132 PHYSIOLOGY. 



When a sufficient quantity of freces has arrived in the lower part 

 of the rectum, there is felt a need of expelling them. During defeca- 

 tion all the organs situated in the abdomen are compressed so that 

 the intestinal contents may be expelled, but the anal sphincter, like 

 the cardiac sphincter of the stomach, offers a resistance, and during 

 the violent efforts the vesical sphincter is relaxed, allowing the urine 

 to escape. The sensory nerve-endings in the mucous membrane of 

 the rectum carry impressions to the ano-spinal center in the lumbar 

 cord, which sends out motor impulses to the muscles of the intestine. 

 At the same time the glottis is closed, the diaphragm and abdominal 

 muscles are set into action, and the act of defecation is accomplished. 



Section of cerebral crura, that is, the locus niger, permits the 

 anal and vaginal sphincters to take on rhythmic contractions. 



Fig. 37a- Rhythmic Systolic Movements of the Anal Sphincter of 

 Cat after Transverse Section of Lumbar Part of Spinal Cord. Rubber 

 Balloon in the Rectum Connected with Marey's Polygraph. (Oxx, from 

 LUCIANI. ) 



At ci, irritation of the sciatic, the rhythm of the sphincter is inhibited. By 

 1), irritation stopped, the rhythm starts up again stronger and somewhat 

 irregular. 



Nutrient Enemata. Enemas have been shown to travel by anti- 

 peristalsis up the large intestine, through the ileo-csecal valve and 

 into the small intestine, where there is no antiperistalsis. The 

 normal antiperistaltic movements of the ascending colon seem to 

 be effective factors in propelling these enemas into the small intes- 

 tine. Hence nutrient enemas are absorbed. 



HUNGER AND THIRST 



The seat of sensations of hunger is located in the epigastrium. 

 The seat of sensations of thirst is located in the pharynx, and is 

 quieted by intravenous injections of water. In every case it is 

 admitted that hunger and thirst are but localized expressions of a 

 general need of the blood for food and drink. The true seat of 



