818 PHYSIOLOGY 



the middle of the balloon, i.e. at the point of greatest tension, and 

 the amplitude of the contractions is augmented by increasing the 

 tension on the walls of the gut. These movements are unaffected 

 by the direct application of drugs such as nicotine or cocaine, which 

 we might expect to paralyse any local nervous structures in the 

 wall of the gut. Bayliss and Starling concluded that these rhythmic 

 contractions were myogenic,* that they were propagated from muscle 

 fibre to muscle fibre, and that they coursed down the gut at the rate 

 of about 5 cm. per second. Since, however, they may apparently arise 

 at any portion of the gut which is subject to any special tension, it 

 is not easy to be certain that a contraction recorded at any point is 

 really propagated from a point two or three inches higher up. These 

 contractions must cause a thorough mixing of the contents of the gut 

 with the digestive fluids. On examining under the Rontgen rays the 

 intestines of a cat which has taken a large meal of bread and milk 

 mixed with bismuth some hours previously, a length of gut may be 

 seen in which the food contents form a continuous column. Suddenly 

 movements occur in this column, which is split into a number of equal 

 segments. Within a few seconds each of these segments is halved, the 

 corresponding halves of adjacent segments uniting. Again contractions 

 recur in the original positions, dividing the newly formed segments 

 of contents and re-forming the segments in the same position as 

 they had at first (Fig. 341). If the contraction is a continuous 

 propagated wave, it is evidently reinforced at regular intervals down 

 the gut, so as to divide the column of food into a number of spherical 

 or oval segments. The points of greatest tension immediately become 

 the points which are midway between the spots where the first 

 contractions were most pronounced. The second contractions 

 therefore start at these points of greatest tension, and divide the 

 first formed segments into two parts, which join with the corresponding 

 halves of the neighbouring segments. In this way every particle of 

 food is brought successively into intimate contact with the intestinal 

 wall. These movements have not a translatory effect, and a column 

 of food may remain at the same level in the gut for a considerable 

 time. 



The onward progress of the food is caused by a true peristaltic 



* Magnus has shown that it is possible to pull off strips of the longitudinal 

 (outer) coat of muscle fibres from the small intestine. Such strips, if they 

 contain Auerbach's plexus, will contract rhythmically if kept in warm oxygenated 

 Ringer's fluid. If, however, the plexus has been left behind in stripping off 

 the muscle, no rhythmic contractions are to be observed, although contraction 

 can still be excited by artificial stimulation. Magnus concludes that even the 

 rhythmic ' pendular ' contractions depend for their occurrence on the integrity 

 of the connection between local ganglionic centres and muscle fibres, and cannot 

 therefore be strictly regarded as myogenic. 



