DEFECATION. 119 



almost any intermediate part of the intestine. A reversed " anti- 

 peristaltic motion," as it is called, only occurs as a result of some 

 intense local stimulation, such as the strangulation of a hernia, etc. 



The motion produced by the substances contained in the intes- 

 tine depends on their character. The solid parts excite more 

 rapid movements, and the more fluid portions but slightly influ- 

 ence the intestinal peristalsis. 



Thus the solids which make their way through the pylorus are 

 seldom to be found in the jejunum, no matter at what period after 

 a meal the animal be killed, whereas the folds of the mucous mem- 

 brane are always bathed in a fluid creamy material during the 

 entire period of digestion, and even for a considerable time after 

 all the food has left the stomach. 



Mechanism of Defsecation. This is a point of much 

 importance, for the evacuation of the lower bowel is intimately 

 connected with feelings of comfort and health, and the insuring 

 of its accomplishment, in illness, forms an essential part of the 

 physician's duty. 



The movements of the intestine cause the various excretions 

 and indigestible parts of the food to pass towards the sigmoid 

 flexure of the colon, where their onward motion is checked for a 

 time by the strong circular muscle of the rectum (called the su- 

 perior sphincter, or tertius by Hyrtl), which does not carry on the 

 peristaltic wave. The materials here get packed into a more or 

 less solid mass, which is gradually augmented after each meal. 



The lower outlet of the alimentary canal is closed by two distinct 

 sphincter muscles. One thin external superficial muscle, made 

 up of striated fibres, belongs to the perineal group, and has little 

 influence on the closure of the anus. The deep or internal sphinc- 

 ter, which is much stronger, surrounds the gut for rather more 

 than an inch (3 centimetres, Henle) in height, and is one-quarter 

 inch thick. It is made of smooth muscle, and therefore capable 

 of prolonged (tonic) contraction. It would appear, however, that 

 this strong sphincter is a supernumerary guard to the anal orifice, 

 which is but rarely called into action, for during the interval of 

 rest between the acts of defecation, the faeces do not come in con- 



