THE DIGESTIVE SYSTEM 87 



is quite simple. The essential factors seem to be: (1) Negative 

 pressure in the thoracic part of the esophagus produced by its 

 relaxation and by the inspiratory phase of the diaphragm; (2) 

 contraction of the rumen and reticulum; (3) contraction of the 

 abdominal muscles. The result is that a portion of the ingesta 

 in the stomach is injected forcibly into the esophagus and carried 

 to the mouth. The r61e of the esophagus is largely passive. As 

 soon as the mass known as the "cud " reaches the mouth, superflu- 

 ous fluid is squeezed out of it and swallowed. Mastication begins 

 at once and continues half a minute to a minute, according to the 

 state of the bolus. The bolus is then swallowed and in three or 

 four seconds another one has reached the mouth. The "cud" 

 weighs about 100-120 grams. There is no evidence to support 

 the view that the remasticated bolus when swallowed passes 

 along the esophageal groove to the omasum, although this is a 

 current statement. 



In the omasum, or third compartment of the stomach of the ox, 

 the food is thoroughly triturated and reduced to a very fine 

 form. This is accomplished by pressure between the laminae 

 and by rasping of the numerous horny papillae which stud their 

 surfaces. The contractions of the omasum are slow but very 

 powerful. 



The movements of the small intestine are more active than 

 those in other portions of the bowels. This is evident when 

 an examination is made of the contents, which are found only 

 in small amounts. As in other organs, a wave of contraction 

 occurs in the muscular coat. This wave progresses from the 

 anterior to the posterior end of the bowel and is called peristalsis. 

 Simultaneously with a relaxation of the circular fibers in the 

 intestinal walls, the longitudinal fibers contract above the point 

 -of relaxation and pull the intestine off the contents as a glove 

 from a finger. The effects of the process are: (1) Food is repeat- 

 edly brought in close contact with different parts of the bowel 

 wall; (2) undigested food is thoroughly mixed with digestive 

 fluids; (3) digested matter is thoroughly exposed to the mucous 

 membrane for absorption; (4) the repeated contractions facilitate 

 the flow of blood and lymph. When the ear is placed upon the 

 flank of a horse the rumble caused by the passage of ingesta can 

 be distinctly heard. If the animal is standing quietly the noise 

 can be distinguished at a distance of several feet. Most physiolo- 



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