28 MANUAL FOR STABLE SERGEANTS. 



assists in insalivation. From the mouth the food is carried back 

 by the tongue to the pharynx. As soon as it reaches the pharynx 

 the act of swallowing becomes involuntary and is completed by the 

 pharynx and esophagus. 



44. When the food reaches the stomach it is subjected to a me- 

 chanical rolling and mixing in the left side of the stomach (macera- 

 tion). It gradually passes to the right side of the stomach, where 

 it is acted upon by the gastric juice. It then passes into the small 

 intestine and is called chyme. 



45. In the small intestine, the chyme is acted on by the bile 

 and pancreatic fluid and is then called chyle. The villi of the small 

 intestines take up those parts of the food that have been rendered 

 absorbable by digestion and the remainder is passed on to the 

 cecum, which is the water reservoir. Here it is soaked and diges- 

 tion continues slowly in the cecum and great colon. The digested 

 parts of the food are absorbed here and the waste materials are passed 

 on into the small colon. In the small colon the moisture is absorbed 

 and the residue is formed into pellets of dung which are stored in 

 the rectum, to be discharged at intervals through the anus (defe- 

 cation). 



THE RESPIRATORY SYSTEM. 



46. The respiratory system consists of the nostrils, nasal 

 chambers, pharynx, larynx, trachea, bronchi, and the lungs, all of 

 which, except the air sacs, are lined with mucous membrane. 



47. A mucous membrane is a thin layer of tissue lining all 

 cavities and canals of the body which communicate with the external 

 air. 



48. Serous membranes are thin, glistening structures which 

 line the cavities of the body and cover to a certain extent the organs 

 therein contained. They secrete a fluid called serous fluid (serum) 

 which moistens their surfaces and prevents friction. The synovia! 

 membranes, the peritoneum, and the plurae are serous membranes. 



49. The nostrils are two oblong openings above the upper lip. 

 They are the anterior openings of the nasal chambers. 



50. The nasal chambers extend from the nostrils to the pharynx 

 and occupy the bony canals above the mouth, from which they" are 

 separated by the hard palate. They are separated from each other 

 by the cartilaginous septum nasi. Each chamber is divided into 



