Spirochœta recurrentis (S. obermeieri) is a delicate spiral filament (7 to 9 by 0-25 /*) provided, as shown by appropriate staining (Loeffler's whip stain), with a long flagellum (5 to 7 yu by O.1 to 0-2 p). The body of the parasite may have three, four, or six bends or turns, the flagellum from three to five. By the Romanow-sky method the body of the parasite usually stains uniformly, with the exception of the extremities, which are pointed and take only a very faint tint. In fresh blood the spirochsetes, propelled by the flagellum, exhibit very active screw-like movement. Some are longer than others, the long forms resulting from end-to-end attachment of two or more parasites. That this is the explanation of the long forms, which may measure from 16 to 100 u, is shown by staining. In those measuring from 16 to 19 /n we find a flagellum at each end of the filament and a pale zone in the middle, the pale zone corresponding to the approximated lightly staining extremities above referred to. The still longer forms admit of a similar explanation. Although the normal habitat of the spirochætes is the liquor sanguinis, occasionally they are seen in the red blood-corpuscles.
Obermeier and von Jaksch describe certain refractile bodies present in the blood during the fever intermissions. The latter author says that he has observed the development of these bodies into short rods from which the typical spirochætes are eventually evolved. This observation has not been confirmed.
Demonstration of the parasite.—— It is necessary to remember that the parasite only occurs in the blood during the febrile stage of the disease, disappearing from it completely during the non-febrile intervals. In some forms and cases of the disease it is present in large numbers in every field of the microscope; in other forms and cases it is so scanty that many fields have to be examined before a single specimen can be discovered. In thin films of fresh blood its presence can usually be recognized from the agitation its movements communicate to the adjacent corpuscles. In dried and fixed films the stains in general use for malaria work suffice for its demonstration. The
