32* DIGESTION 



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 longi- 

 tudinal 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 de- 

 veloped 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, ileo-colic sphincter), 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 de- 

 veloped 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 fibres, 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, pharynx, and 

 oesophagus, the glands lie in the submucosa, as do the glands of Brunner 

 in the duodenum; everywhere else they are confined to the mucous 

 membrane proper. Between the openings of the glands the mucous 

 membrane is lined with a single layer of columnar epithelial cells, some- 

 times (in the small intestine) arranged along the sides of tiny projec- 

 tions or villi. When the intestine is contracted the villi are long and 

 cylindrical in shape, when it is relaxed or distended they are flat and 

 conical. At the ends of the alimentary canal, viz., in the mouth, 

 pharynx, and oesophagus, and at the anus, the epithelium is stratified 

 squamous, and not columnar. 



The purpose of food is to supply the waste of the tissues, to 

 replenish the stores of material from the oxidation of which the 

 energy required for the running of the bodily machine is derived, 

 and thus to maintain the normal composition of the body. In the 

 body we find a multitude of substances marked off from each other, 

 some by the sharpest chemical differences, others by characters 

 much less distinct, but falling upon the whole into the few fairly 

 definite groups already described (p. i). 



Now, although it is by no means necessary that a substance in 

 the body belonging to one of these great groups should be derived 

 from a substance of the same group in the food, it has been found 



