Page:Popular Science Monthly Volume 75.djvu/257

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253
ORIGIN OF THE NERVOUS SYSTEM

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

PROFESSOR OF ZOOLOGY, HARVARD UNIVERSITY

III. Central Nervous Organs

IN dealing with the differentiation of nervous organs, the earthworm affords a good example of a simple type of well-centralized nervous system. The central nervous organs in this animal (Fig. 1) consist of a brain or cerebral ganglion situated anteriorly and dorsal to the buccal cavity, right and left œsophageal connectives extending from the brain ventrally to the ventral nerve-cord which stretches as a segmented organ from near the anterior end of the worm over its ventral line posteriorly to the tail. The segments in the ventral cord agree in

PSM V75 D257 Head of an earthworm in longitudinal section.png
Fig. 1. Head of an Earthworm in longitudinal Section, b, brain; m, mouth; o, œsophagus; vn, ventral nerve-cord.

number and position with those of the worm's body and from each segment three pairs of nerves pass out to the integument and muscles of the adjacent region.

The essential nervous elements of the ventral cord can be made out in transverse sections (Fig. 2). In such sections the integument will be seen to be filled with sense-cells, each of which ends peripherally in a sensory bristle and gives rise centrally, in addition to a few sub-epithelial processes, to a single nerve-fiber which passes inward between the muscles and enters the ventral ganglion by one of its three nerves; finally this fiber spreads out in the fibrillar substance or neuropile of the ganglion. This cell-body in the integument with its processes including the nerve-fiber constitutes a primary sensory neurone. These neurones usually do not spread beyond the ganglion with which they are directly connected, but in exceptional cases they may extend into the ganglion anterior or posterior to this one.

In the ventral and lateral portions of each ganglion are numerous large nerve-cells from which coarse processes extend through the neuro-