CHAPTER XVI. 



THE NERVOUS SYSTEM. 



THE nervous system consists of the following parts : firstly, of large 

 masses of nervous matter situated within the bony cranium and spinal 

 column, and constituting the brain and spinal cord; secondly, of 

 smaller masses of nervous matter, situated for the most part in the 

 abdominal and thoracic cavities, but also in the neck and head, and 

 constituting what are known as sympathetic ganglia; thirdly, of cords 

 of nerve-fibres which connect the central nervous system with the 

 periphery and with the so-called sympathetic ganglia, which are not in 

 reality a system independent of the brain and cord as was formerly 

 taught, but are really part and parcel of the same system ; and fourthly, 

 of peripheral organs in connection with the beginnings or endings of the 

 nerves at the periphery of the body. 



It w T ill be necessary to consider these several parts of the nervous system 

 seriatim; it will be most useful for the understanding of the subject, 

 however, to proceed first of all with the consideration of the properties 

 of nerve-fibres, as this forms the most elementary portion of the subject. 



Nerve-fibres. The structure of the different kinds of nerve-fibres 

 has been already dealt with (p. 91, et seq.) ; their function remains to be 

 considered here. 



FUNCTION OF NERVE -FIBRES. 



The office of nerve-fibres is to conduct impressions. From the 

 account of nervous action previously given (p. 52? et'fieq.) it will be 

 readily understood, that nerve-fibres may be stimulated to act by any- 

 thing which, with sufficient suddenness, increases their irritability; 

 they are incapable of originating of themselves the condition necessary 

 for the manifestation of their own energy. The stimulus produces it* 

 effect upon the termination of the nerve stimulated, being conducted to 

 it by the nerve-fibre. The effect of the stimulus upon a nerve therefore 

 depends upon the nature of its end-organ. A length of a nerve trunk 

 when freshly removed from the body, if stimulated midway between its 

 extremities, will, as shown by the deflection of the needle of a galvanometer 

 at either end, conduct the electrical impressions in either direction, and 

 it may be considered therefore only an accidental circumstance as it 

 were, whether when in situ it has conducted impressions to the central 



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