294 INNEKVATION. [CHAP. X. 



of the brain. This takes place not only between the smaller rami- 

 fications, but likewise between the primary trunks. The former 

 is evident all over the surface of the cerebrum and cerebellum. 

 The latter constitutes the well-known circle of Willis. This ana- 

 stomosis encloses a space, somewhat of an oval figure, within which 

 are found the optic nerves, the tuber cinerum, the infundibulum, 

 the corpora mamillaria, and the interpeduncular space. The 

 anterior communicating artery, between the anterior cerebral arte- 

 ries, completes the circle in front. The lateral portion of the circle 

 is formed by the posterior communicating artery, and it is com- 

 pleted behind by the bifurcation of the basilar into the two pos- 

 terior cerebral arteries. Thus, a stoppage in either carotid, or in 

 either vertebral, would speedily be remedied. The coalescence of 

 the vertebrals to form the basilar, affords considerable security to 

 the brain against an impediment in one vertebral ; and, should the 

 basilar be the seat of obstacle, the anastomoses of the inferior cere- 

 bellar arteries with the superior ones would ensure a sufficient 

 supply of blood to that organ. If either or both carotids be stopped 

 up, the posterior communicating arteries will supply a considerable 

 quantity of blood to the intracranial portions of them ; or, if one 

 carotid be interrupted, the anterior communicating branch will be 

 called into requisition to supply blood from the opposite side. 



Obstruction to the circulation in both carotids and both verte- 

 brals is productive of a complete cessation of cerebral action, and 

 death immediately ensues, unless the circulation can be quickly 

 restored. This was proved clearly by Sir A. Cooper's experiments 

 on rabbits. The circulation may, however, be interrupted in both 

 carotids, or in both vertebrals, without permanent bad effect ; or 

 in one carotid or in one vertebral, provided the condition of the 

 remaining vessels be such as not to impede the circulation in them. 

 In cases where the neighbouring anastomotic branches are not 

 sufficient to restore the circulation to a part from which it has been 

 cut off by the obliteration of its proper vessel, the cerebral 

 substance of that region is apt to experience a peculiar form of 

 softening or wasting, which is distinguished by the absence of any 

 discoloration by the effusion of blood, and of any new matter. 



The four great channels of sanguineous supply to the brain are 

 continued up straight from the aorta itself, or from an early stage 

 of the subclavian. The contained columns are propelled very 

 directly towards the base of the brain, through wide canals. 

 Were such columns to strike directly upon the base of the brain, 

 there can be no doubt it would suffer materially. Considerable 



