CHEMISTRY OF ANIMALS. 65 



tain complex organic compounds, and a small proportion 

 of certain mineral or inorganic substances. The presence 

 of all these three constituents appears to be essential to the 

 existence of living matter, and vital action can not appar- 

 ently be carried on in the absence of any one of the three. 

 It is to be remembered, however, that though we can make 

 such a rough analysis as the above of the matter of living 

 beings, we know really very little of the mode in which 

 these constituents are combined in the living body. Thus 

 it is uncertain how much of the water is in a state of chem- 

 ical combination with other constituents of the tissues; and 

 we are still more ignorant of the exact mode in which the 

 mineral or inorganic constituents occur. It is certain, how- 

 ever, that some, at any rate, of the mineral substances are 

 chemically combined with the organic compounds ; whilst 

 it is quite certain that both groups of substances are essen- 

 tial to life. 



In the following, a very brief outline will be given of the 

 more important facts as to the elementary chemistry of 

 animals and plants respectively : 



CHEMISTRY OF ANIMALS. 



The number of elements which have been recognised 

 in animal bodies is not very large, the chief, if not the 

 only ones, being carbon, hydrogen, oxygen, nitrogen, sul- 

 phur, phosphorus, chlorine, fluorine, calcium, magnesium, 

 aluminium, potassium, sodium, iron, manganese, copper, 

 and silicon. The first four elements of this list are some- 

 times spoken of as the " essential elements," as they occur 

 in most tissues ; whilst the remainder are very improperly 

 termed the "incidental elements," as occurring only in 

 small quantities and in special tissues. Some, however, of 

 these "incidental elements" are essential constituents of 

 the compounds formed by the so-called " essential ele- 

 ments," and most of them are just as necessary to life as 

 the latter are. 







