114 INHIBITION. [BooK i. CHAP. in. 



which the muscular walls of various arteries are relaxed ; or 

 augmented, whereby the tonic contraction of various arteries is 

 increased. 



The most striking instance of inhibition is offered by the heart. 

 If when the heart is beating well and regularly, the pneumogastric 

 be divided, and the peripheral portion be stimulated even for a very 

 short time with an interrupted current, the heart is immediately 

 brought to a standstill. Its beats are arrested, it lies perfectly 

 flaccid and motionless, and it is not till after some little time that 

 it recommences its beat. Here again it is usually said that the 

 pneumogastric contains efferent cardio-inhibitory fibres, impulses 

 passing along which from the medulla stop the automatic actions 

 of the cardiac ganglia; the respiratory inhibitory fibres of the 

 same nerve are afferent, i.e. impulses pass along them up to the 

 medulla. 



Though inhibition is most clearly seen in the case of automatic 

 actions, other actions may be similarly inhibited. Thus, as we 

 shall see later on, the reflex actions of the spinal cord may, by 

 appropriate means, be inhibited. 



To sum up, then, the most fundamental properties of nervous 

 tissues. 



Nerve-fibres are concerned in the propagation only, not in the 

 origination or transformation, of nervous impulses. As far as is at 

 present known, impulses are propagated in the same manner along 

 both sensory and motor nerves. Sensory impulses differ from 

 motor impulses inasmuch as the former are generated in sensory 

 organs and pass up to the central nervous cells, while the latter 

 pass from the central nervous cells to the muscles or to some 

 other peripheral organs. 



The operations of the nerve-cells are either automatic or reflex. 

 In both an automatic and a reflex action, the diversity and the 

 co-ordination of the impulses are determined by the condition of 

 the nerve-cells. During the passage of an impulse along a nerve- 

 fibre, there is no augmentation of energy ; in passing through a 

 nerve-cell, the augmentation may be, and generally is, most con- 

 siderable. 



When afferent impulses reach a centre already in action, the 

 activity of that centre may, according to circumstances, be either 

 depressed or exalted, may be ' inhibited ' or ' augmented.' 



