Bilateralism. 151 



cordingly, in all right handed persons the left hemisphere of the brain is 

 the largest, weighing, on an average, one-eighth of an ounce the most, 

 and very often it presides over the faculty of speech, becoming spokes- 

 man for both sides, to the exclusion of the right lobe. The two sides 

 of the several segments of the brain are connected with each other by 

 cross fibers called commissures, and, ordinarily, there is more or less 

 cooperation and correlation between the opposite sides of the brain. 

 But the two sides may and do differ in activity and function, as the two 

 hands differ. The actions of the individual ma} r be governed by the 

 strong side exclusively, or by the two alternately. One side may be 

 authority in some departments, the other in other departments, so that 

 in case of disease to one side the individual may be all right on some 

 subjects and quite insane on others. The eyes also often differ from 

 each other in size and in focal adjustment. 



Notwithstanding the mutual dependence of the two active sides of the 

 vertebrate body, there is, at the same time, a certain degree of inde- 

 pendence. In general, wherever the organs are double the animal func- 

 tions can be performed and life sustained for a time with one. Either 

 of the external senses may be active with only one eye or ear or nostril, 

 respiration may go on with a single lung or even less, reproduction with 

 a single ovary and testis, and mental action with a single cerebral hem- 

 isphere. 



One whole side of the body may be paralyzed, including one-half the 

 brain, without the total destruction of any function. 



The curious human habit of preferring the right hand to the left and 

 its consequences are paralleled by similar circumstances among other 

 vertebrates. A remarkable example is the Flat Fish family. They have 

 a habit of swimming near the bottom and on one side instead of the 

 belly. As this position brings one eye on the underside looking toward 

 the bottom, it is useless, and the fish from the habit of twisting the 

 head and neck to bring this eye into a useful position on top, has after 

 many generations of effort, assisted by the hereditary transmission of 

 each accumulation of deformity, succeeded in Bringing his optics around 

 almost the quarter of a circle. To do this the bones of the cranium and 

 face are twisted over and the median line, instead of being straight as in 

 most fishes, is distorted and crooked. This want of symmetry is often 

 shared by the backbone and limbs, one fin being larger than its mate. 



The Monochirus, a member of this family resembling the Sole, has 

 only one small pectoral fin on the same side with the eyes which is the 

 right side in this genus the opposite fin being aborted. Some of the 

 genera of the Flat fishes usually lie on the right, others on the left side, 

 and the eyes are on the top side, whichever that is. 



In the Soles the eyes are on the right side, the mouth is twisted 



