594 NERVOUS SYSTEM. 



General Properties of the Nerves. 



Numerous experiments have been made, especially upon the cerebro-spinal nerves, 

 with regard to their action under different kinds of stimulation, the probable nature of 

 the nervous agent or nerve-force, the extent and duration of their excitability and sensi- 

 bility, etc., which have developed facts of more or less physiological interest and impor- 

 tance. As far as the nerves of general sensibility are concerned, the phenomena of con- 

 duction of impressions are essentially the same in all, if we except certain variations in 

 different nerves as regards the degree of sensibility. The motor nerves all respond in 

 the same manner to stimulation ; and it is upon this portion of the nervous system that 

 the most important observations have been made. This being the case, it is evident that 

 the cerebro-spinal nerves, in their behavior under the experimental conditions above 

 mentioned, possess certain general properties, and that the functions of special nerves 

 are to be studied, after a full consideration of these general properties, in connection 

 with their anatomical distribution to the different organs in the economy. 



The points to be considered, aside from the simple division of the nerves into motor 

 and sensory, are as follows : 



1. The conditions of excitability and sensibility of the nerves, or what is known as 

 nervous irritability. 



2. The nature of the nervous agent, or the so-called nerve-force. 



3. Certain phenomena following the application of electricity to the nerves. 



Nervous Irritability. We have already alluded in a general way to what is known 

 as nervous irritability. The term is used by physiologists to express the condition of 

 nerves which enables them to respond to artificial stimulation, or to conduct the natural 

 stimulus or external impressions. So long as a nerve retains this property it is said to 

 be irritable. Of course, while in a normal condition and during life, irritability, as 

 applied to nerves, simply means that these parts are capable of performing their peculiar 

 functions ; but, after death, for a certain time the nerves will respond to artificial stimu- 

 lation ; and it is to this property that the term " irritability " seems to be most applicable. 

 At a certain time after death, varying in different classes of animals with the activity of 

 their nutrition, the irritability of the nerves disappears. This occurs very soon in warm- 

 blooded animals, but it is later in animals lower in the scale, so that the latter present 

 the most favorable conditions for experimentation. Most observations upon nervous irri- 

 tability, indeed, have been made upon frogs and other cold-blooded animals. Analogous 

 facts have already been noted with regard to the muscular system, although, as we have 

 seen, the irritability of the muscular tissue is entirely distinct from that of the nerves. 



Immediately or soon after death, when the irritability of the nerves is at its maxi- 

 mum, they may be excited by mechanical, chemical, or galvanic stimulus, all of these 

 agents producing contraction of the muscles to which the motor filaments are distributed. 

 Mechanical irritation, simply pinching a portion of the nerve, for example, produces a 

 single muscular contraction ; but, if the injury to the nerve be such as to disorganize its 

 fibres, that portion of the nerve will no longer conduct a stimulus. Among the irritants 

 of this kind, we may cite the extremes of heat and cold. If an exposed nerve be cau- 

 terized, a vigorous muscular contraction follows. The same effect, though less marked, 

 may be produced by the sudden application of intense cold. Among chemical reagents, 

 there are some which excite the nerves and others which produce no effect; but these 

 are not important from a physiological point of view. Suffice it to say, that mechanical 

 irritation and the action of certain chemicals are capable of exciting the nerves ; but 

 that, when their action goes so far as to disorganize the fibres, the conducting power of 

 these fibres is lost. While, however, irritation of the nerve above the point of injury has 

 no effect, stimulation between this point and the muscles is still followed by contraction. 



The most convenient method of exciting the nerves in physiological experiments is 



