DIGESTION 281 



various names of pharynx, oesophagus, stomach, small intestine, large 

 intestine, and rectum, till it ends at the anus. Its walls are largely 

 composed of muscular fibres ; its lumen is clad with epithelium, and 

 into it open the ducts of glands, which, morphologically speaking, 

 are involutions or diverticula formed in its course. In virtue of its 

 muscular fibres it is a contractile tube; in virtue of its epithelial 

 lining and its special glands it is a secreting tube ; in virtue of both 

 it is fitted to perform those mechanical and chemical actions upon 

 the food which are necessary for digestion. Its inner surface is in 

 most parts richly supplied with bloodvessels, and in special regions 

 beset with peculiarly-arranged lymphatics ; by both of these channels 

 the alimentary tube performs its function of absorption. From the 

 beginning of the oesophagus to the end of the rectum the muscular 

 wall consists, broadly speaking, of an outer coat of longitudinally- 

 arranged fibres, and a thicker inner coat of fibres running circularly 

 or transversely around the tube. Between the layers lies a plexus of 

 non-medullated nerves and nerve-cells (Auerbach's plexus). In the 

 stomach the longitudinal fibres are found only on the two curvatures, 

 and a third incomplete coat of oblique fibres makes its appearance 

 internal to the circular layer. In the large intestine, again, the 

 longitudinal fibres are chiefly collected into three isolated strands. 

 In the pharynx the typical arrangement is departed from, inasmuch 

 as there is no regular longitudinal layer ; but the three constrictor 

 muscles represent to a certain extent the great circular coat. The 

 muscles of the mouth and of the pharynx are of the striped variety. 

 So is the muscle of the upper half of the oesophagus in man and the 

 cat, and of the whole oesophagus in the dog and the rabbit. In the 

 rest of the alimentary canal the muscle is smooth, except at the very 

 end, where the external sphincter of the anus is striped. In certain 

 situations the circular coat is developed into a regular anatomical 

 sphincter, a definite muscular ring, whose function it is to shut one 

 part of the tube off from another (sphincter pylori), or to help to 

 close the external opening of the tube (internal sphincter of anus). 

 Elsewhere a tonic contraction of a portion of the circular coat, 

 not anatomically developed beyond the rest, creates a functional 

 sphincter (cardiac sphincter of stomach). 



Throughout the greater part of the digestive tract the peritoneum 

 forms a thin serous layer, external to the muscular coat. Internally 

 the muscular coat is separated from the mucous membrane, the lining 

 of the canal, by some loose areolar tissue containing bloodvessels, 

 lymphatics and nerves (Meissner's plexus), and called the submucous 

 coat. Between the mucous and submucous layers, but belonging to 

 the former, in the whole canal below the beginning of the oesophagus, 

 is a thin coat of smooth muscular fibre, the muscularis mucosae, con- 

 sisting in some parts, e.g., in the stomach, of two, or even three, 

 layers. Between this and the lumen of the canal lie the ducts and 

 alveoli of glands, surrounded by bloodvessels and embedded in 

 adenoid or lymphoid tissue, which in particular regions is collected 

 into well-defined masses (solitary follicles, Peyer's patches, tonsils), 

 extending, it may be, into the submucous tissue. In the mouth, 



