The intestinal canal is the habitat of the majority of animal parasites found in man. Frequently their presence leads to no morbid symptoms, local or general; nor are the symptoms, when they do arise, always characteristic of the presence of Animal parasites. parasites alone. Discovery of their bodies, or of their eggs, in the stools is in most instances the only satisfactory proof of their presence. The parasites found in the bowel belong principally to two natural groups, Protozoa and Metazoa. The great class of the Protozoa furnish amoebae, members of Sporozoa and Infusoria. The amoebae are almost invariably found in the large intestine; one species, indeed, is termed Amoeba coli. The frequently observed relation between attacks of dysentery and the presence of amoebae in the stools has led to the proposition that an Amoeba dysenterica exists, causing the disease—a theory supported by the detection of amoebae in the contents of dysenteric abscesses of the liver. No symptoms of injury to health appear to accompany the presence of Sporozoa in the bowel, while the species of Infusoria found in it, the Cercomonas, and Trichomonas intestinalis, and the Balantidium coli, may or may not be guilty of prolonging conditions within the bowel as have previously set up diarrhoea.

The Metazoa supply examples of intestinal parasites from the classes Annuloida and Nematoidea. To the former class belong the various tapeworms found in the small intestine of man. They, like other intestinal parasites, are 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.

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 Vegetable parasites. 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.

Males. Females. Both Sexes.
Organ or Tissue in
Order of Frequency.		 Per-
centage		Organ or Tissue in
Order of Frequency.		 Per-
centage		Organ or Tissue in
Order of Frequency.		 Per-
centage
 1 Stomach 22.56  1 Stomach 22.37  1 Stomach 22.49
 2 Lip 12.94  2 Rectum 17.24  2 Rectum 13.12
 3 Rectum 11.57  3 Liver 15.50  3 Liver 10.02
 4 Tongue 11.36  4 Peritoneum 7.86  4 Lip 9.89
 5 Oesophagus 10.90  5 Oesophagus 5.33  5 Oesophagus 9.29
 6 Liver 7.80  6 Sigmoid 4.53  6 Tongue 8.96
 7 Jaw 6.38  7 Pancreas 3.52  7 Jaw 5.65
 8 Mouth 2.88  8 Tongue 3.12  8 Peritoneum 2.94
 9 Tonsils 2.09  9 Omentum 2.98  9 Sigmoid 2.56
10 Sigmoid flexure 1.77 10 Lip 2.57 10 Mouth 2.40
11 Parotid 1.10 11 Jaw 1.97 11 Pancreas 1.80
12 Pancreas ” 12 Colon 1.84 12 Tonsils 1.35
13 Caecum 0.94 13 Abdomen ” 13 Omentum 1.25
14 Peritoneum ” 14 Intestine 1.56 14 Parotid 1.12
15 Colon 0.89 15 Caecum 1.37 15 Colon ”
16 Pharynx 0.79 16 Mouth 1.18 16 Caecum 1.08
17 Intestine (site unknown) ” 17 Parotid ” 17 Intestine 1.00
18 Abdomen 0.71 18 Splenic flexure 0.98 18 Abdomen ”
19 Mesentery 0.55 19 Jejunum and ileum 0.78 19 Pharynx 0.62
20 Omentum ” 20 Tonsils 0.68 20 Mesentery 0.52
21 Hepatic flexure 0.39 21 Pharynx 0.40 21 Jejunum and ileum 0.44
22 Submaxillary gland 0.31 22 Hepatic flexure ” 22 Hepatic flexure ”
23 Jejunum and ileum ” 23 Mesentery ” 23 Splenic flexure ”
24 Duodenum 0.23 24 Submaxillary 0.20 24 Submaxillary 0.28
25 Splenic flexure 0.15 25 Duodenum ” 25 Duodenum 0.22
Note.—The figures where several organs are bracketed apply to each organ separately.

In a healthy new-born infant the mouth is free from micro-organisms, and very few are found in a breast-fed baby, but Bacillus lactis may be found where the child is bottle fed. If there is trouble with the first dentition and food is allowed to collect, staphylococci, streptococci, pneumococci and colon bacilli may be present. Even in healthy babies Oidium albicans may be present, and in older children the pseudo-diphtheria bacillus. From carious teeth may be isolated streptothrix, leptothrix, spirilla and fusiform bacilli. Under conditions of health these micro-organisms live in the mouth as saprophytes, and show no virulence when cultivated and injected into animals. The two common pyogenetic organisms, Staphylococcus albus and brevis, show no virulence. Also the pneumococcus, though often present, must be raised in virulence before it can produce untoward results. The foulness of the mouth is supposed to be due to the colon bacillus and its allies, but those obtained from the mouth are innocuous. Also to enable the Oidium albicans to attack the mucous membrane there must be some slight inflammation or injury. The micro-organisms found in the stomach gain access to that organ in the food or by regurgitation from the small intestine. Most are relatively inert, but some have a special fermentative action on the food (see [Nutrition]). Abelous isolated sixteen distinct species of organism from a healthy stomach, including Sarcinae, B. lactis, pyocyaneus, subtilis, lactis erythrogenes, amylobacter, megatherium, and Vibrio rugula.

Hare-lip, cleft palate, hernia and imperforate anus are physical abnormalities which are interesting to the surgeon rather than to the pathologist. The oesophagus may be the seat of a diverticulum, or blind pouch, usually situated in its lower half, which in Physical abnormalities most instances is probably partly acquired and partly congenital; a local weakness succumbing to pressure. Hypertrophy of the muscular coat of the pyloric region is an infrequent congenital gastric anomaly in infants, preventing the passage of food into the bowel, and causing death in a short time. Incomplete closure of the vitelline duct results in the presence of a diverticulum—Meckel’s—generally connected with the ileum, mainly important by reason of the readiness with which it occasions intestinal obstruction. Idiopathic congenital dilatation of the colon has been described.

Table II.