98 PHYSIOLOGY OF MUSCLES AM) NKRVKS. 



take place simultaneously throughout the length of the 

 tube; but, commencing at one point, it continually 

 propagates itsdf' along fivsh lengths of the tube, so thai 

 the contents an- slowly driven forward. The principal 

 agents in this are the circularly arranged fibres, which 

 atone point completely close the tube, while, bv the 



< traction of the longitudinal fibres, the wall of the 



tube is drawn back over its contents, thus providing for 

 the propulsion of the contents. This is called peri- 

 sf, i/t',c Hill/inn. It takes place along the whole of the 

 digestive canal, from the throat to the other end, and 

 in this ca-e affects the forward motion of the food, as 

 aUo, finally, the expulsion of the undigested re-idue. 



5. Peristaltic motion may be very well observed by 

 laying bare the throat of a dog, and then placing water 

 in the mouth of the animal, so that the motion of swal- 

 lowing takes place. It may also be seen in the intes- 

 tines when laid bare, as also in the urinary duct, in 

 which each drop of urine leaving the kidneys produces 

 a wave which propagates itself from the kidneys to ihj 

 urinary bladder. Such movements may also be artifi- 

 cially elicited by mechanically or electrically irritating 

 some one point of the intestine, urinary duct, or other 

 >uch part, or by irritating the nerves appropriate to 

 these parts. The most striking feature is the slowness 

 with which these motions take place. No| only does a 

 Long time, observable without any artificial aid, elapse 

 after the application of (he irritant be lore the motion 

 begins, bui, even if tin- irritation is sudden and in- 

 stantancoii-, the motion excited ;it one point passes 

 along very gradually, idowly increa-ing up to a definite 

 point, and then again gradually decreasing. This slow- 

 ii' ss of motion es>entially distinguishes sniooih from 



