244 Preference of the Right Arm? [Book IX. 



claviah and carotid. The refiftance being fmaller, the 

 impetus and velocity pf.the'blood muft be leis affedted ; 

 and as the ftrength of the mufcles is as the quantity of 

 blood fent into them in a given time, thofe of the right 

 arm will be ftronger than thofe of the left. This there- 

 fore accounts in fome meafure for the preference which 

 is generally given to the right arm, though it muft be 

 acknowledged that it is difficult, from this reafoning, 

 to account for the preference which fome children give 

 to the left. The right fubclavian and carotid fome- 

 times arife feparately like die left, but it has not been 

 afcertained that this exception to the ufual ftrufture 

 happens more frequently in left than in right handed 

 perfons. In quadrupeds we obferve fomething of 

 the fame preference of the right limbs, and attended 

 with the fame diftribution of the arteries. In birds, 

 which muft be nicely balanced, the arteries of both 

 fides come off alike. 



The two carotid arteries proceed upwards on each 

 fide of the windpipe, behind the fterno- cleidomaftoi- 

 deus mufcle, and the platifma myoides, as high as the 

 larynx, without a divifion. About this part the carotid 

 artery is divided into two others, called external and 

 internal carotid arteries. The external carotid fupplies 

 the parts about the larynx, the face, the external parts 

 of the head and the dura mater. The other divifion 

 of the carotid is diftributed almoft entirely on the brain, 

 and is therefore called the internal carotid artery. It 

 firft proceeds to the lower orifice of the great canal of 

 the pars petrofum of the temporal bone. After being 

 contorted according to the courfe of this pafiage, it 

 at length enters the cavity of* the cranium, at the fide 

 of the fella turcica. As it leaves the bony canal^ it 

 fends off an artery, which fupplies the contents of the 



orbit* 



