Page:Popular Science Monthly Volume 75.djvu/342

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338
THE POPULAR SCIENCE MONTHLY

THE ORIGIN OF THE NERVOUS SYSTEM AND ITS APPROPRIATION OF EFFECTORS
By G. H. PARKER

PROFESSOR OF ZOOLOGY, HARVARD UNIVERSITY

IV. The Appropriation of Effectors

IN the preceding articles in this series the origin and development of the neuromuscular mechanism has been broadly sketched in a succession of representative stages. The first stage was that of the independent effector, the muscle which was brought into action by the direct influence of environmental changes as seen in the pore sphincters of sponges. The second stage was that of the combined receptor and effector in which the receptors, in the form of diffuse sensory epithelia or specialized sense-organs, served as delicate triggers to set the muscles in action and thereby render the effectors responsive to a wider range of stimuli than they would be under independent stimulation. Finally, the third stage is seen in the complete neuromuscular mechanism in which a central nervous organ or adjustor has developed between the receptors and the effectors. This adjustor serves as a switchboard for nervous transmission and a repository for the effects of nervous activities.

This line of progressive differentiation from the muscle to the complete nervous system is complicated by the fact that in the more complete examples of the third stage the nervous system is found connected not only with such effectors as muscles, but with electric organs, chromatophores, glands, luminous organs, etc. If the history of the growth of the neuromuscular mechanism as it has been sketched in these articles is a correct one, the effectors just named must be regarded in the light of relatively recent acquisitions and in my opinion they illustrate an invasion and appropriation on the part of the nervous system of territory that was not originally under its control. This principle of appropriation results not only in the acquisition of totally new forms of effectors such as glands, etc., but also in gaining control over independently and newly developed muscles. Examples of this kind will be taken up first in discussing this question of nervous appropriation.

The differentiation of the central nervous organs is in large part a process that goes on hand in hand with the differentiation of the muscles. This is well seen not only in the higher invertebrates, but also in the vertebrates. The differentiation of a single muscle into a group of muscles and the consequent and corresponding changes in the nerv-