436 SPECIAL HISTOLOGY. 



OF THE DIGESTIVE ORGANS. 



I. OF THE INTESTINAL CANAL. 



128. THE intestinal canal is composed fundamentally of the so- 

 called membranes of the intestine. The innermost of these, the mucous 

 membrane, membrana mucosa, corresponds in its structure with the skin, 

 and like it possesses, (1) a non-vascular investment composed of cells 

 the epithelium ; (2) the mucous membrane, more strictly speaking, com- 

 posed of connecting and elastic tissues ; containing vessels, nerves, 

 smooth muscular fibres, and different forms of minute glands, and often 

 presenting peculiar processes (papillce, villi)', and (3) an external layer 

 of loose connective tissue, the submucous cellular tissue. The second 

 intestinal tunic, the muscular membrane, tunica muscularis, is provided, 

 for a certain distance at the commencement and at the termination of 

 the intestine, with striated fibres, but in the remainder of its extent the 

 muscles are everywhere of the smooth kind, and form in general two 

 distinct layers ; an external, with longitudinal, and an internal, with 

 transverse fibres ; more rarely there are three separate layers. The 

 third membrane, the serous, tunica serosa, exists only upon those por- 

 tions of the intestine which occupy the cavities of the abdomen and 

 pelvis ; it is a delicate, transparent membrane, poor in nerves and ves- 

 sels, and provided with an epithelium ; it invests the intestinal canal, and 

 connects it with the walls of the abdominal cavity and with the other 

 viscera. 



OF THE ORAL CAVITY. 

 A. OF THE MUCOUS MEMBRANE OF THE ORAL CAVITY. 



129. The commencement of the intestine may be said to have only 

 one tunic, the mucous membrane, which is applied more or less closely 

 to the bones and muscles bounding the oral cavity ; and is distin- 

 guished by its not inconsiderable thickness, by its red color, arising from 

 the abundance of its vessels, and by its numerous nerves and papillae. 



The proper mucous membrane, although it is continuous with, and 

 gradually passes into, the cutis upon the lips, is more transparent and 

 softer than the corium ; however, it possesses considerable firmness and 

 is still more extensible. Like the thinnest portions of the cutis, it con- 

 sists of a single layer, 0-1-0*2 of a line in thickness, and presents a 

 great number of papillae, like those of the skin, upon its outer surface; 

 they are in general simple but occasionally bifurcated (when hypertro- 

 phied they may possess even more processes), are conical or filiform, 



