i 7V 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY III 



there is no rete mirabile and the external carotid pro- 

 vides qo significant source. In the dog and cat the 

 chief supply is from the external carotids by way of a 

 well developed rete mirabile and from the vertebrals; 

 the internal carotids are of minor significance in the 

 dog and vestigial in the cat. 



The primates, including man, demonstrate the 

 best isolation between the intracranial and extra- 

 cranial blood supply, with only negligible anasto- 

 moses between them, the supply being largely via the 

 internal carotids with the vertebrals contributing 

 significantly. In the rhesus monkey the fractions of 

 total flow provided by these two sources are three 

 quarters and one quarter, respectively (14). For these 

 reasons the more reliable studies on cerebral circula- 

 tion have usually been performed in primates or in 

 man since studies in lower animals require an exten- 

 sive surgical exposure of the vessels with ligation of 

 those not involved in the cerebral circulation. 



The main arteries supplying the brain communicate 

 with each other in a system of anastomoses, desig- 

 nated the circle of Willis, involving fairly large-sized 

 vessels at the base of the brain. There is evidence, 

 however, that, under normal conditions, littlecommu- 

 nication occurs between the various arterial sources 

 at the circle of Willis (71) which is not unexpected 

 since the pressure in all of the contributing vessels 

 must be approximately the same. Experiments in 

 lower animals (71) and the experience of many 

 radiologists with cerebral arteriography in man (44) 

 leave little doubt that, for the most part, each cerebral 

 hemisphere is supplied by the carotid on that side, 

 while the structures in the posterior fossa receive 

 their blood supply from the basilar artery. There is 

 evidence that in man the circle of Willis functions in 

 emergencies, for when one internal carotid is ligated, 

 tin- pressure distal to tin- ligature is maintained at 

 approximately half its previous value because of the 

 contributions through the circle of Willis (107). In 

 young individuals, ligation of one internal carotid 



doe 1 cause a reduction in the total circulation to 



the I nain, although such a diminution may occur 

 and be associated with neurological symptoms in 

 elderly individuals where the component vessels of 

 the circle may be sclerotic and unable to dilate (100). 



The Capillary supply to the brain is rather rich 

 (8, 11), although not nearly as rich as that to other 

 organs such as the heart. In cerebral cortex there is 



an average of about 1000 capillaries per mm 2 of 

 [-section and in white matter about 300 ( 1 J, 15, 



1 1 j ), as COmp.iM-'l '" 1 1 Kite than -,ooo capillaries per 

 mm- in the myocardium. Studies of the capillary 



structure of brain reveal no evidence of arteriovenous 

 anastomoses (8, 24, 86, 112) such as occur in many 

 other tissues. 



In the cerebral venous system the blood flow tends 

 to lose its laterality to some extent (4) since the 

 superior and inferior sagittal sinuses drain blood from 

 both sides of the brain. The inferolateral surfaces of 

 the hemispheres, however, drain unilaterally through 

 the respective lateral sinus to the ipsilatcral internal 

 jugular vein, and the superior and inferior petrosal 

 sinuses drain directly into the ipsilateral jugular bulb. 



Because of its relevance to methods of measuring 

 cerebral blood flow, the question concerning the 

 validity of sampling a single internal jugular bulb 

 merits some attention (55). The superior sagittal 

 sinus appears to drain the cortex of both cerebral 

 hemispheres, continuing, in most individuals, as the 

 right lateral sinus and right internal jugular vein, 

 while the inferior sagittal sinus, draining the sub- 

 cortical masses, passes into the left internal jugular. 

 This has suggested to some (43) that in the average 

 man, blood in the right internal jugular may be pre- 

 dominantly of cortical origin while that in the left 

 arises from subcortical regions. A number of addi- 

 tional anatomical considerations, however, militate 

 against that conclusion. The superficial cortical veins 

 at their origin dip into subcortical Livers (10) so th.it 

 these, as well as the superior sagittal sinus which 

 drains them, must contain drainage from both white 

 and gray matter. In at least two thirds of individuals 

 there is a competent continence of the sinuses ( torcular 

 Herophili) (18, 30) in which a reasonable amount of 

 intermixing between the superior, inferior and the 

 occipital sinuses occurs 1 here are, in addition, cer- 

 tain anastomotic veins which cross from the superior 

 sagittal sinus over the surface of the hemispheres to 

 drain into the lateral sinus on each side, while the 

 inferior surface of the hemispheres together with the 

 cerebellum, pons and medulla drain into each internal 

 jugular by way of the lateral or petrosal sinuses A 

 large number of studies in man on samplings taken 

 simultaneously from both internal jugulars in the 

 waking state (31, 32, ;,",, 74, 89, 1 1 ; 1 .mil even under 

 anesthesia (43, 109) have disclosed no systematic pre- 

 ponderance of the right over the left with respect to 

 blood flow or oxygen consumption. Furthermore, 

 individual differences between the two side-, were 

 within or close to the experimental error in every 

 series save one (4 3), suggesting that for most purposes 

 values obtained bv sampling one jugular bulb are 

 adequately representative of the brain as a whole. 

 Where the best estimate in an individual case is re- 





