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TTTNNY. 
heart itself above and below; and they pass into the cavity of 
the heart to become united to its walls on the opposite side. 
This cavity has several lateral recesses which do not commu- 
nicate with any vessel ; but they appear to serve as an enlargement 
of the cavity, without allowing of the weakening of its muscular 
walls; which latter are very thick and full of blood. There 
are three valves with cords at the beginning of the aorta (or 
vessel which conveys the blood from the heart to other parts 
of the body.) The general cavity of the heart and all its 
smaller recesses are crossed and re-crossed with tendinous fibres, 
to render them additionally strong. Such a powerful circulation 
of the blood must render necessary a proportional supply of 
pure air to maintain its energy; and this again an organization 
by which the power of abstracting it by breathing from the 
surrounding water shall be rendered efficient; a measure which 
is accomplished by a high development of the nerves supplying 
the gills 
It is worthy of notice to observe how near sometimes the 
ancients have approached to an important discovery without 
having been able to make it. They could not fail to know 
that the breathing of air was necessary to the life of animals 
of the land; and they observed an action somewhat similar to 
breathing in fishes, but were not able to suppose that air could 
be obtained, or be necessary in the case of the latter. According 
to Aillian, therefore, (B. 9, C. 64,) it was the doctrine of 
Democritus, Aristotle, and his followers Theophrastus and 
Empedocles, that the function of the gills in fishes was to 
extract from the salt water of the sea a portion of fresh water, 
which they supposed to be mingled with it; and that such a 
separation of elements, and appropriation of fresh water, was 
essential to their life in the same manner as the obtaining of 
air is to creatures inhaling it. 
