558 HANDBOOK OF PHYSIOLOGY. 



tioned only a moderate discharge from the centre, the movement being 

 centre may be not only not to set it into activity, but to prevent or 

 stop an action already going on. On the other hand, the action of 

 afferent impulses upon a nerve-centre may be to augment, render more 

 powerful or extensive, and increase in a certain direction an action 

 already in course. Such may be well illustrated by the action of the 

 to a certain extent co-extensive with the strength of the stimulus. 



The time taken in a reflex action has been found to be .066 to .058 

 second, but this is only a rough and arbitrary estimation. 



Automatism. A second function which appears to be possessed In- 

 certain nerve-centres and not by others is that of automatic action or 

 automatism. By this is meant that it is not dependent for its discharge 

 upon any afferent stimuli, but that it is capable of sending out of itself- 

 efferent impulses of various kinds. The centre may be supposed to do 

 this by the nature of its own metabolism, anabolism or building up of the 

 explosive substance being followed by katabolism or its discharge. So 

 that the centre sends out its impulses to muscles rhythmically. Such 

 a power of automatism we have seen is attributed to the respiratory cen- 

 tres in the bulb. 



Inhibition and Augmentation. Not only may movements of 

 muscles, discharge of secretion from gland-cells and the like be produced 

 by afferent impulses reaching nerve-centres, but also inhibition of action 

 which is already taking place. This is well seen in the matter of the 

 inhibitory action of the vagus upon the cardiac contractions. The vagi 

 convey to the heart impulses from the cardio-inhibitory centres which 

 have a restraining action upon the contractions of the heart, as is seen 

 by the increase in the frequency of the heart-beats when the vagi are 

 divided; but we have seen that appropriate afferent stimuli, as, for 

 example, when applied to the abdominal sympathetic, may increase the 

 action of the centre to such an extent that the heart may be altogether 

 stopped in diastole. In such a case the result of the afferent stimuli 

 upon the centre has been to produce complete inhibition and not mus- 

 cular contraction. This is not the only example of inhibition which 

 might be instanced; the action of almost any centre may be inhibited 

 by impulses reaching it; indeed the effect of afferent impulses upon a 

 vagi upon the respiratory centres to which attention has been drawn in 

 the chapter upon respiration. 



Membranes of the Brain and Spinal Cord. The Brain and Spinal Cord are 

 enveloped in three membranes (1) the Dura Mater, (2) the Arachnoid, (3) 

 the Pia Mater. 



(1) The Dura Mater, or external covering, is a tough membrane composed of 

 bundles of connective-tissue which cross at various angles, and in whose inter- 

 stices branched connective- tissue corpuscles lie : it is lined by a thin elastic mem- 

 brane, and on the inner surface and where it is not adherent to the bone, on the 



