500 DIGESTION 



same experimental conditions. Associated with the variations in fre- 

 quency are also differences in the amplitude of the contractions of the in- 

 testinal muscle. As the contractions become less frequent their ampli- 

 tude increases to the extent that the ratio between the amplitude of the 

 contractions of duodenal and ileal segments is as 3 to 20. Tone and irrita- 

 bility diminish progressively from duodenum to ileum. According to 

 Alvarez the underlying cause of the decline in contraction rate is to be 

 found in a study of the metabolism of the intestinal muscle in the differ- 

 ent regions. The determination of the metabolic changes in different 

 parts of the intestinal muscle from pylorus to the lower end of the ileum 

 showed a gradual descent in the curve of energy output (metabolic gra- 

 dient) which ran a course parallel to that of the intestinal contractions, 

 a slow or a rapid metabolism being associated with a slow or a rapid 

 contraction rate, respectively. This direct relationship between the fre- 

 quency of rhythm and the metabolism rate of intestinal muscle is in favor 

 of the view that the intestinal contractions are myogenic, moreover, 

 the graded activity of the muscle with regard to rhythm, tonus and ir- 

 ritability is probably an important factor in the production of peristalsis 

 and in the determination of the direction which this movement shall take. 



THE PERISTALTIC MOVEMENTS 



The other movement observed in the small intestine is that known as the 

 peristaltic wave. It occurs in two forms: (1) as a slowly advancing con- 



Fig. 162. Intestinal contractions (balloon method) after excision of the abdominal ganglia and 

 section of both vagi. Mechanical stimulation above (/) and below (2) the balloon causes relaxa- 

 tion and contraction respectively. (From Starling.) 



traction (1 to 2 cm. per minute), preceded by an inhibition of the walls, 

 and proceeding only through a short distance in a coil (4 to 5 cm.); and 

 (2) as a swift movement called the peristaltic rush, which sweeps with- 

 out pause for much longer distances along the canal. 



Further analysis of the peristaltic wave can readily be made by the 



