284 



THE EVACUATION OF FECES. 



is surrounded by both sphincter-muscles, of which the upper or internal 

 is formed of unstriated and the external of striated muscle-fibers. 



Immediately after the act of defecation the external sphincter (Fig. 

 1 08, S; Fig. 109) usually contracts, and remains contracted for some time. 

 When the muscle relaxes, the elasticity of the parts surrounding the 

 anal opening, particularly of both the sphincter-muscles, is sufficient to 

 insure closure of the anus. In the interval of rest or until the pressure 

 of the feces again occurs, there is no evidence of a permanent contrac- 

 tion (tonic innervation) of the anal sphincters. As long as the fecal 

 matters lie above the rectum, they give rise to no conscious sensation. 



FIG. 108. The Perineum and its Muscles: i, anus; 2, coccyx; 3, ischial tuberosity; 4, tuberososacral ligament; 

 5, acetabulum; B, bulbocavernosus muscle; Ts, superficial transverse perineal muscle; F, fascia of the deep 

 transverse perineal muscle; J, ischiocavernosus muscle; O, internal obturator muscle; S, external sphincter 

 ani muscle; L, levator ani muscle; P, pyriformis muscle. 



It is only their descent into the rectum that causes the feeling of 

 tenesmus. At the same time the stimulation of the sensory nerves of 

 the rectum causes reflex stimulation of the sphincters. The center for 

 this reflex (Budge's anospinal center) is situated in the lumbar cord. 



In animals, after division of the spinal cord above this center, the anal opening 

 closes actively when touched ; but soon after this reflex contraction the sphincters 

 relax, and the anus may thus remain open for a time. This is due to the fact 

 that the active voluntary contraction of the external sphincter-muscle, already 

 mentioned, under the control of the will (cerebrum), which keeps the anus 

 closed for some time after each evacuation of the bowel, is absent. In dogs, in 



