42 Proceedings of the Royal Irish Academy. 



left arm than what proceeds to the right. If, therefore, the size and 

 activity of an organ depend, as asserted with respect to the hemi- 

 spheres, on its blood supply, it would follow from the structure- 

 argument that in right-handed people the left hand is, per se, the 

 stronger and more active ! 



It might possibly, but improbably, be argued that the influence of 

 the left hemisphere counteracts this left lateral strength. But if the 

 hemispheres are removed dogs can yet walk, and birds fly, and fi'ogs 

 swim, on being stirred by external stimulus, whence it is inferred that, 

 though the seat of the volition be in the hemispheres, these do not 

 impart the power of motion. The cerebrum being concerned with 

 Tolition, a greater size and complexity of one of its hemispheres may 

 certainly indicate greater volitional activity there ; but we should look 

 further down, to the brachial centres, in order to discover if there be 

 inequality here discernible. That these are correlated with use of the 

 fore-extremities is declared by the extraordinary size of the myelo- 

 brachial enlargement in birds, which depend on their fore-arms to 

 raise and carry the entire burthen of their bodies, in birds, which 

 walk, the pelvic enlargement is the greater of the two. It is true that 

 the corpus striatum is. unusually large in birds, forming the greater 

 bulk of the hemisphei'e ; and being, according to one theory, the motor 

 terminal, it might have attracted notice in this discussion. But there 

 is no allegation that the corpora striata are of unequal sizes in the 

 human subject, nor has histological research taken pai'ticular notice 

 of inequality in the cornua of the cervical enlargement, which might, 

 however, reasonably be expected. Use might result in augmentation, 

 but that use would not, in the spinal cord, be the consequence of un- 

 equal blood-supply, but the effect of volition, habit, and heredity. 



There is, besides, another and more emphatic way of meeting 

 all possible and impossible objections in reference to this question, 

 which I will now state : — If right-handedness in man be due to the 

 fact that one hemisphere of his brain is better supplied with blood 

 than the other — if it be due, in any way whatever, to the mode in 

 which the brachial and cephalic arteries come off from the aorta, then 

 this conclusion is imperative : in every other animal, if any other there 

 be, in which a similar vascular arrangement is found, there must be 

 a similar exhibition of dextral predominance, or right-handedness. If 

 not, then dextral predominance does not depend on vascular arrange- 

 ment. ]N"ow, what are the animals which come into the same group with 

 man , when classed by the standard of arterial identity in this region ? 

 They are : Monotremata (Oruithorynchus), many Marsupials (Phas- 

 colomys, Wombat), the Edentata (Bradypus, Dasypus), Hyperoodon, 

 and Whales, Beavers, Eats, most claviculate Rodents, Seals, Prosimians 

 (Tarsius), and the Chimpanzee.^*, ^^ To these must be added the 



1* Owen: Comp. Anat. and Fhys. of Vertebrates, vol. iii. p. 535. 

 1^ Gegenbaur : Comp. Anat. § 244. 



