WHY ARE WE RIGHT-HANDED ? 87 



velopment of the left hemisphere as to weight, bulk, and number of 

 convolutions ; as a consequence, when there is need of muscular ac- 

 tion, the child naturally uses those muscles which possess the more 

 powerful nerve-supply, for muscles are only strong in proportion to 

 their nerve-supply ; as a consequence, the nervous energy is dispatched, 

 in those cases where there can be a choice, from the left hemisphere ; 

 as a consequence, the right hand and right leg will be the more likely 

 used, since this side of the body is innervated by the left hemisphere. 

 Thus, predisposition primarily, and use afterward, influencing and 

 strengthening each other, fix upon us a habit almost unchangeable 

 how firmly, let those who ever attempted to break the habit in a left- 

 handed boy testify. 



But this leads to another question. Why are there left-handed 

 people ? Before we answer this question we will again look at the 

 diagram. We have seen that, as the aorta rises from the heart, it 

 arches from right to left, and the first large artery it gives off is the 

 innominate, which in turn is divided into the right common carotid 

 and right subclavian arteries. Farther on, we find the left common 

 carotid and the left subclavian ai'teries arising separately from the 

 aorta. Now, in making their dissections, anatomists have found that 

 in a certain proportion of their subjects the aorta arches from left to 

 right, in which cases the innominate is on the left side, and the com- 

 mon carotid and subclavian separate on the right. This arrangement 

 would favor the growth of the right hemisphere, and would predispose 

 to the use of the left hand. 



Unfortunately, there have been no post-mortem examinations made 

 for the purpose of observing whether this arrangement of blood-ves- 

 sels and the use of the left hand really do occur in the same individual, 

 nor is it necessary that it should be found in every case, for there are 

 other anomalies in vessel-branching which would favor the growth of 

 the right hemisphere. Apropos of speaking of the preponderance of 

 the right over the left hemisphere, it might not be amiss to mention 

 here that recent investigations have shown this condition of the brain 

 to be characteristic of certain forms of insanity. This does not prove, 

 however, that because a person is left-handed he is necessarily in any 

 degree insane, as some dexterous reader may superciliously infer. Now, 

 if the reason of our choice of a hand is due to an organic cause, how 

 unwise is it to fight against nature, unless we commence at the be- 

 ginning, and trust that habit will overcome the predisposition to the 

 use of the left hand ! Undertaken later, the result is often to spoil 

 the skill of the left hand, without training the right to do its work as 

 well. In conclusion, from what we have seen above, in answer to the 

 question, Why are we right-handed ? it might be said, because we are 

 left-headed. 



