234 



EXCRETION OF YJECAL MATTER. 



particularly of the two sphincters, suffices to keep the anus closed. In the interval 

 between two evacuations, there does not seem to be a continued tonic contraction 

 of the sphincters. As long as the faeces lie above the rectum, they do not excite 

 any conscious sensations, but the sensation of requiring to go to stool occurs when 

 the faeces .pass into the rectum. At the same time, the stimulation of the sensory 

 nerves of the rectum causes a reflex excitement of the sphincters. The centre for 

 these movements (Budge's centrum anospinale) lies in the lumbar region of the 

 spinal cord ( 3G2) ; in the rabbit between the sixth and seventh, and in the dog 

 at the fifth lumbar vertebra (Masiits). 



In animals whose spinal cord is divided above the centre, a slight touch in the region of the 

 anus causes this orifice to contract, but after this lively refiex contraction, the sphincters relax 

 again, and the anus may remain open for a time. This occurs because the voluntary impulses 

 which proceed from the brain to"*cause the contraction of the external sphincter are absent. 



Fig. 172. 

 The perinseum and its muscles. 1, anus ; 2, coccyx ; 3, tuberosity ; 4, sciatic ligament ; 5, 

 cotyloid cavity ; B, bulbo-cavcrnosus muscle ; Ts, superficial transverse perineal muscle ; 

 F, fascia of the deep transverse perineal muscle ; J, ischio-cavernosus muscle ; M, obturator 

 internus ; S, external anal sphincter ; L, levator ani ; P, pyriformis. 



Landois observed that in dogs with the posterior roots of their lower lumbar and sacral nerves 

 divided, the anus remained open, and not unfrequently a mass of faeces remained half ejected. 

 As the sensibility of the rectum and anus was abolished in these animals, the sphincters could 

 not contract reflexly, nor could there be any voluntary contraction of the sphincters, the result 

 of sensory impulses from the rectum. 



The external sphincter can be contracted voluntarily, like any voluntary muscle, 

 but the closure of the anus can only be effected up to a certain degree. When 

 the pressure from above is very great, the energetic peristalsis at last overcomes 



