88 PRINCIPLES OF VETERINARY SCIENCE 



gists believe that the efficiency of digestion depends as much 

 upon peristalsis as upon the chemical processes produced by the 

 digestive juices. 



The movements of the large intestine are similar to, but slower 

 than, those of the small intestine. There is no good reason 

 to support the idea that material can pass from the small intestine 

 directly into the colon. This supposition was based on the 

 anatomical arrangement of the openings into the cecum. Con- 

 siderable energy is necessary to move the great masses of ingesta 

 in the large bowel. The energy is furnished by the wide muscular 

 bands in the walls of the viscus. In the horse the feces are 

 molded into the characteristic balls by the contractions of these 

 bands in the small colon. 



Defecation is the term applied to the act of emptying the rectum 

 of the undigested and indigestible parts of the food and other 

 wastes. The rectum serves as a pouch for the collection of the 

 feces. When it is sufficiently full a reaction occurs in its walls 

 from pressure, which causes it to be evacuated. The act of 

 defecation is normally involuntary in horses and cattle and is due 

 to the contraction of the smooth muscles in the walls of the rectum. 

 In man fixation of the diaphragm and closure of the glottis are 

 necessary. The sphincter ani muscle forms a purse-string-like 

 protection at the anal opening. 



The points that have been covered in the foregoing sections of 

 this chapter show: (1) what structures are concerned with diges- 

 tion and (2) how they move ingesta. In the succeeding sections 

 we will learn how the processes of secretion, digestion, absorption, 

 and nutrition are concerned with the life of the animal. 



THE PHYSIOLOGY OF SECRETION 



Secretion is the process of preparing an active substance in 

 solution for use in assisting digestion. In a broad sense it would 

 also include the elaboration of all substances produced by glandu- 

 lar activity, e.g., tears, milk, sweat, and secretions of the ductless 

 glands. 



Glands are the organs which produce secretions. They are 

 soft structures composed of secreting tissue supported by a 

 connective-tissue framework that forms a capsule and dips into 

 the deeper parts to form septa. A typical gland shows on its 



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