it may join at an unusual place, 

 or may fail to anastomose with the vessel at all. In case of 

 undevelopment of this vessel it is almost invariably compensated 

 for by enlargement of either the posterior communicating, or the 

 anterior choroid, but in obstruction of the internal carotid of the 

 same side the effects are disastrous. See Case No. 1909. 



In my list the posterior cerebral of the right side was noted as 

 smaller than normal in fifteen cases; the left in ten, and both in 

 twenty-one cases. The right artery was absent in one case, and 

 in two instances the main distribution was supplied by an enlarged 

 anterior choroid artery. Nos. 2163 and 2177. 



WiNDLE claims that in twenty-four of his cases this vessel orig- 

 inated from the carotid artery; eleven on the right side; nine on the 

 left; and four on both sides. I have not been able to satisfy my- 

 self that this was so in any of my cases, as it seems to me that all 

 may be regarded as compensating enlargements of the posterior 

 communicating and anterior choroid arteries. 



In one of Windle's cases and one of my own the third nerve 

 was divided by this artery. 



Anterior and Posterior Choroid Arteries. — The anterior choroid 

 artery is fairly constant but may be represented by mere threads. 

 In two instances among my cases I found this vessel in part taking 

 the place of the posterior cerebral. Nos. 2163 and 2177. The 

 posterior choroid arteries sometimes arise from the posterior cere- 

 brals; sometimes from the superior cerebellar arteries. They are 

 somewhat irregular in their development but were not carefully 

 studied. 



