THE NERVOUS SYSTEM. 



557 



the result of the discharge as it were of energy from the centre, it may 

 be supposed that sometimes the centre is in a more explosive condition 

 than at another; this is shown for example in the case of a frog poisoned 

 by strychnine, when the slightest stimulus applied to the skin will pro- 

 duce the most violent and general tetanic spasms, while under ordinary 

 circumstances the contraction of a few muscles only would result. Wo 

 must also suppose that the centres are particularly sensitive to particu- 

 lar kinds of stimuli, sometimes producing very extensive and violent 



Fig. 349A. Showing the arrangement of a simple reflex mechanism composed of a motor and 

 sensory neuron, sg, Posterior spinal ganglion; sand sth, sensory root; m, motor nerve cell; mw, 

 motor root. 



muscular actions in response to a slight stimulus of a special kind. 

 Such a condition is illustrated in the violent and general muscular 

 spasms occurring when a small particle of food passes into the larynx, 

 violent expiratory spasms accompanied by contractions of other muscles 

 taking place. 



A nerve-centre must be considered as capable by its connections 

 with efferent nerves of producing most extensive muscular movements, 

 and when from any reason, either by the intensity of the afferent 

 stimuli reaching it, or by the special nature, extent, or point of appli- 

 cation of the afferent stimuli, or by special changes in its own metabol- 

 ism brought abont by poison or by some other means, a maximum dis- 

 charge takes place, the resulting movements are most extensive. Under 

 ordinary conditions, however, a slight stimulus produces, as above men- 



