e destitute of any power of active digestion,
simply absorbing the nutritious proceeds of the digestive processes of
their hosts. Nematode worms infest both the small and large intestine;
_Ascaris lumbricoides_, the common round worm, and the male _Oxyuris
vermicularis_ are found in the small bowel, the adult female _Oxyuris
vermicularis_ and the _Tricocephalus dispar_ in the large.

The eggs of the _Trichina spiralis_, when introduced with the food,
develop in the bowel into larval forms which invade the tissues of the
body, to find in the muscles congenial spots wherein to reach maturity.
Similarly, the eggs of the Echinococcus are hatched in the bowel, and
the embryos proceed to take up their abode in the tissues of the body,
developing into cysts capable of growth into mature worms after their
ingestion by dogs.

Vegetable parasites.

Numbers of bacterial forms habitually infest the alimentary canal. Many
of them are non-pathogenic; some develop pathogenic characters only
under provocation or when a suitable environment induces them to act in
such a manner; others may form the _materies morbi_ of special lesions,
or be casual visitors capable of originating disease if opportunity
occurs. Apart from those organisms associated with acute infective
diseases, disturbances of function and physical lesions may be the
result of abnormal bacterial activity in the canal; and these
disturbances may be both local and general. Many of the bacteria
commonly present produce putrefactive changes in the contents of the
tract by their metabolic processes. They render the medium they grow in
alkaline, produce different gases and elaborate more or less virulent
toxins. Other species set up an acid fermentation, seldom accompanied by
gas or toxin formation. The products of either class are inimical to the
free growth of members of the other. The species which produce acids are
more resistant to the action of acids. Thus, when the contents of the
stomach possess a normal or excessive proportion of free hydrochloric
acid, a much larger number of putrefactive and pathogenic organisms in
the food are destroyed or inhibited than of the bacteria of acid
fermentation. Diminished gastric acidity allows of the entry of a
greater number of putrefactive (and pathogenic) types, with, as a
consequence, increased facilities for their growth and activity, and the
appearance of intestinal derangements.

TABLE I.

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