96 TUNNY. 



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 commn- 

 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 ^lian, 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 suj^posed 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. 



