3i6 TRAVELS IN AFRICA, 
affecting any other part of the animal economy. The effect 
of opium is feen on the blood and mufcular fibres ; of mer- 
cury on the glands and lymphatics ; of cantharides on the 
nerves : and too great a portion of thefe, taken into the body, 
may have a pernicious effedt on the eyes, but always through 
the medium of other parts. The whole materia medica^ per- 
haps, furnifhes no drug or mineral that is known, when taken 
into the ftomach, to have a local and partial effed: on the eyes. 
Such an effedt is even irreconcileable with the general and 
conftantly obferved operation of all remedies applied to the 
human body. 
Befides, if the injury were folely or even in part to arifefrom 
the ufe of the Nile water, all thofe who drink it muft be equally 
affected, allowing for the different degree of firmnefs in the fta- 
mina of each. But certain orders of men are rarely attacked 
by this difeafe, and they too who are continually ufmg the river 
water both internally and externally. 
Rice is one of the mofl: nutritive and falubrious of the farina- 
ceous aliments, and certainly does not operate to render the 
humours acrid, and thereby to inflame the eyes. It is ufed as 
a main article of food by the natives of a large portion of Afia, 
and forms no inconfiderable part of the confumption in other 
countries, without being obferved to produce any fuch efFed: as 
is here attributed to it ; and may therefore fairly be denied to 
have any fuch power. 
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