1907.] on Dexterity and the Bend Sinister: 64B 



but in the healthy human brain I have found it almost exactly alike 

 in the two hemispheres, 1036 in the frontal, 1038 in the parietal, 1040- 

 in the occipital and 1038 in the tempero-sphenoidal lobes. 



The next cause of right-handedness brought forward when it was 

 reahsed that the greater weight of the left hemisphere of the brain and 

 its higher specific gravity would not do, was vascular supply. The left 

 hemisphere, it was alleged, has a better blood supply than the right. 

 It is, therefore, better nourished, is functionally more active, and 

 therefore energises the right arm and hand more copiously than the 

 right hemisphere does the left. Of the two great arteries that supply 

 the fore part of the brain — the carotids — that on the left side rises 

 directly from the arch of the aorta, while that on the right branches 

 off the innominate artery, through which the blood for it and the sub- 

 clavian artery has been carried (Fig. 17). The jet from the left ventricle 

 of the heart to the brain is, therefore, more direct and less deflected 

 than on the right side ; and it has been inferred, therefore, that the 

 left hemisphere is better irrigated with arterial blood than the right : 

 but the important point is the calibre of the arteries that enter the 

 skull, and as regards that Professor Cunnyngham has found that 

 moulds taken of the carotid canal through which the internal carotid 

 enters gave 583|- square millimetres on the left and 583 on the right 

 side. Dr. Sidney Martin and I found that rings cut from the internal 

 carotid arteries within the skull gave in adult males an average 

 diameter of 2*8 mm. on the right and 2*75 mm. on the left side, 

 while the vertebral arteries supplying the hind brain gave an average 

 diameter of 2"2 mm. on the right and 2*75 on the left side. In view 

 of these facts and of the beautiful arrangement for the equahsation 

 of the cerebral circulation, called the Circle of Willis, it is clear the 

 arterial blood supply to the left hemisphere is not ampler than that 

 to the right, and that right and left-handedness cannot be connected 

 with cerebral circulation, especially as the vessels exist for the brain 

 and not the brain for the vessels. 



But apart from their weight and blood supply, there is a feature^ 

 and a prominent feature, in which the two hemispheres of the brain 

 differ from each other, and that is their convolutional development, 

 and through that we approach nearer to the fountain-head of dextral 

 and sinistral differentiation. The hemispheres in the higher animals 

 are folded or puckered, as it were, into ridges with hollows between 

 them, and thus the superficies, affording accommodation for the 

 cortical layer, grey matter or active substance is extended according 

 to the degree to which the pHcation is carried. In humble creatures 

 like the rabbit (Fig. 18), in which a Little intellect goes a long way, 

 the surface of the hemispheres is smooth without foldings, and then, 

 speaking generally, as we ascend in the scale of inteUigence through 

 the animal kingdom, the number and complexity of the convolutions 

 increases. It is in civilised man that the cerebral convolutions reach 

 their richest development. 



2 T 2 



