92 NEKVOUS SYSTEM. 



of these general properties, in connection with their anatom- 

 ical distribution to the different organs in the economy. 



The points to be considered, aside from the simple divis- 

 ion 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 elec- 

 tricity to the nerves. 



Nervous Irritability. We have already alluded in a 

 general way to what is known as nervous irritability. 1 The 

 term is used by physiologists to express the condition of 

 nerves which enables them to respond to artificial stimula- 

 tion, or to conduct the natural stimulus or external impres- 

 sions. So long as a nerve retains this property it is said to 

 be irritable. Of course, while in a normal condition and dur- 

 ing 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 stimulation ; 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 is later in animals lower in the scale, so that the latter 

 present the most favorable conditions for experimentation. 

 Most observations on nervous irritability, indeed, have been 

 made upon frogs and other cold-blooded animals. Analo- 

 gous facts have already been noted with regard to the mus- 

 cular system, although, as we have seen, the irritability of 

 the muscular tissue is entirely distinct from that of the 

 nerves. 51 



Immediately or soon after death, when the irritability of 



1 See page 66. 2 See vol. iii., Movements, p. 464. 



