OF ANIMAL BODIES. 
97 
one valve. Their joint action fulfils the office of a valve: 
for instance; over the entrance of the right auricle of the 
heart into the right ventricle, three of these skins or mem¬ 
branes are fixed, of a triangular figure, the bases of the 
triangles fastened to the flesh; the sides and summits 
loose; but, though loose, connected by threads of a deter¬ 
minate length, with certain small fleshy prominences ad¬ 
joining. The effect of this construction is, that, when the 
ventricle contracts, the blood endeavouring to escape in all 
directions, and amongst other directions pressing upwards, 
gets between these membranes and the sides of the heart; 
and thereby forces them up into such a position, as that, 
together, they constitute, when raised, a hollow cone, (the 
strings, before spoken of, hindering them from proceeding 
or separating farther;) which cone, entirely occupying the 
passage, prevents the return of the blood into the auricle. 
A shorter account of the matter may be this: So long as the 
blood proceeds in its proper course, the membranes which 
compose the valve are pressed close to the side of the ves¬ 
sel, and occasion no impediment to the circulation: when 
the blood would regurgitate, they are raised from the side 
of the vessel, and, meeting in the middle of its cavity, shut 
up the channel. Can any one doubt of contrivance here; 
or is it possible to shut our eyes against the proof of it? 
This valve, also, is not more curious in its structure, 
than it is important in its office. Upon the play of the 
valve* even upon the proportioned length of the strings or 
fibres which check the ascent of the membranes, depends, 
as it should seem, nothing less than the life itself of the 
animal. We may here likewise repeat, what we before ob¬ 
served concerning some of the ligaments of the body, that 
they could not be formed by any action of the parts them¬ 
selves. There are cases in which, although good uses ap¬ 
pear to arise from the shape or configuration of a part, yet 
that shape or configuration itself may seem to be produced 
by the action of the part, or by the action or pressure of 
adjoining parts. Thus the bend, and the internal smooth 
concavity of the ribs, may be attributed to the equal pres¬ 
sure of the soft bowels; the particular shape of some bones 
and joints, to the traction of the annexed mufecles, or to 
the position of contiguous muscles. But valves could not 
be so formed. Action and pressure are all against them. 
The blood, in its proper course, has no tendency to pro¬ 
duce such things; and, in its improper or reflected current, 
has a tendency to prevent their production. Whilst we 
see, therefore, the use and necessity of this machinery, we 
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