CHAP. VII.] ACTION OF THE SPHINCTERS. 191 



resistances must be brought into relation with the muscular sense 

 through the organ of touch, which is adapted to this purpose by 

 its superficial position on the body. But the powers of the 

 muscular sense, isolated from tact, are exhibited, in its enabling 

 one to estimate the weight of a tumour developed in the interior 

 of the limb, and in general the resistance afforded by the weight 

 of one part of the body, or the action of one muscle or set of mus- 

 cles, to that of another. Hence a principal source of the marvellous 

 power which all animals possess of associating the various parts 

 of their bodies in numberless combinations of harmonious move- 

 ment. 



Of some Varieties of Muscular Movement. Having described the 

 differences between the movements of active and passive contrac- 

 tion, we shall now be more able to refer to their proper causes 

 those varieties of movement by which certain muscles or classes 

 of muscles are distinguished. In briefly adverting to these, we 

 shall have to glance at some collateral considerations regarding 

 the mode of their connexion with the nervous system, which can- 

 not be fully understood without reference to what will be after- 

 wards said under that head. 



The Action of the Sphincters of the anus and bladder seems, at 

 first, peculiar. They are constantly contracted, except during the 

 passage of the contents ; and yet no fatigue attends this persistent 

 action. The explanation is very simple. They remain contracted 

 unless the contained matters are forced within them by a superior 

 power. Now, their mass, and therefore their contractility, is supe- 

 rior to that of the wall of the cavity above ; consequently their 

 passive contraction endures while that of the parts above is being 

 gradually mastered by the accumulation of the faeces or urine. But, 

 when these excretions at length excite active contraction in the 

 walls of the cavity containing them, this overcomes the passive 

 contraction of the sphincters, and the evacuation occurs. The 

 sphincters have striped fibres and voluntary nerves, by means 

 of which we can for a time add active to passive contraction, and 

 thus retard the expulsion ; but, as the accumulation proceeds, this 

 power is diminished or lost, and the sphincters yield. The levator 

 and sphincter ani frequently aid the accumulation of the faeces 

 by temporary active contractions, by which the faeces tending to 

 dilate the sphincter are pushed backwards for a while. The rectum 

 is thus preserved empty until the period immediately preceding 

 defaecation. 



In paralysis of the lower part of the body from disease or injury 



