e with an increasing tendency to sleep, hence
the name sleeping sickness. As the stupor deepens the patient looses all
desire or power of exertion and as little food is taken he rapidly
wastes away and finally succumbs for after this final stage is reached
there is no relief.

It is definitely known that a species of tsetse-fly, _Glossina palpalis_
(Fig. 112), which somewhat resembles our stable-fly, is responsible for
the dissemination of the disease, and some recent investigators have
suggested that certain species of mosquitoes may also carry the parasite
from one host to another. There still remains some doubt as to the exact
manner in which the fly transmits the disease, but it seems altogether
likely that it is an alternative host and does not serve as a simple
mechanical carrier. In this respect it is like the mosquito which is one
of the necessary hosts of the malaria parasites, and unlike the
house-fly which carries the germs of various diseases in a purely
mechanical way without serving as a definite necessary host for the
parasite.

The tsetse-fly is found only in tropical Africa and is limited in its
distribution there to certain very definite, narrow, brushy areas along
the water's edge. If these places can be avoided there seems to be
little danger. Those who are fighting the disease have found that if the
brush in the vicinity of watering-places and ferry-landings is cleared
away such places become comparatively safe. These flies do not lay eggs
but produce full-grown larvae which soon pupate in the ground.

ELEPHANTIASIS

In many tropical regions human blood as well as that of other animals is
the normal habitat of certain worm-like parasites (Nematodes). They are
not entirely confined to the tropics but may extend far up into the
subtropical regions. Five or six different species of these parasites
are known, only one of which, however, has been shown to be of any
pathological importance, as far as human beings are concerned.

[Illustration: FIG. 111--_Trypanosoma gambiense_; various forms from
blood and cerebrospinal fluid. (After Manson.)]

[Illustration: FIG. 112--Tsetse-fly. (After Manson.)]

This species, _Filaria bancrofti_, is not only very widely distributed,
but in regions such as some of the South Sea Islands a very large per
cent of the natives have the filariae present in their blood. When these
parasites are withdrawn from the circulation and placed on a slide for
study they are s