360 POPULAR SCIENCE MONTHLY. 



DEXTRALITY AND SINISTRALITY. 



By De. GKORGE M. GOULD, 



PHILADELPHIA. 



rTIHE theories that have been advanced as to the origin of dextrality 

 -■- and sinistrality are: 



1. A natural provision. (Sir Charles Bell, and others.) 



2. The left-sided location of the heart. (Referred to by Wilson.) 



3. A greater supply of nerve force to the muscles because of an 

 earlier and greater development of the brain upon one side. (Pro- 

 fessor Gratiolet.) 



4. Obstruction to the flow of blood in the vena cava, by the pulsa- 

 tion of the aorta. (Dr. Barclay.) 



5. Inspiration produces, mechanically, a superior efficacy of the 

 muscles of the right side. (Professor Buchannan.) This theory is 

 based upon the observation of the anatomic peculiarities of the liver, 

 lungs, etc., and their supposed influence upon the center of gravity 

 of the body. (So far as pertains to the center of gravity, the theory 

 has been adopted by Dr. Struthers and by Dr. Allis.) 



6. The center of gravity theory. The influence of the weight of 

 the viscera of the two sides of the body, upon the position of the center 

 of gravity. (Dr. Struthers, accepted by Buchanan, Allis, etc.) 



7. The origin of the subclavian arteries, the left before the right 

 in the- left-handed, with superiority of blood-supply to certain struc- 

 tures. (Professor Hyrtl.) 



8. The development of one cerebral hemisphere more than the other. 

 (Wilson.) 



9. The Topsy theory — 'just growed.' 



These theories merit little argument in rebuttal. No. 3 and No. 8 

 are essentially the same, and, of course, are mere avoidances of an 

 explanation. No. 2, No. 4, No. 5, No. 6 and No. 7 are not based upon 

 facts, and contain fallacies of observation, rendering them at least 

 of insufficient reach and validity. No. 9 is almost as good as any 

 or all of the rest, and we are left with the frank confession of Dr. 

 Struthers, that the mystery 'has baffled satisfactory explanation.' 

 Carlyle said it was 'a question not to be settled, not worth asking 

 except as a kind of riddle.' It is, however, of great and practical 

 importance in medicine and in social life. 



In a large way and notwithstanding a certain number of exceptions, 

 it is an illuminating truth of biology that 'the ontogeny repeats the 



