THE RABBIT. 91 



said to be afferent or sensory ; if entirely of efferent fibres, it 

 is efferent. Most nerves contain both kinds and are called 

 mixed. 



Afferent fibres have their peripheral terminations in 

 sense-organs that is, in organs which are affected by 

 external influences such as light, change of temperature, 

 sound-waves, contact of external bodies, pressure, etc. ; and 

 the afferent fibres convey impulses set up by these external 

 agents to the central nervous system. Efferent fibres, 

 on the other hand, end in connexion with muscle-fibres, 

 gland-cells, and other cells which do work of any kind, 

 and convey the impulses which cause them to contract, 

 secrete, etc., or, in the case of unstriped muscle-fibres, 

 impulses that tend to increase or to decrease their rhythmical 

 contraction. Efferent fibres and efferent nerves are often 

 called "motor," but this is a misleading term. Efferent 

 fibres are (1) motor, when distributed to striped muscles, 

 (2) accelerator, or (3) inhibitor when distributed to un- 

 striped muscles, (4) secretory when distributed to glands. 

 Even this does not quite exhaust the varieties of efferent 

 nerve-fibres, but it covers most of them. 



10. Nerve-impulses. A muscle-fibre or gland-cell 

 may be compared to the charge of explosive in a loaded 

 gun : it contains a store of energy and is capable of doing 

 a definite piece of work, but in order to start it to work 

 some relatively small amount of work the pulling of the 

 trigger must first be done upon it by an external agent. 

 So a muscle (or, at least, a striped muscle) will not contract 

 until it receives a stimulus from without, which, as it were, 

 makes it explode. Effective stimuli of various kinds can 

 be artificially applied to muscles, a sharp blow on the 

 bare muscle, a drop of acid, or an electric discharge. But 

 under normal circumstances the stimulus is always an 

 impulse sent from some nerve cell along its axis-cylinder, 

 which ends in a fine ramification over the surface of tho 

 muscle-fibre. 



What the nature of this propagation of a stimulus may 

 be we do not know. It may perhaps be comparable to the 

 firing of a train of gunpowder, if we could imagine only 



