66 



NUTRITIONAL PHYSIOLOGY 



another furnishing the requisite stimuli. We cannot go 

 through the series of movements unless there is at least 

 a little saliva to be swallowed, and we cannot arrest the 

 march of events when it is once begun. 



The contractile tissue in the upper part of the esophagus 

 is._ol a skeletal variety. Lower down this gives place to 

 typical visceral muscle. Hence it is not strange to find that 

 the advance of the bolus becomes progressively slower as 

 it descends. The movement which is here taking place 



i JL m 



Fig. 12. An exaggerated representation of peristalsis. I and II 

 are successive views of the same portion of the alimentary tube: 

 P is the zone of contraction shifting downward and always pre- 

 ceded by the zone of unusual relaxation (N). Ill is an imaginary 

 section through II, showing the food bolus (b) slipping along in 

 advance of the contracting region, its advance being facilitated 

 by the relaxation below. 



is what is known as a peristalsis, and it is highly important 

 that its principle should be understood. The most ob- 

 vious feature is a ring of contraction setting in above the 

 enclosed pellet, causing it to slide onward, and following 

 it down by involving in succession each level of the tube. 

 The mechanical application can be simply illustrated by 

 propelling a glass bead through a soft-rubber tube by 

 pinching repeatedly behind it with thumb and finger. A 

 strict analysis of what occurs in the esophagus obliges us to 

 recognize that the process is not so simple as it first appears. 



