PHYSIOLOGY OF DIGESTION 477 



in health to lubricate the surface, but in disease becoming very profuse and 

 greatly altered in character. 



The muscular coat of the hollow viscera varies in thickness ; but 

 the whole of it belongs to the unsti'iped division of muscles, and its action 

 is purely involuntary. In all but the large intestines the fibres are 

 arranged in a circular direction, but in these they are divided into sets, one 

 circular, and the other collected in separate longitudinal bands. By the 

 consecutive action of these fibres (called peristaltic), the food is driven on- 

 wards from one end to the other of the alimentary canal. 



The mucous membrane, which lines the whole length of the alimentary 

 canal, from the mouth to the anus, is continuous with the skin at these two 

 orifices — with the mucous membrane lining the air-passages of the lungs at 

 the entrance to the larynx — with that investing the nasal passages and 

 cavities at the antero-superior part of the pharynx — and, lastly, with the 

 internal ear through the eustachian tubes which open into the back of 

 the pharynx. It is also reflected into the ducts of the salivary glands, 

 which open into the mouth, and into those of the liver and pancreas, so 

 that it has very extensive communications with these several organs. Like 

 the skin, this membrane has a base composed of primary membrane, called 

 the coriutn, on which are scattered the glands that secrete the gastric juice, 

 imbedded in loose areolar tissue. In the intestines we shall find it exten- 

 sively supplied with absorbents, which open upon its velvety pile or villi, 

 and the whole protected by epithelium, which serves an important part in 

 the production of the mucus everywhere found upon its surface when in a 

 healthy state. In the oesophagus it is thick, and disposed in longitudinal 

 folds, allowing of lateral distension. In the stomach it exists in coarse 

 folds or rugce, and in the intestines it is gathered into sharp folds, chiefly 

 manifested in the duodenum. It is extensively supplied with blood 

 throughout its whole surface, but especially where it lines the stomach and 

 small intestines, and it is also liberally furnished with nerves, chiefly derived 

 from the great sympathetic system. 



The abdominal viscera are supplied with blood by branches from the 

 aorta, passing between the folds of the peritoneum to reach their destination, 

 excepting in the cases of the kidneys and pancreas, which have no such 

 folds. The same folds also include the veins returning the blood to form 

 the vena portie (see page 466), and also the lymphatics and chyliferous ab- 

 sorbents, to be hereafter described. 



The nerves are chiefly derived from the great sympathetic system ; but 

 branches from the cerebro-spinal system are also distributed to the contents 

 of the abdomen, and especially to the stomach, by means of the pneumo- 

 gastric nerve. 



PHYSIOLOGY OF DIGESTION 



Before proceeding to examine into the anatomy of the abdominal 

 organs, it may be well to investigate the nature of the processes which are 

 carried out by them. To do this, the food must be traced from its 

 prehension by the lips and teeth to its expulsion from the anus. Thus, 

 commencing with the mouth, we find it there ground into a coarse pulp, 



