2. Filaria Sanguinis Hominis.--Of the several varieties of this worm, _Filaria nocturna_ is most common and {195} most important clinically. The adults are thread-like worms about 8 to 10 cm. long. They are rarely seen. They live in pairs in the lymphatic channels and glands, especially those of the pelvis and groin, and often occur in such numbers as to obstruct the flow of lymph. This is the most common cause of elephantiasis. Infection is very common in tropical countries, especially in Samoa, the West Indies, Central America, and the Isthmus of Panama. It is said that in Samoa 50 per cent. of the natives are infected. The female is viviparous, and produces vast numbers of embryos, which appear in the circulating blood. These embryos are very actively motile, worm-like structures, about as wide as a red corpuscle and 0.2 to 0.4 mm. long (Fig. 107). They are found in the peripheral blood only at night, appearing about 8 P.M. and reaching their maximum number--which is sometimes enormous--about midnight. If the patient change his time of sleeping, they will appear during the day. Infection is carried by a variety of mosquito, which acts as intermediate host. Diagnosis rests upon detection of embryos in the blood. They can be seen in stained preparations, but are best found in fresh unstained blood. A rather large drop is taken upon a slide, covered, and examined with a low power. The embryo can be located by the commotion which its active motion produces among the corpuscles. This motion consists almost wholly in apparently purposeless lashing and coiling movements, and continues for many hours. 3. Trypanosoma Hominis.--Various trypanosomes are common in the blood of fishes, amphibians, birds, and {196} mammals (Fig. 81). They live in the blood-plasma and do not attack the corpuscles. In some animals they are apparently harmless; in others they are an important cause of disease. [Illustration: FIG. 81.--Trypanosomes from blood of gray rat (Boston).] _Trypanosoma hominis_ is an actively motile, spindle-shaped organism, two or three times the diameter of a red corpuscle in length, with one end terminating in a long flagellum. It can be seen with medium power objectives in either fresh or stained blood. Human trypanosomiasis is common in Africa. As a rule, it is a very chronic disease. "Sleeping sickness" is a late stage when the organisms have invaded the cerebrospinal fluid. Infection is carried by a biting fly, _Glossina palpalis_. VIII. SERUM REACTIONS