494 journal of Comparative Neurology and Psychology. 



developmental defects should be considered apart from the changes 

 produced by arterial disease. In most cases this is not difficult, 

 and in many instances the acquired condition is clearly inadequate 

 to produce the anomalies present. 



In order better to comprehend the significance of the arterial 

 anomalies of the circle of Willis and the great basal trunks it is 

 necessary to refer to the normal condition of the circulation as in- 

 dicated by the diagram, Fig. i. Normally the carotid system and 

 the vertebral system are separated by a balance, or meeting of the 

 two currents, in the middle of the two posterior communicating 

 arteries, while at the same time the circulation of the two cerebral 

 hemispheres is virtually distinct on account of a normal balance 

 which exists in the anterior communicating artery, and by the direc- 

 tion of the blood current in the first part of the posterior cerebral 

 artery. Under normal conditions of current and pressure it must be 

 extremely improbable for any part of the cerebrum to receive even 

 temporarily any blood supply but from the proper vessels. The 

 separation of the cerebellar circulation from that of the cerebrum 

 under normal conditions is equally distinct, the current from the 

 carotid system being opposed by that in the basilar artery. The 

 pontine and upper cerebellar arteries have a more common blood 

 supply, being in keeping with the fusion of the vertebrals into the 

 basilar, though the two posterior inferior cerebellar vessels must 

 normally receive their blood supply from the corresponding verte- 

 brals. Anastomoses between the cerebellar arteries are also more 

 common than in the cerebral trunks though usually the obstruction 

 of a cerebellar artery results in softening of its proper area. 



In abnormal developmental conditions this regulation of circu- 

 lation is of course disturbed and the direction of the current in a 

 vessel may be permanently reversed, while in disease of the vessels 

 these anomalies may greatly add to the gravity of the situation. 



As a basis for this study I have noted the conditions found in 

 220 consecutive cases of mental disease; and, as a matter of some 

 additional interest I have given the age, sex, pathological condition 

 of the vessels, and the form of mental affection in each case. I must 

 claim, however, as the foundation of my opinion, the examina- 

 tion of over 2220 cases rather than the smaller number given as 

 the basis for the statistics. 



I have also given somewhat in detail three cases of anomalies of 

 unusual interest, not included in the list, and have introduced a 



