_Causes._ Beside the origin from the removal or general disease or degeneration of the thyroid, it has been attributed to digestive troubles, associated with fermentations and the production of toxic matters, to rheumatism, infection and malaria, to rachitis and unhygienic conditions. The systemic changes and trials of growth and development, of pregnancy and lactation, seem to be factors in certain cases. A peculiar irritable nervous organization transmitted by heredity is undoubtedly a potent cause, and upon this, bacteridian, leucocytic and other poisons operate so as to rouse the paroxysms. _Symptoms._ There are usually prodromata in the shape of dullness, prostration, weariness, and some dullness of the special senses. Fever is commonly absent and the contractions tend to affect both flexors and extensors, but as the force of the first predominates, the affected member is usually held more or less rigidly semi-flexed. The spasm appears suddenly, often taking occasion of some voluntary movement, and may last for several minutes or hours. It is followed by an interval of relaxation of equally uncertain duration. Though usually attacking the limbs and causing the victim to walk on the toes, it may extend to the face, neck or trunk, and constitute an intermittent trismus, oposthotonos, or emprosthotonos. _Diagnosis._ Tetany is to be distinguished from tetanus by the complete intermissions of the spasms, and by the voluntary development of these by compression of the presiding nerve or artery. Pressure on the nerve arouses its excitability, and compression of the artery shutting off the supply of blood from the disordered and susceptible muscles, tends to increase their irritability. Ligature of an artery supplying healthy muscles causes simple trembling of such organs. From the spasms of cerebral, spinal or meningeal inflammation tetany is distinguished by the absence of fever, and the complete intermissions of the paroxysms. The spasms of ergotism bear the closest relation to those of tetany and in the absence of proof of the ingestion of ergot, might well be confounded with them. _Treatment._ This consists mainly in doing away with the causes, when these can be ascertained. Portions of thyroid may be grafted if complete thyroidectomy has been performed, or thyroid extract may be given. In the human subject recoveries have followed the expulsion of intestinal worms, the cure of gastric dilation, dyspepsia, fermentations, diarrhœa, rachitis, menstrual irregularity, or auto-intoxication. Fíré has seen recovery follow the extraction of a carious tooth. The spasms may be met by the internal administration of antispasmodics (chloral, belladonna, bromides, opiates), and the external application over the affected muscles of anodynes and antispasmodics (belladonna, opium, chloroform, oil of cajeput, oil of peppermint, menthol, etc.). Warm or tepid baths are often of great value and a mild electric current has been found useful.