The origin of the nervous system and its appropriation of effectors / by G.H. Parker.

  • George Howard Parker
Date:
1909
Catalogue details

Licence: In copyright

Credit: The origin of the nervous system and its appropriation of effectors / by G.H. Parker. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    7' THE ORIGIN OF THE NERVOUS SYSTEM AND ITS APPROPRIATION OF EFFECTORS I. Independent Effectors1 By G. H. PARKER PROFESSOR OF ZOOLOGY, HARVARD UNIVERSITY THE physiological unit in the operations of the nervous system is the reflex. Broadly understood, this consists of the chain of consequences that begins with the reception of a stimulus on the sur- face of the animal and, leading through the central nervous organs, ends in the excitation of a reaction by some such organ as a muscle. The term reflex is made to apply nowadays to nervous operations involving conscious states as well as to those that are carried out uncon- sciously. In its greatest simplicity the conventional reflex involves at least two nervous cells or neurones and some form of reacting organ v3- Fig. 1. Transverse Section of the Ventral Nervous Chain and surround- ing Structures of an Earthworm (modified from Retzius). cm, circular muscle; ep, epidermis; 1m, longitudinal muscle; me, motor cell-body; mf, motor nerve-fiber; sc, sensory cell-body; sf, sensory nerve-fiber; vg, ventral ganglion. such as a muscle-fiber. The first neurone, as exemplified in the nervous structure of such an animal as the earthworm, is often the body of a sense-cell on the surface of the animal and the sensory nerve- fiber to which this cell body gives rise and which leads to the central nervous organ. The second neurone is a nerve-cell whose body lies within the central nervous organ and whose process, a motor nerve- fiber, extends from the central organ to the muscle-cells which it con- 1 The four articles in this series represent four lectures given at the University of Illinois between March 30 and April 3, 1909.
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