550 SUTHERLAND SIMPSON 



whereas that secreted by the thyroid is a hormone, or exciting agent. 

 Normally the two neutralize each other; when one is absent, the other 

 produces its action uncontrolled. Tkis theory was first advocated by 

 Kiidinger. An obvious objection to it is the fact that the nervous excita- 

 tion which follows removal of the parathyroid is not affected by the sub- 

 sequent destruction of the thyroid, as it should be, if the two glands were 

 normally antagonistic. The statement of Vassale and Generali that the 

 symptoms of parathyroidectomy are more severe than those of thyro- 

 parathyroidectomy has not been generally upheld. Frequently it is found 

 that removal of the parathyroids is followed, from the beginning, by de- 

 pression instead of overexcitation, not of the same nature but to some 

 extent resembling that of simple thyroidectomy, and this cannot be readily 

 explained on the above theory. 



The calcium deficiency theory has been advocated mainly by Mac- 

 Callum and his collaborators. According to MacCallum, the secretion of 

 the parathyroid glands regulates the metabolism of calcium in the body in 

 some way at present unknown, and when this control is interfered with by 

 the absence of these glands, there is a depletion of calcium in the tissues. 

 This reduction in the calcium content of the central nervous system is the 

 essential cause of the acute neuromuscular disturbances seen in parathy-- 

 roid tetany ; for it has been recognized since the work of Sabbatani that 

 calcium in excess depresses the excitability of the cerebral motor cortex, 

 and that its diminution or absence has the opposite effect. How the cal- 

 cium content of the tissues is affected by the parathyroid secretion is not 

 clear, but one suggestion by MacCallum is that "it must govern the con- 

 version of a non-dialyzable and ineffective combination of calcium into a 

 dialyzable form, which is essential for the control of the excitability of 

 the nerve." 



The main argument in support of this idea is the undoubted fact that 

 the injection of soluble salts of calcium gives immediate relief from the 

 nervous symptoms of parathyroid tetany. This, however, is only pallia- 

 tive and when repeatedly applied may prolong the life of the animal, but 

 does not preserve it. The strongest objection to it is that other elements 

 are as effective as calcium, and particularly, that simply bleeding the 

 animal and then replacing the blood shed by an equal amount of calcium- 

 free isotonic solution of sodium chloride will also bring prompt relief. 



The fact that bleeding alone will relieve the symptoms produced by 

 absence of the parathyroids suggests the toxin theory of parathyroid func- 

 tion. This, in its present form, which may be termed the Guanidin 

 Theory, was first suggested by the observation of Koch that methylguani- 

 din is present in the urine of dogs suffering from parathyroid tetany. It 

 is advocated by Noel Paton and his pupils, and is supported by an impos- 

 ing array of facts brought forward by them. These are presented in an 

 imjxirtant series of contributions published in the Quarterly Journal of 



