Owing to the difficulty of excluding swallowed sputum, the presence of the tubercle bacillus is less significant in the feces than in other material. It may, however, be taken as evidence of intestinal tuberculosis when clinical signs indicate an intestinal lesion and reasonable care is {243} exercised in regard to the sputum. Success in the search will depend largely upon careful selection of the portion examined. A random search will almost surely fail. Whitish or grayish flakes of mucus or blood-stained or purulent particles should be spread upon slides or covers and stained by the method given upon p. 127. In the case of rectal ulcers, swabs can be made directly from the ulcerated surface. 6. Crystals.--Various crystals may be found, but few have any significance. Slender, needle-like crystals of fatty acids and soaps (Fig. 32) and triple phosphate crystals (Fig. 90) are common. Characteristic octahedral crystals of calcium oxalate (Fig. 47) appear after ingestion of certain vegetables. Charcot-Leyden crystals (Fig. 6) are not infrequently encountered, and strongly suggest the presence of intestinal parasites. Yellowish or brown, needle-like or rhombic crystals of hematoidin (Fig. 32) may be seen after hemorrhage into the bowel. 7. Ova of Parasites.--The stool should be well mixed with water and allowed to settle. The ova will be found in the upper or middle portions of the sediment. Descriptions will be found in the following chapter. {244} CHAPTER VI ANIMAL PARASITES Animal parasites are common in all countries, but are especially abundant in the tropics, where almost every native is host for one or more varieties. Because of our growing intercourse with these regions, the subject is assuming increasing importance in this country. Many parasites, hitherto comparatively unknown here, will probably become common. Some parasites produce no symptoms, even when present in large numbers. Others cause very serious symptoms. Only those which have clinical interest will be considered here. The illustrations will give a better idea of their appearance than any description. They belong to three classes: I. Protozoa. II. Vermes. III. Arthropoda. I. PROTOZOA