1. Tetany occurs spontaneously in many forms and may also be produced by the destruction of the parathyroid glands. Recent researches tend to demonstrate an intimate relation between the various forms of tetany and relative or absolute insufficiency of the parathyroid gland.
2. The parathyroid glands are independent organs with definite specific function. Whether or not this function is intimately related to that of other organs of internal secretion is not as yet proven.
3. The number and distribution of the parathyroid glands varies. Failure to produce tetany experimentally is probably due to the fact that some parathyroid tissue remains after an apparently complete extirpation. When extirpation is complete tetany appears, even in herbivora. Only a very small amount of parathyroid tissue is required to prevent this.
4. The effect of the extirpation of the parathyroid glands may be annulled by the reintroduction of an extract of these glands even from an animal of widely different character. The active principle is associated with a nucleo-proteid in the extract and may be separated with this nucleo-proteid from the remaining inert albuminous substances. Its effect in counteracting tetany appears some hours after injection and lasts several days.
5. The parathyroid glands contain no considerable amount of iodine. The parathyroid extract is not an iodine containing compound.
6. In tetany there is apparently some disturbance of the composition of the circulating fluids ordinarily prevented by the secretion of the parathyroid, which disarranges the balance of the mineral constituents of the tissues. Possibly this consists in the appearance of an injurious substance of an acid nature for such tetany may he relieved by extensive bleeding with replacement of the blood by salt solution. No actual poisonous material has, however, been demonstrated by the transference of the blood of a tetanic animal to the veins of a normal one.
7. Numerous researches have shown the important relation of the calcium salts to the excitability of the central nervous system. Their withdrawal leaves the nerve cells in a state of hyperexcitability which can be made to disappear by supplying them with a solution of a calcium salt.
8. Tetany may be regarded as an expression of hyperexcitability of the nerve cells from some such cause.
9. The injection of a solution of a salt of calcium into the circulation of an animal in tetany promptly checks all the symptoms and restores the animal to an apparently normal condition.
10. Injections of magnesium salts probably have a similar effect but these effects are masked by the toxic action of the salt.
11. The injection of sodium or potassium salts has no such beneficial effect but rather tends to intensify the symptoms. This is true also of the alkaline salts of sodium which were studied especially in respect to their basic properties.
12. The effect of calcium is of value in human therapeutics in combating the symptoms of spontaneous forms of tetany and in relieving the symptoms in cases of operative tetany and thus tiding over the period of acute parathyroid insufficiency until remnants of parathyroid tissue can recover their function or new parathyroid tissue can be transplanted. It is in this way an important and convenient ally of the method of injecting parathyroid extract.
13. Studies of the metabolism in parathyroidectomized animals show:
1. A marked reduction in the calcium content of the tissues especially of the blood and brain, during tetany.
2. An increased output of calcium in the urine and fæces on the development of tetany.
3. An increased output of nitrogen in the urine.
4. An increased output of ammonia in the urine with
4a. an increased ammonia ratio in the urine.
5. An increased amount of ammonia in the blood.
Much of this affords evidence of the existence of some type of acid intoxication. Its effects are, however, not neutralized by the introduction of alkaline sodium salts and may perhaps be regarded as especially important in producing a drainage of calcium salts from the tissues which can be remedied by the reintroduction of calcium salts.
14. Emphasis must be laid upon the remarkable difference which exists between the alterations inmetabolism following thyroidectomy and those following parathyroidectomy. In myxoedema there is lowered metabolism, decreased respiratory changes and lowered nitrogen output with depression of body temperature. In tetany there is increased metabolism, probably increased respiratory changes, certainly increase in nitrogen output and elevation of the temperature.
15. It is important, therefore, that in any experiments upon metabolism in relation to the thyroid and parathyroid gland, these glands should be clearly distinguished as structures exercising very different and in large part contrary effects upon metabolism.
16. In general the role of the calcium salts in connection with tetany may be conceived of as follows: These salts have a moderating influence upon the nerve cells. The parathyroid secretion in some way controls the calcium exchange in the body. It may possibly be that in the absence of the parathyroid secretion, substances arise which can combine with calcium, abstract it from the tissues and cause its excretion and that the parathyroid secretion prevents the appearance of such bodies. The mechanism of the parathyroid action is not determined, but the result, the impoverishment of the tissues with respect to calcium and the consequent development of hyperexcitability of the nerve cells, and tetany is proven. Only the restoration of calcium to the tissues can prevent this.
17. This explanation is readily applicable to spontaneous forms of tetany in which there is a drain of calcium for physiological purposes, or in which some other condition causes a drain of calcium. In such cases the parathyroid glands may be relatively insufficient.