4 THE HEART AND RESPIRATION. PART in. 



more animal food or hydrocarbon is requisite for the 

 production of heat. 



The waste of the tissues, and the aeration of the vital 

 juices, that is, the exchange of the respiratory gases, 

 are common to all animals. The heart, upon whose ex- 

 pansions and contractions the circulation of the blood 

 depends, is represented in the lower animals by pro- 

 pelling organs of a variety of forms ; and the organs of 

 respiration differ exceedingly, according to the medium 

 in which the animals live. Water, both fresh and salt, 

 though a suffocating element to land animals, contains 

 a great deal of air, not only in the state of gas, but also 

 in solution, the quantity in solution being directly as 

 the pressure ; so that animals living in the deepest re- 

 cesses of the ocean breathe as freely as those that live 

 on land, but with respiratory organs of a very different 

 structure. In the lowest classes, which have no respi- 

 ratory organs at all, the gases are exchanged through 

 their thin delicate skins. 



The mechanical forces act within the living being 

 according to the same laws as they do in the external 

 world : the chemical powers too, which are the cause of 

 digestion, heat, and respiration, follow the same laws of 

 definite and quantitative proportion as they do in inert 

 matter ; but neither the mechanical forces, nor the 

 physical powers, could create a germ ; nor could they 

 even awaken its dormant state to living energy, unless a 

 vital power existed in it, the origin of which is beyond 

 the reach of man. 



Animals are endowed with nerve-force, in addition to 

 mechanical force and the physical powers which are 

 common to them and vegetables ; a force which consti- 

 tutes their prime distinction, which is superior to all 

 the other powers from its immediate connection with 

 mind, and which becomes more evident, and more evi- 

 dently under the control of the animal, in proportion as 



