208 SECTIONAL ADDRESSES. 
improvement follows the adjustment of the mineral balance in the food. 
Thus, Kellner obtained an increased rate of growth in calves by the addition 
of a calcium salt to a diet on which the animals grew at an average rate 
and showed no signs of obvious ill-health. In some work, not yet reported, 
we have found that the addition of traces of iodine to the diet of stall-fed 
calves in winter increased the rate of growth as compared with that of 
control animals whose condition would be regarded as normal. Further, 
it has been shown recently that the supply of minerals in the food of 
the mother may have a profound influence on the vitality of the young 
at birth and for some time after, even where there may be no obvious 
effect on the mother. Thus Hart, Steenbock and Humphrey have noted 
that deficiency of calcium in the diet of cows may lead to the birth of 
dead or weakly calves, and Ennis Smith has shown that deficiency of 
iodine in the food of pigs may lead to the birth of dead young, although 
there is not, in either case, any obvious pathological condition apparent 
in the mothers. 
Some attention has recently been devoted to the question of an in- 
creased susceptibility to certain infectious diseases in cases of malnutrition 
due to deficiencies of minerals. We have noted in feeding experiments 
that the mortality from intercurrent infections is much higher in the groups 
fed on diets which, for experimental reasons, are ill-balanced or deficient 
in mineral matter. Meigs has noted a similar increased incidence of 
diseases in cows on diets deficient in calcium. In clinical medicine the 
administration of inorganic salts of calcium, iodine and manganese has 
been advocated in the treatment of certain bacterial infections. It is 
very probable that such treatment is efficacious where these salts are 
deficient in the body and that its efficacy is due to the making good of that 
deficiency. It has been suggested that deficiency of calcium may be a 
causative factor in producing a lowered resistance to tuberculosis. If 
this is correct 1t is of great economic importance on account of the inci- 
dence of tuberculosis in dairy cows. It is known that, at the height of 
lactation, there is usually a loss of calcium from the body. This is greater 
the higher the yield of milk, and tuberculosis seems to be more liable 
to occur in heavy-milking cows. 
Though these results of disordered mineral metabolism can be pro- 
duced under experimental conditions, it does not necessarily follow that 
they occur to any considerable extent under practical conditions. There 
is, however, strong circumstantial evidence that deficiency of one or more 
mineral elements is a common cause of malnutrition in farm stock. Indeed, 
in some cases, it has been possible to identify the deficiency ; in these 
cases marked beneficial results follow the addition to the ration of the 
elements present in insufficient amount. This evidence, which has been 
accumulating during the past few years, now warrants the attention 
of the practical expert. 
It may be of interest to consider some reasons which can be adduced 
for believing that the danger of deficiencies of minerals in the food of 
farm animals has been increasing in recent years. During the past half- 
century the types of animals after which breeders have been striving — 
are those whose young have a very rapid rate of growth, or whose females — 
have a great capacity for producing the constructive materials required — 
P 
