UPON THE SERIES OP PREHISTORIC CRANIA. 671 



of the brains of human beings of various known capabilities and 

 ages with each other and with the brains of the lower animals 

 which are nearest to man casts a diffused, if not a concen- 

 trated light upon the entire enquiry. Arguments bearing upon 

 this question may be procurable from other sources ; of the four 

 specified the first furnishes us with the least ambiguous and the 

 most readily verifiable indications ; with these the indications fur- 

 nished by the other three, if not connected in the way of effect with 

 cause, are at all events correlated in the way of concomitant growth. 



A survey of the distribution of the several arteries 1 supplying the 

 cerebrum appears to show unmistakably that the particular zone of 

 convolutions in question is at unmistakable disadvantage in the 

 matter of irrigation as compared with the segments of the hemi- 

 sphere which lie in front of it, and that of two brains of equal or 

 approximately equal length that one is the more favourably con- 

 ditioned which has this segment contributing the smaller factor 

 towards making up its total length. 



The belt of convolutions which interposes itself between the line 

 of the lambdoid suture and another line drawn parallel to either half 

 of that suture over the parietal tuberosity of that side receives its 

 main arterial supply from the terminal twigs of that branch of the 

 internal carotid which is known in this country as the ' middle 

 cerebral ' artery, but which for the present purpose might bear one 

 of its foreign names, viz. ' arteria fossa Sylvii* For it is only after 

 having supplied the very numerous and extensive convolutions 

 which form the floor and the walls and the margins of this great 

 fossa that terminal branches of this artery emerge on to the ex- 

 terior convex surface of the brain and distribute themselves to this 

 zone of the hemisphere. In following up the distribution of the 

 two terminal branches of the carotid we are impressed with the 

 differentially favourable condition as regards vascular irrigation of 



1 Some difference of opinion exists between the two investigators, MM. Duret and 

 Heubner, who have (Archives de Physiologic Normale, 1874, torn, vi., and Centralblatt 

 fur die Med. Wiss., 1872, Die luetische Erkrankung der Hirnarterien, 1874, pp. 

 172-175) been investigating the cerebral circulation, as to the degree of freedom with 

 which the arteries in question anastomose with each other. Heubner however, who 

 specially insists upon the formation in the pia mater of a common retif orin reservoir 

 by all the brain-supplying arteries in a common solidarity, nevertheless allows, as the 

 facts of pathological embolism even more than those of experimental injection compel 

 him, that the different parts of this reticulation are filled from the different main 

 arteries with differing facility ; ' von dem entfernterem naturlich scJiwerer und lang- 

 samer als von dem naheren' This is all that need be asked for justifying the argu- 

 ment in the text. 



