86 



THE POPULAR SCIENCE MONTHLY 



WHY ARE WE EIGHT-HANDED? 



By W. C. CAHALL, M. D. 



THE reader has no doubt often wondered why people almost inva- 

 riably use their right hand in preference to the left. Is it not 

 remarkable that, through all time and in all lands, man has been a 

 right-handed being ? The individual exceptions only prove the rule. 

 What is the reason ? It can not be simply imitation or heredity, for 

 in those children who are disposed to use the left hand these influences 

 will not avail in changing the inclination, even, in many instances, 

 when supplemented by persuasion or force. 



In my belief, there is a physical cause for this uniform habit a 

 cause that is demonstrable by anatomical and physiological facts. 

 These, for the sake of brevity, are expressed in the following state- 

 ments : 



1. The brain (cerebrum) is divided into two hemispheres. 



2. The nerve-force and nerve-fibers which produce muscular action 

 on the one side of the body have their origin in the opposite hemi- 

 sphere of the brain. 



3. The left hemisphere, from the earliest period, is larger and 

 heavier than its counterpart, and the convolutions of gray matter the 



reservoirs of nervous energy are more 

 numerous on this side than on the right. 



4. This superior development of the 

 left hemisphere as to weight, size, and 

 richness of convolutions, may be attrib- 

 uted to a peculiar arrangement of the 

 blood-vessels, by means of which a greater 

 blood-supply is distributed to the brain- 

 substance of this side. 



5. The arrangement of the blood-ves- 

 sels to which I refer is the manner of 

 origin of the right and left common carot- 

 id arteries. The carotid artery is a branch 

 of the innominate artery on the right side, 

 while it springs direct from the aorta on 

 the left. 



This directness of communication, in 

 addition to a larger caliber of the left 

 carotid, gives the left hemisphere a decided advantage in the race of 

 development. 



To reverse these statements we would have : as a consequence of 

 the greater capacity of the left carotid the left hemisphere of the brain 

 has a greater blood-supply ; as a consequence, there is a greater de- 



1, 2, 3, aorta. 



1. Ascending part, of aorta. 



2. Transverse part and arch of aorta 



3. Descending part of aorta. 



4. Innominate artery. 



5. Right subclavian artery. 



6. Right common carotid artery. 



7. Left common carotid artery. 



8. Left subclavian artery. 



