44 PHYSIOLOGY 



Little is known about the significance, if any, of the other elements which 

 I have mentioned as occasional constituents of living beings. Silicon, which 

 is of universal distribution, is assimilated as silica, probably in colloidal 

 solution, and is distributed in minute quantities through all plant and 

 animal tissues. It forms a very large percentage of the mineral basis of 

 grasses, but even here it does not seem to be indispensable, since these will 

 grow in a medium devoid of silica as luxuriantly as under normal con- 

 ditions. 



Fluorine is found in the enamel of the teeth and in minute traces in other 

 tissues of the body. 



Bromine, though present in quantity in some seaweeds, appears to play 

 no part in the economy of higher animals. 



Iodine is found in large quantities in many seaweeds and is present as an 

 organic iodine compound in the skeleton of certain horny sponges. An 

 organic iodine compound is also found in the thyroid gland of the higher 

 animals, and may possibly be the active principle by means of which these 

 glands are able to affect the nutrition of the whole body. Iodine, therefore, 

 would seem to be an essential constituent of the higher animals. 



Aluminium is found in large quantities in certain lycopods. Whether 

 it is essential to their growth is not known. 



Copper is certainly not a necessary constituent of a large number of 

 plants and animals. In one class, the cephalopods, it appears to take the 

 part of iron in the formation of a blood pigment. The hsemocyanine, which 

 was described by Fredericq, plays the same part in the blood of cephalopods 

 that is played by haemoglobin in the blood of vertebrates. When oxidised 

 it is of a blue colour, but gives off its oxygen and is reduced to a colourless 

 compound on exposure to a vacuum. 



Among these elementary constituents of the body, a definite line of 

 demarcation can be drawn between the carbon and hydrogen on the one 

 hand and all the other constituents on the other. The first two elements are 

 built up in a deoxidised form into the living structure of the protoplasmic 

 molecule. The products of their complete oxidation are volatile, namely, 

 carbon dioxide and water, and leave the body in these forms. The nitrogen 

 set free by the breaking down of the proteins will pass off as free nitrogen or 

 as ammonia. The sulphuric acid formed by the oxidation of the sulphur 

 combines with the bases to form non- volatile salts. We may therefore divide 

 the ultimate constituents of the body into those which are combustible and 

 are driven off on heating, and those which are left behind as the ash. 



