THEOMBOSIS OF CEEEBEAL AETEEIES. 171 



The size of the communicating branches of the circle of Willis 

 materially influences the results of blocking of the main feeding 

 trunks. If the communicating trunks are healthy and of fair 

 size and the blood pressure remains high, the integrity of the 

 brain may be maintained by an adequate collateral circulation. 

 If the vessels are diseased or the blood pressure is low, as after 

 severe haemorrhage, serious cerebral symptoms may be expected. 

 Thus cerebral thrombosis, a condition in which the vessels as 

 a rule are extensively diseased, is more serious than embolism 

 of a healthy trunk ; also ligature of the carotid after severe 

 haemorrhage is more likely to induce hemiplegia than when 

 there has been no loss of blood. 



The posterior communicating artery may be able to supply 

 sufficient blood to the middle and anterior cerebral arteries of the 

 same side when the carotid artery is blocked in the cavernous 

 sinus, and in this connexion it is important to remember that 

 the right posterior communicating artery is almost invariably 

 larger than its fellow. Abnormal arrangements of the arteries 

 at the base of the brain may be responsible for anomalous 

 distribution of cerebral softening. 



THROMBOSIS OF CEREBRAL ARTERIES. 



Thrombosis of the cerebral arteries is usually the result of 

 pre-existing vascular disease. The arterial disease is in most 

 cases due to syphilis, but occasionally other forms of arteritis 

 produce the same result. Karely the pressure of tumours or 

 spread of inflammation from the meninges causes vascular 

 occlusion. The large vessels at the base of the brain, such as the 

 basilar artery and the middle or posterior cerebral arteries, are 

 most likely to suffer. These vessels lie in the subarachnoid cisterns 

 and their prolongations, have but little external support, and the 

 pressure in them is high. The perforating arteries to the interior 

 of the brain are given off from the large basal vessels and do not 

 anastomose, hence cerebral softening is an inevitable consequence 

 of the occlusion of these small branches. Owing to the proximity 



