THE DIGESTIVE SYSTEM 69 



in the pharynx — viz.: (1, 2) the posterior nares, which com- 

 municate with the nasal chambers; (3) the mouth orifice; (4) the 

 esophagus; (5) the larynx; (6, 7) the eustachian tubes, slit-like 

 openings situated high up on the lateral walls of the pharynx. The 

 latter have a cartilaginous flap which closes during swallowing. 



The esophagus is a fibromuscular tube which connects the 

 pharynx with the stomach. It is about 4| feet long and runs down 

 the left side of the neck. Upon reaching the thorax it assumes a 

 niedian position, passes between the two lungs, pierces the dia- 

 phragm, and at once enters the stomach. The lining membrane is 

 always thrown up in folds except during the act of swallowing. 



The Mechanism of Swallowing. — Direct observations made 

 by Sisson show that when the horse swallows the following move- 

 ments occur: The soft palate is shortened and is raised so as to 

 close the openings into the nasal cavity. The pharynx is short- 

 ened and its end toward the mouth is dilated. The root of the 

 tongue and the larynx are brought toward each other, so that the 

 epiglottis touches the former. The arytenoid cartilages of the 

 larynx are approximated and the glottis is closed, thus preventing 

 aspiration of food particles. The bolus of food is "shot," so to 

 speak, through the pharynx by an energetic contraction of the 

 muscles which raise the tongue. This is followed by a so-called 

 "clearing up" phase, in which remnants which remain are removed 

 from the pharynx by the constrictor muscles of that organ. The 

 first stage of deglutition is voluntary, but the rest of the act is not 

 under the control of the will. 



The stomach is a U-shaped sac placed between the esophagus 

 and intestines (Fig. 17) . It has a capacity of 2 to 4 gallons. The 

 inlet or cardia and the outlet or pylorus are close together. It is 

 situated in the abdominal cavity opposite the fourteenth to six- 

 teenth ribs, behind the diaphragm and liver, and above the large 

 colon, so it does not come in contact with the walls of the cavity. 

 The stomach is held in position mainly by the pressure of the sur- 

 rounding organs and by the esophagus. Like other viscera in the 

 abdominal cavity, it has three layers to the wall — the mucous, the 

 muscular, and the serous. The mucous layer is clearly divided 

 into two parts (Fig. 18). That which lies to the left contains no 

 glands, is similar to the lining of the esophagus, and is covered 

 with thick, stratified epitheUum. The glandular part is subdivided 

 into three zones, according to the types of glands which it contains, 



