44 
ON THE VETERINARY MATERIA MEDICA. 
There are two varieties of the extract: one in dark, rounded, 
hard masses, chiefly from Bombay; and the other in square pieces, 
lighter coloured, and more easily pulverised, from Bengal; but 
often both varieties are found in the same package. The darker 
contains more of the tannin principle, and less of extractive matter, 
and is, on the whole, preferable; but both are mixed with earthy 
matter and sand^ from carelessness in the preparation, and often 
fraudulently to increase the weight. 
The old name of this drug was Terra Japonica, or Japan earth; 
it is, however, a vegetable extract, often, indeed, adulterated 
with earthy matter. It is sparingly soluble in water, and the 
quantity of residuum, after the catechu has been triturated with 
ninety times its weight of water, will indicate the degree of adul¬ 
teration. 
Catechu ranks in the human pharmacopoeia as an unirritating 
and powerful astringent. There is considerable difficulty in ex¬ 
plaining the mode of action as it regards this class of medicines ; 
but every day's experience abundantly proves that certain drugs, 
internally administered, have the power of restraining unhealthy 
and immoderate discharges of various kinds. The effect seems 
to be in some degree mechanical : the relaxed and open ves¬ 
sels being contracted and closed, the term astringent was used, 
or, possibly, some chemical action might be exerted on the fluid 
within the mouths of these vessels, some coagulation of the fluid 
might take place within the vessels, and thus the flux or discharge 
might be arrested. It is not merely a tonic power which these medi¬ 
cines exert, removing the debility of the vessels or parts; for many of 
the most active and permanent stimulants have little or no astrin¬ 
gent efficacy, and some of the best astringents exert no influence 
on the pulse, or have any power as tonics. Astringents, then, 
may properly be considered as forming a distinct class of medi¬ 
cines, somewhat allied to the tonics, but differing materially from 
them. 
There has been found widely diffused among plants a prin¬ 
ciple recognised by chemists under the name of tannin . Among 
other properties, it possesses the important one of combining with 
gelatine or jelly, and converting it into a condensed and firm, 
and almost insoluble compound ; and thus it is that the tannin in 
oak bark unites with the skins of animals, and converts them 
into leather . 
The student may make some pleasing and satisfactory experi¬ 
ments respecting this affinity between tannin and gelatine. Into 
a weak solution of isinglass let a few drops of a filtered solution 
of catechu be poured, and a considerable precipitate will be thrown 
down. If a great proportion of the catechu be used, a co- 
