CHAPTEE XVII 



THE AUTONOMIC NERVOUS SYSTEM 



ALLUSION has frequently been made in the foregoing chapters to 

 sympathetic nerves. These nerves govern the processes in the body 

 over which we have no voluntary control. They innervate cardiac 

 muscle, the plain muscle in the walls of blood-vessels, and in the 

 walls of other contractile viscera such as the stomach and intestine, 

 the bladder, and the organs of generation. Secretory nerve-fibres 

 also come into the same category. In the chapters which im- 

 mediately follow this one, we shall be studying such organs, organs 

 which carry on the vegetative functions of life as it was formerly the 

 custom to call them. It is therefore desirable that, at the outset, we 

 should obtain some general idea of the nervous mechanism involved 

 in controlling and regulating these functions. 



The sympathetic system proper consists of a chain of ganglia or 

 collections of nerve-cells, situated on each side of the vertebral 

 column. These ganglia correspond roughly with the spinal seg- 

 ments; the uppermost is called the superior cervical ganglion, and the 

 next the inferior cervical ganglion ; these are the only two ganglia in 

 the cervical region in the dog ; in man there is a middle cervical 

 ganglion in addition. The inferior cervical ganglion is connected to 

 the first thoracic ganglion (a large ganglion sometimes called the 

 ganglion stellatum) by fibres, some of which go in front of, and others 

 behind the subclavian artery; this ring around the artery is called 

 the annulus of Vieussens ; after this the correspondence of the 

 ganglia to the spinal nerve roots is more exact, and we finally reach 

 the ganglion at the end of the chain, the ganglion coccygeum. 



All these ganglia (with the possible exception in some animals of 

 the inferior cervical ganglion) send bundles of nerve-fibres to the 

 spinal nerves, and the communicating strands between the ganglia 

 and the spinal nerves are termed the rami communicantes. The 

 rami communicantes are divided into white and grey. The white 

 rami consist of medullated fibres of small diameter ; the grey rami 

 consist mainly of non-medullated nerve-fibres. 



The sympathetic chain, then, is a system of ganglia longitudinally 



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