206 SECTIONAL ADDRESSES. 
even reverse processes being carried out by means of the colloidal 
mechanism. 
In the animal body these changes in the concentration of the inorganic 
ions can be correlated with changes in the functions of the organs. Thus, all 
the organs regulated by the central nervous system depend for the integrity 
of their functions upon the maintenance of definite ratios of calcium, 
potassium and sodium in the fluids within the nerve tissues. Changes in 
the relative proportions of these are accompanied by alteration in the 
excitability of nerve and in the irritability of muscle. The classical 
experiments of Ringer on the perfused heart show that minute changes 
in the concentrations of calcium or potassium in the perfusing fluids have 
a profound effect on the activity of the heart. These examples are 
merely illustrations of the general law that any disturbance of the normal 
physiological balance of the salt solution of the body is accompanied by a 
correlated impairment of function. 
There is both experimental and clinical evidence to show that changes 
in the physiological balance are involved in many pathological conditions 
in which the symptoms can be associated with excess or deficiency of 
specific ions. Some of these pathological conditions, such as rickets, 
can be produced experimentally in animals fed on ill-balanced diets, and 
the symptoms can be relieved by adjusting the diet so that the essential 
mineral elements are absorbed from the intestine in the right amounts and 
proportions. 
The body is remarkably efficient in maintaining this balance, in spite 
of the fact that there is a continuous loss in the excreta, and the mineral 
matter of the food is liable to be very different in composition from that 
found in the blood. Within limits, the elements present in excess tend to 
be excreted and those deficient to be conserved. The bones act as a 
reservoir, especially for calcium and phosphorus, the two required in 
largest amounts. Reserves can be deposited in the bones when the supply 
is ample, and mobilised in times of need. It is probable that this function 
of the bones, viz. regulating the supply of mineral elements to the body 
fluids, is as important as the more obvious one of providing arigid framework. 
It is probably more fundamental, for, when the available mineral matter 
is insufficient to maintain both the physiological balance in the blood 
and the rigidity of the skeleton, it is rigidity which is sacrificed. There 
is little doubt that in most diseases affecting the bones, the skeletal symp- 
toms are only secondary manifestations of the influence of some factor, 
often a dietary one, which upsets the balance of mineral elements in 
the blood. 
We have seen that the presence in the body fluids of mineral elements 
in definite amounts and proportions is a necessary condition of health, 
and that the body possesses means of regulating this physiological balance. 
We must now consider what occurs when there are in the diet deficiencies 
or excesses of mineral elements greater than can be dealt with by the 
regulating mechanisms of the body. 
Let us note first the widespread and drastic results of more or less 
complete privation of all mineral elements. Forster studied the effects 
of feeding a diet from which the mineral salts had been removed as com- 
pletely as possible. He found that on this diet_animals died sooner than, 
