Sometimes imbedded within the substance of the thyroid in the neck, sometimes placed directly behind it upon the windpipe, are four tiny glands, each about the size of a wheat seed, the parathyroids.  For long they were swamped in the nearness of their great neighbor, and considered merely a variable part of it.  There are some who contend that even today.  But it has been proven that they are separate, individual glands, with a structure and function of their own, and a definite importance to the body economy.

On the animal family tree they appear early, contemporaneously with the thyroids.  In the embryo they develop from about the same sites.  And very often they look very much alike under the microscope, especially when the cells are in certain quiescent stage of secretion.  Yet they are wholly independent in nature, activity and business.

First experimenters upon the effects of removal of the thyroid were confused by contradictory findings with different animals because in some they would take out the parathyroids at the same time without knowing it, and in others they would not.  That possibility suggested, more careful dissectors accomplished the job of extirpating the thyroid while leaving the parathyroids intact and vice versa.  In consequence some definite information about the parathyroids is available, even though their internal secretion has never been isolated, or its existence established as more than an inference.

When the parathyroids are removed, an astounding increase in the excitability of the nerves follow.  It is as if the animal were thoroughly poisoned with strychnine.  The slightest stimulus will make him jump, or throw him into a spasm.  When the excitability of the nerves is measured by an electrical instrument it is found augmented by from five hundred to one thousand per cent.  The reflexes, those automatic responses of brain and spinal cord to certain stimuli and situations, become enormously sensitive, so that merely letting the light into a darkened room will make the subject of the experiment go into a series of convulsions.

On the chemical side, an explanation for these nervous phenomena has been advanced.  Lime in the blood and cells appears to be necessary in a number of ways.  In the making of bone and teeth, in the coagulation of the blood, in the keeping of fluid within the blood vessels, and in maintaining the tone of the nerves, it plays a major role.  Now the parathyroids, among all the glands of internal secretion, seem to act as the prime regulators of the amount of lime held within the blood and cells.  For when the parathyroids have been completely and aseptically excised, without injuring any other organ, immediately the body begins to lose lime.  Something has gone out of it that helped it to bind lime, and without that essential something, the internal secretion presumably of the parathyroids, the lime departs.  As a conspicuous consequence the teeth fail to develop properly, particularly as to their enamel, for which lime is an essential constituent.  Hair is lost, there is a general wasting, the nails get brittle, and the bones soften, and the animal dies.  Supplying lime directly, particularly by direct injection into the blood, will relieve the symptoms.