the elements necessary for protoplasmic
upkeep and activity. Morbid lesions of the respiratory and circulatory
systems are frequently capable of compensation through increased
activity elsewhere, and the symptoms they give rise to follow chiefly
along one line; diseases of the digestive organs are more liable to
occasion disorders elsewhere than to excite compensatory actions. The
digestive system includes every organ, function and process concerned
with the utilization of food-stuffs, from the moment of their entrance
into the mouth, their preparation in the canal, assimilation with the
tissues, their employment therein, up to their excretion or expulsion in
the form of waste. Each portion resembles a link of a continuous chain;
each link depends upon the integrity of the others, the weakening or
breaking of one straining or making impotent the chain as a whole.

The mucous membrane lining the alimentary tract is the part most subject
to pathological alterations, and in this connexion it should be
remembered that this membrane differs both in structure and functions
throughout the tract. Chiefly protective from the mouth to the cardia,
it is secretory and absorbent in the stomach and bowel; while the
glandular cells forming part of it secrete both acid and alkaline
fluids, several ferments or mucus. Over the dorsum of the tongue its
modified cells subserve the sense of taste. Without, connected with it
by the submucous connective tissue, is placed the muscular coat, and
externally over the greater portion of its length the peritoneal serous
membrane. All parts are supplied with blood-vessels, lymph-ducts and
nerves, the last belonging either to local or to central circuits.
Associated with the tract are the salivary glands, the liver and the
pancreas; while, in addition, lymphoid tissue is met with diffusely
scattered throughout the lining membranes in the tonsils, appendix,
solitary glands and Peyer's patches, and the mesenteric glands. The
functions of the various parts of the system in whose lesions we are
here interested are many in number, and can only be summarized here.
(For the physiology of digestion see NUTRITION.) Broadly, they maybe
given as: (1) Ingestion and swallowing of food, transmission of it
through the tract, and expulsion of the waste material; (2) secretion of
acids and alkalis for the performance of digestive processes, aided by
(3) elaboration and addition of complex bodies, termed enzymes or
fe