20 PHYSIOLOGY CHAP. 



Name. s\inll. Atomic Weight. 



Copper . . Cu 63-17 



Lead . Pl> 206-47 



Zinc . . ZIL 64-90 



With the exception of silicon, which is widely distributed in 

 both kingdoms, fluorine, which in small but constant quantities- 

 enters into the chemical composition of the enamel of the teeth, 

 and iodine, which has lately been found in one of the constituents 

 of the thyroid gland, it is probable that all these elements are 

 without physiological significance to the cell-body in which they 

 are found, and that they enter accidentally, like many other 

 extraneous elements, e.g. drugs, toxins, or such as are merely 

 indifferent bodies. 



It is worth noting that the twelve principal elements that enter 

 constantly into the composition of cells have all a low atomic 

 weight. Nine of them, in fact, belong to the first three series of 

 MendelejefFs Periodic System, and only three (potassium, calcium, 

 iron) belong to the fourth series of the system. Further, these 

 are all found either in the state of elements or as very simple 

 inorganic combinations, which are widely diffused in the air, in 

 water, and in the upper layers of the soil the only habitat of flora 

 and fauna. 



VI. The chemical compounds of which the cell is built up may 

 be divided into organic and inorganic. Organic substances are 

 distinguished as nitrogenous and non-nitrogenous : the former 

 include the Proteins and their derivatives, the latter the Fats and 

 Carbohydrates. 



Proteins are the most important organic substances, and are 

 indispensable in the constitution of living protoplasm. They are 

 essentially distinct from carbohydrates and fats in their element- 

 ary composition, for in addition to carbon, hydrogen, and oxygen 

 they contain nitrogen and sulphur. Their molecular structure, 

 and the exact number of atoms of the several elements which 

 enter into their constitution, are still unknown to us. There is, 

 however, no doubt that the molecular structure of these sub- 

 stances is highly complex ; more so, perhaps, than that of any 

 other chemical substance, since the ratio of the number of the 

 various atoms reaches a very considerable figure. 



It should be noted that the five elements above mentioned are 

 found in the different proteins in much the same proportions, 

 as appears from the following table, which gives the limits 

 between which the percentages of the various elements of protein 



oscillate : 



C 50 - 55 mean 52 per cent. 

 H 9-5- 7-3 7 

 N 15 -17-6 16 

 O 19 -24 23 

 S 0-3 - 2-4 2 



