CEEEBEAL EMBOLISM. 169 



patent foramen ovale or a defect in the ventricular septum might 

 conceivably allow a clot from one of the systemic veins to reach 

 the brain, but such an occurrence must be exceedingly rare. 

 In certain cases microbic emboli may be carried from the lungs 

 by the pulmonary veins and set up suppuration in the brain. 



Emboli are frequently arrested at the points where vessels 

 bifurcate or become suddenly narrowed by giving off large 

 branches. As examples may be cited the internal carotid where 

 it breaks up into its branches near the base of the brain and the 

 division of the middle cerebral artery into a leash of cortical 

 vessels. Branches which form a more or less direct continuation 

 of the main trunk from which they spring are more likely to 

 receive plugs than those which come off at sharp angles. The 

 left common carotid is in a direct line with the first part of the arch 

 of the aorta, whilst the innominate trunk from which the right 

 common carotid is derived comes off from the aorta at a con- 

 siderable angle, consequently emboli are rather more frequent in 

 the left middle cerebral artery than in the right. Further, an 

 embolus travelling in the internal carotid artery is much more 

 likely to enter the middle cerebral artery than the anterior cere- 

 bral, since the former is a direct continuation of the main trunk. 



The left vertebral artery, arising from the left subclavian artery 

 where that is distinctly ascending, is a more direct continuation 

 of the subclavian than is the right vertebral artery which comes 

 off the subclavian nearly at a right angle, so the left vertebral 

 artery is much more likely to receive an embolus. Since the 

 vertebral arteries unite to form the basilar trunk, and that in turn 

 bifurcates symmetrically, each of its posterior cerebral branches 

 is equally liable to be plugged. 



The cerebellar arteries come off their parent trunks at acute 

 angles and are rarely the sites of embolisms. The basilar artery 

 too is rarely occluded since a clot which can traverse the vertebral 

 artery is unlikely to be arrested in the larger basilar trunk formed 

 by the union of the two vertebrals, but such clots are sometimes 

 entangled at the point of bifurcation of the vessel. 



The results of embolism differ in different regions of the brain 



