SiGERSON — On Dextt-al Pre-eminence. 41 



II. The arguments from structure require particular attention. 

 In 1810 it was first laid clown, so far as I have ascertained, that the 

 preferential use of the right hand is due to a natural peculiarity in 

 the form of the right subclavian and carotid arteries, and that a simi- 

 lar preference is traceable in some dogs and horses as regards the fore 

 extremity. ^^ This statement was omitted from subsequent editions of 

 the Encyclofcedia. In 1871 Dr. Ogle gave a development of this 

 theory. Maintaining that the left hemisphere of the brain is pre- 

 eminently more complex in right-handed people; and conversely, 

 that the right hemisphere predominates in left-handed persons, he 

 argued that the cause of the greater development, as a rule, of the 

 left hemisphere probably depends on its receiving a freer supply of 

 blood than the right. He observes that the left arteries of the neck 

 are, as a rule, slightly larger than the right, and that (independently 

 of the size of the vessels) the stream of blood is less hindered on the 

 left side than on the right. Man, he added, is not the only animal 

 ■with a tendency to use one side preferentially, as monkeys and parrots 

 also exhibit it. This structural explanation Dr. Ogle regarded as 

 corroborated by the peculiarities of the cerebral blood supply in such 

 animals. ^^ 



JSTow, the anastomoses of the vertebrals and carotids at the base of 

 the brain must mitigate, if not eliminate, any lateral inequality ; but 

 that inequality, if it exist, has appeared to some anatomists to favour 

 the right side, not the left. Thus, Mr. Erasmus Wilson, describing 

 the common carotids, says — " It follows, therefore, that the right 

 carotid is shorter than the left ; it is also more anterior, and, in con- 

 sequence of proceeding from a branch instead of from the main trunk, 

 it is larger than its fellow." Under such circumstances, the right 

 hemisphere should have the better blood supply. It is to be remarked 

 that no notice is taken of the compensatory action of the vertebrals, 

 any more than of the interposition of the Circle of "Willis. 



The second branch of the argument deserves peculiar attention. 

 It is asserted that the preferential use of the right extremity depends 

 upon the mode of origin of the brachial and cephalic arteries. In 

 man (usually) there is a right brachio-cephalic trunk which bifurcates 

 into the right common carotid and right brachial, or sub-clavian ; on 

 the left, the common carotid comes off direct from the arch of the 

 aorta, so likewise does the left brachial. Now, if we admit that, on 

 account of this arrangement, the left side of the head receives a freer 

 and fuller supply of blood than the right side, we must logically go 

 further, and assert that not only does the right side of the head receive 

 a less free and full supply of blood than the left, but also (be it noted) 

 we must maintain that a freer and fuller supply of blood goes to the 



^^ Encyclopcedia Britannica, 1810: Art. — Comparative Anatomy — cited by Mr. 

 Pearson, of Emmanuel College — Lancet, November 27, 1875. 

 '3 Lancet, July 8, 1871. 



R. I.A. PROC, SEE. II., VOL. IV. — SCIENCE. G 



