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Page:Popular Science Monthly Volume 44.djvu/628

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urged as reasons for right-handedness are not present; animals do not carry their young, nor pat them to sleep, nor do animals shake hands! It must therefore be shown that animals are right or left handed, or that they differ in some marked respect in regard to function, in their nervous make-up, from man. Admitting the need of meeting these requirements; admitting again that we have little evidence that animals are dextral in their functions; admitting also the known results as to the control of the two halves of the muscular system by the opposite brain hemispheres respectively; admitting further that the motor speech function is performed by the hemisphere which controls the stronger side of the body, and is adjacent to the motor arm center in that hemisphere; and admitting, finally, that the speech function is one in which the animals have little share—all these admissions lead us at once to the view that there is a fundamental connection between the rise of speech and the rise of right-handedness.[1]

Looking broadly at the methods of nervous and muscular development, and accepting all the results of neurology we are able to gather, we may say that in the differentiation of functions in the animal series certain principles may be recognized: 1. The deep-seated vital functions represent least nervous differentiation, as is seen in the simple organs known as the lower nervous centers. 2. New unsymmetrical functions give a differential or twofold organic development, the great instance of which is found in the cerebral hemispheres. 3. New symmetrical or unilateral functions find their counterpart each in one of three kinds of nervous adaptation: (a) co-ordination of the hemispheres in a single function—i. e., functions which are crippled if either hemisphere is damaged; (b) co-ordination of particular functions in each hemisphere—i. e., functions which are not crippled unless both hemispheres are damaged) and (c) co-ordination of particular functions in one hemisphere only—i. e., functions which are crippled if one selected hemisphere is damaged. All these kinds of co-ordination exist.

It is easy to see that both speech and right-handed function belong under the last head of the last class—co-ordinations of particular functions in one hemisphere only—and that they belong in the same hemisphere. Why is this? What have they in common?

A very essential kind of hand movements are the so-called "expressive" movements, meaning those which serve to convey a meaning, or express a state of consciousness. Of course, speech is

  1. This much has been before surmised (see Mazel, Revue Scientifique, 1892, i, p. 113). He makes no attempt, however, to account for the association, except by calling both functions expressive.