48 PHYSIOLOGY 



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 conditions. 



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 hsemo- 

 cyanine, 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 oxida- 

 tion 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. 



