56 



THE POPULAR SCIENCE MONTHLY 



THE OKIGIN OF THE KEKVOUS SYSTEM AND ITS 

 APPKOPKIATION OF EFFECTOKS 



I. Independent Effectors 1 



By G. H. PARKER 



PROFESSOR OP 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 



•Fig. 1. Transverse Section op the Ventral Nervous Chain and surround- 

 ing Structures op an Earthworm (modified from Retzius). cm, circular muscle; 

 ep, epidermis ; \m, longitudinal muscle ; mc, 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. 



