MOVEMENTS OF THE INTESTINES. 449 



ring of constriction so as to shorten the intestine and to cause a rolling- 

 motion in the intestine itself. The contractions of the circular fibres of 

 the small intestine, as a rule, commence immediately below the pylorus 

 and progress downward toward the large intestine, so tending to force 

 the contents of the small intestine onward toward the ileo-caecal valve ; 

 but while the pylorus is ordinarily the commencing point of this series 

 of contractions, the wave of contraction frequently miy seem to com- 

 mence at other points, and normally invariably moves downward toward 

 the large intestine. It is stated that occasionally the wave of contraction 

 moves in both directions, the upward movement being then spoken of 

 as an antiperistaltic movement. Such a contraction occurs in cases of 

 obstruction of the intestines, but it is extremely doubtful as to whether 

 such an antiperistaltic movement ever occurs during health. 



It is a peculiarity of unstriped muscular fibre that when stimulated 

 its contraction is preceded by a very long latent period and lasts a con- 

 siderable time, relaxation taking place but slowly afterward. In such 

 cases as the intestine and ureter, a stimulation of any one point not only 

 causes contraction of the muscular fibres in that locality, but that con- 

 traction is transmitted to the parts below it, the contraction being prop- 

 agated from fibre to fibre, so producing a wave of contraction which 

 passes along both circular and longitudinal coats of such tubular struc- 

 tures. 



The peristaltic movements of the intestine are, in all probability, due 

 to the contact of food with the interior, and yet, as in the case of the 

 stomach, it is probable that this stimulation is not of a purely mechanical 

 nature ; for while the insertion of foreign bodies into the small intestine 

 starts up a wave of contraction, that contraction soon passes off as the 

 intestine becomes accustomed to the presence of the body. So, also, the 

 intestinal movements are frequently more energetic when the intestine is 

 comparatively empty than when distended with food. 



The principal cause of the movements of the muscular coat of the 

 bowels is without doubt to be found in the condition of the blood circu- 

 lating through the vessels in the walls of the alimentary tube. Closure 

 of the aorta causes active peristaltic motion of the intestine. If the 

 intestinal tube be already in motion, closure of the aorta increases the 

 vigor of the intestinal movement. Closure of the vena cava, or portal 

 vein, and dyspnoea likewise increase peristalsis. An increase in the 

 amount of carbon dioxide in the blood, or a decrease in the amount of 

 oxygen, leads to powerful peristaltic movements, and this condition will 

 probably explain the activity of the intestinal movements seen in animals 

 recently killed. In this case, however, the exposure to the cold air is 

 also concerned in the production of this movement. The manner in 

 which these changes in blood supply influence intestinal movement has 



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