CHAPTER I. 



THE HISTORY OF THE INVOLUNTARY NERVOUS SYSTEM. 



THE muscular tissue of the body is divisible into two well- 

 marked groups, the voluntary muscles and the involuntary 

 muscles ; terms which in themselves imply a distinction between 

 the nervous arrangements connected with these two groups of 

 muscles. We may therefore look upon the nervous system as 

 composed of two parts, the one connected with the movements 

 of voluntary muscles, and the other with those of involuntary 

 muscles. The former part may be termed shortly the voluntary 

 nervous system ; and the latter, the involuntary nervous system. 

 Our knowledge of the methods, by which sensory stimuli are 

 converted into action, is chiefly based upon investigations of the 

 voluntary nervous system ; and has led to the conception that 

 the nervous system is built up of a series of neurons, through 

 which all the complicated movements can be effected. In the 

 vertebrate a reflex action in the spinal cord is caused by a 

 sensory stimulus, which travels from the periphery along a 

 sensory nerve to a nerve cell in the ganglion on a posterior root, 

 and thence by a root fibre to make communication with some 

 cell or cells in the central nervous system itself. This primary 

 or sensory neuron has been called the receptor element, and its 

 parts consist of a sensory nerve cell, a sensory nerve fibre and a 

 sensory root fibre ; the last is not confined to its own segment in 

 the cord, but communicates with other segments, some near, 

 some far away. In all cases these root fibres ultimately make 

 connexion with another neuron. In the simplest case this other 

 neuron may consist of a motor cell with dendrites and with an axon 

 which goes direct to the muscles and forms its motor nerve fibre. 

 This neuron has been called the excitor element. However, in 

 most cases, possibly in all, the receptor element does not connect 

 directly with the excitor element, but an intermediate neuron in 



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