A MANUAL OF PHYSIOLOGY. 



INTRODUCTION. 



1 Life is a power superadded to matter ; organization arises 

 from, and depends on, life, and is the condition of vital action ; 

 but life never can arise out of, or depend on, organization.' 

 JOHN HUNTER. 



LIVING matter, whether it is studied in plants or in animals, 

 has certain peculiarities of chemical composition and struc- 

 ture, but especially certain peculiarities of action or function 

 which mark it off from the unorganized material of the dead 

 world around it. 



Chemical Composition of Living Matter. Although we cannot 

 analyze the living substance as such, we can to a certain, 

 but limited, extent reconstruct it, so to speak, from its ruins. 

 When subjected to analytical processes, which necessarily 

 kill it, living matter invariably yields bodies of the class of 

 protcids, which have approximately the following composition: 

 Carbon, 51*5 to 54*5 per cent.; oxygen, 20*9 to 23*5 per 

 cent. ;. nitrogen, 1^2 to 17 per cent.; hydrogen, 6*9 to 7*3 

 per cent., with small quantities of sulphur and generally of 

 phosphorus. Nucleo -proteids, which are compounds of 

 proteid with nucleins, a series of bodies very rich in phos- 

 phorus, are also constantly met with. Certain carbo-hydrates, 

 composed of carbon, hydrogen, and oxygen (the last two in 

 the proportions necessary to form water), of which glyccgen 

 (C 6 H 10 O 5 ) may be taken as a type, appear to be always 

 present. Fats, which consist of carbon, hydrogen, and 

 oxygen, and of which tristearin, a compound of stearic acid 

 with glycerine, of the formula C 3 H 5 , 3(C 18 H 35 O 2 ), may be 

 given as an example, are often, but perhaps not always, 

 found. Finally, water and certain inorganic salts, such as 



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