2 9 8 A MANUAL' OF PHYSIOLOGY 



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 ana- 

 tomical 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 tubs 

 (internal sphincter of anus). Elsewhere a tonic contraction of a 

 portion of the circular coat, not anatomically developsd 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 con- 

 fined to the mucous membrane proper. Between the openings of 

 the glands the mucous membrane is lined with a single layer of 

 columnar epithelial cells, sometimes (in the small intestine) arranged 

 along the sides of tiny projections or villi. At the ends of the alimen- 

 tary 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 char- 

 acters much less distinct, but falling upon the whole into the 

 few fairly definite groups already described (p. i). Thus, 

 there are bodies like serum-albumin, serum-globulin, myosinogen, 

 and so on, which are so much alike that they can all be placed 



