126 TRACE ELEMENTS IN ANIMALS 



were never found; these were antimony, arsenic, beryllium, 

 germanium, gold, mercury and titanium. It will be observed 

 that the spectrographic methods will not allow the detection of 

 chlorine, bromine and iodine. 



It thus appears that in addition to the generally recognized 

 indispensable elements a large number of others have been 

 found in animals. Of these copper would appear to be a constant 

 constituent of animals while almost all animals examined con- 

 tain manganese in significant amount. There is no indication 

 that any other of the spectrographically recognizable trace 

 elements are universally present although some of them may 

 occur in relatively considerable amounts in a limited number of 

 species. Of the few elements not recognizable by the spectro- 

 graph iodine is known to be widely distributed. Keilin and 

 Mann (1940) state that 'it is now well established that zinc is a 

 true and general microconstituent of living organisms ' . In view 

 of Webb's work, this statement would appear to be somewhat 

 premature, though it may well be that the view of the universal 

 presence of zinc in both plants and animals will ultimately prove 

 to be correct, and the essentiality of zinc for mammals appears 

 to be well established. 



But as with plants, so with animals, the presence of an 

 element in the organism is no evidence of its necessity. Thus 

 although Webb found that lithium, boron, strontium, aluminium 

 and silver were frequently present, he emphasized that there 

 never appeared to be any appreciable accumulation of these 

 elements from the environment. Even accumulation is no 

 evidence of essentiality, which can only be proved by observa- 

 tion of the deleterious effects produced by the exclusion of the 

 element from the animal's diet and the recovery of the animal 

 from the ill-effects of deficiency when a supply of the element 

 is restored. 



Although the investigation of trace-element effects in animals 

 has not been carried nearly as far as with plants, there are 

 nevertheless a few trace elements which are well-established 

 as essential for certain groups of animals. The best known of 

 these are perhaps copper and iodine, the former of which is 

 known to be needed for the utilization of iron in haemoglobin 

 formation, while iodine is essential for the functioning of the 



