244 PHYSIOLOGY CHAP. 



from above, and auginentor or excitatory influences from below. 

 The activity of the intestinal muscles at any time will depend 

 on the relative influence of these two sets of impulses (Bayliss 

 and Starling). 



Exner and his pupil A. Miiller made an interesting contribu- 

 tion to our knowledge of the reflex co-ordination of the intestinal 

 movements. Exner observed that when a pin was introduced 

 into a loop of intestine point forwards, it was regularly found 

 after a certain time to be inverted, i.e. head forward, point behind. 

 This mechanism of defence is effected by a complicated alternation 

 of relaxation and contraction of the muscle walls ; at the point 

 where the mucous membrane is pierced by the pin, the circular 

 coat relaxes, while the longitudinal contracts, so that a tumefac- 

 tion is formed which surrounds the pointed end of the pin. 



At the same time a ring of constriction forms behind the pin, 

 so that it is first turned transversely to the loop, and subsequently 

 reversed, with the head foremost. Miiller found that this com- 

 plicated reflex mechanism acted just as well when the vagi and 

 the solar plexus were divided. It therefore seems as if this 

 mechanism were carried out by the local nervous mechanisms, i.e. 

 the plexuses of Meissner and Auerbach. 



To illustrate the details of the ingenious theory of intestinal 

 peristalsis formulated by Bayliss and Starling, one of their most 

 important experiments may be quoted. In order to excite 

 peristalsis in an isolated loop of intestine, they insert, about one 

 inch from the upper end, a bolus made of cotton-wool covered 

 with vaseline. Shortly after putting in the bolus, the contractions 

 of the segment of intestine immediately above the bolus undergo 

 increasing augmentation, until the intestine at this point enters 

 into a strong tonic contraction. This presses the bolus onwards, 

 and as the bolus moves the ring of constriction follows it up 

 until it has expelled the bolus through the lower opening of 

 the coil. In some cases, after the bolus has been expelled, a 

 second slow peristaltic wave of contraction may pass from one 

 end of the coil to the other, as if to expel any detached portions 

 of the bolus that may be left behind. This progression occurs 

 only in one direction, from above downwards. If the bolus be 

 inserted from below and pushed up the gut it will be returned by 

 the way it has entered. If, however, the intestine is in good condi- 

 tion, the latter experiment becomes impossible. On attempting to 

 push up the lump of cotton-wool, the intestinal wall contracts 

 strongly above it and resists the upward passage of the bolus. 



If two enterographs are placed at right angles to each other at 

 a point about the middle of a coil of intestine, so as to record 

 the contractions of both longitudinal and transverse coats, the 

 synchronous activity of the two coats is seen to be altered by the 

 passage of a bolus introduced from the upper end. Fig. 82 records 



