98 PHYSIOLOGY OF MUSCLES AND NERVES. 



take place simultaneously throughout the length of the 

 tube ; but, commencing at one point, it continually 

 propagates itself along fresh lengths of the tube, so that 

 the contents are slowly driven forward. The principal 

 agents in this are the circularly arranged fibres, which 

 at one point completely close the tube, while, by the 

 contraction 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- 

 staltic motion. It takes place along the whole of the 

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

 in this case affects the forward motion of the food, as 

 also, finally, the expulsion of the undigested residue. 



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 w^hen 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 tlu 

 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 

 such part, or by irritating the nerves appropriate to 

 these parts. The most striking feature is the slowness 

 with which these motions take place. Not only does a 

 long time, observable without any artificial aid, elapse 

 after the application of the irritant before the motion 

 begins, but, even if the irritation is sudden and in- 

 stantaneous, the motion excited at one point passes 

 along very gradually, slowly increasing up to a definite 

 point, and then again gradually decreasing. This slow- 

 ness of motion essentially distinguishes smooth from 



