CHAP. XI.] ACTION OF THE SPHINCTER. 337 



will nevertheless be paralysed. In the former instances, the centre 

 of volition in the cranium is diseased ; in the latter, the defect con- 

 sists in the destruction of the communication of the brain with that 

 portion of the cord in which the nerves of the sphincter muscle are 

 implanted. 



An examination of the action of the sphincter will shew that the 

 anus is kept closed ordinarily by the passive contraction of the muscle 

 itself (see p. 191); but that its active contractions are mainly 

 excited by voluntary influence, allowance being made for some slight 

 action which may be produced by the stimulus of sudden distention, 

 as in other circular muscles. Now, as a stimulus to sentient nerves 

 constitutes no necessary part of any of these actions, it is probable 

 that the motor nerves of the sphincter have little or no connexion 

 with the sentient ones: and, consequently, that muscle is not 

 excitable to contraction by a stimulus applied to a sentient surface. 

 Hence, whenever the influence of the will upon the lumbar portion 

 of the cord is suspended, this muscle ceases to act, whether a 

 mental or a physical stimulus be exerted. 



Dr. Hall, indeed, cites two experiments which imply that the 

 action of the sphincter is dependent on the cord. In both, how- 

 ever (one on ahorse, the other on a turtle), the observations were 

 made immediately after division of the cord. By the division, the 

 whole organ was thrown into an excited state, both above and 

 below the section, and therefore manifested phenomena similar to 

 those excited by volition. Indeed, we have seen the sphincter 

 repeatedly contracting after division of the cord without the appli- 

 cation of any new stimulus to it ; and the dog continuing to raise 

 and depress his tail as long as the irritation of the cord produced 

 by the section has continued. 



On the same principle, animals will exhibit movements of volun- 

 tary character for some time after decapitation. A bird thus 

 treated will fly for some distance, and with considerable energy, 

 and will flap its wings if the cut surface of the cord be irritated. 

 A fly decapitated flies for some way immediately after the removal 

 of the head ; and Walckeiiaer observed a singular fact respecting 

 the Cerceris ornata, a wasp which attacks a bee that inhabits holes : 

 " at the moment that the insect was forcing its way into the hole 

 of the bee, Walckenaer decapitated it ; notwithstanding which, it 

 continued its motions, and, when turned round, endeavoured to 

 resume its position, and enter the hole."* The change in the 



* Quoted in Miiller's Physiology; by Baly, vol.i. p. 787*, 2ud ed. 

 VOL. I. Z 



