THE NERVOUS SYSTEM—GENERAL CONSIDERATIONS. 915 
tain than ever, so that the result stated above (3) must not be 
interpreted too rigidly. 
Similar doubts hang about the other cases of apparent au- 
tomatism. 
As regards the various comparatively isolated collections of 
cells known as ganglia, the evidence, so far as it goes, is against 
their possessing either automatic or reflex action; and new 
views of their nature will be presented in due course. 
Nervous Inhibition. — If the pneumogastric nerve passing 
from the medulla to the heart of vertebrates be divided and 
the lower (peripheral) end stimulated, a decided change in the 
action of the heart follows, which may be in the direction of 
weakening or slowing, or positive arrest of its action. 
Assuming, for the present, that the cells (center) of the me- 
dulla have the power to bring about the same result, it is seen 
that such nervous influence is preventive or inhibitory of the 
normal cardiac beat, so that the vagus is termed an inhibitory 
nerve. Such inhibition plays a very important part in the 
economy of the higher animals, as will become more and more 
evident as we proceed. The nature of the influences that pro- 
duce such remarkable results will be discussed when we treat 
of the heart. 
An illustration will probably serve in the mean time to make 
the meaning of what has been presented in this chapter more 
clear and readily grasped. 
In the management of railroads a very great variety of 
complicated results are brought about, owing to system and 
orderly arrangement, by which the wishes of the chief mana- 
ger are carried out. 
Telegraphing is of necessity extensively employed. Sup- 
pose a message to be conveyed from one office to another, this 
may (1) simply pass through an intermediate office, without 
special cognizance from the operator in charge; (2) the operator 
may receive and transmit it unaltered ; (3) he may be required 
to send a message that shall vary from the one he receives in a 
greater or less degree ; or (4) he may arrest the command alto- 
gether, owing to the facts which he alone knows and upon. 
which he is empowered always to act according to his best dis- 
cretion. 
In the first instance, we have an analogy with the passage 
of a nervous impulse through central fibers, or, at all events, 
unaffected by cells; in the second, the resemblance is to cells 
acting as conductors merely ; in the third, to the usual behavior 
