XV. 



THE DIGESTIVE TEACT. 



THE digestive tract is a continuous canal extending from 

 the mouth to the anus, widening into the cavities of the 

 mouth, the pharynx, and the stomach. Its walls are composed 

 of connective tissue, and either striated or smooth muscle-fibers, 

 and lined with epithelium, which is partly stratified and partly 

 arranged in a single layer. The beginning and termination of 

 the digestive tract are under the control of voluntary striped 

 muscles. The flat muscle-layers keep the canal closed, unless 

 solid, liquid, or gaseous material separates its walls and tem- 

 porarily makes the caliber patent. All portions of the canal are 

 in a high degree extensible. 



The characteristic feature of the mucous membrane covering 

 the whole length of the tract are stratified epithelium in the 

 walls of the mouth, the pharynx, the oesophagus, and the lowest 

 portion of the rectum j flat epithelium in the wall of the stomach, 

 and a single columnar epithelium throughout the intestines. 

 Delicate fibrous, and partly myxoinatous, connective tissue, 

 freely supplied with blood- and lymph- vessels, is found in the 

 walls of the oral cavity, forming papillae, which reach their 

 highest development in the mucosa of the tongue j and are also 

 present in that of the pharynx and oesophagus. Connective 

 tissue also produces the filiform elevations in the small intestine 

 and the projections and folds in the large intestine ; besides all 

 folds occluding the caliber of the canal when in an empty con- 

 dition. The connective tissue surrounds the epithelial prolonga- 



