CU. XXV.] THE PARATHYROIDS 339 



The discovery of the relationships between the thyroid and these 

 morbid conditions is especially interesting, because important practical 

 results in their treatment have followed close on the heels of experi- 

 mental investigation. The missing internal secretion of the thyroid 

 may be replaced in these animals and patients by grafting the thyroid 

 of another animal into the abdomen; or more simply by injecting 

 thyroid extract subcutaneously ; or even by feeding on the thyroid 

 of other animals. This treatment, which has to be kept up for the 

 rest of the patient's life, is entirely successful. Chemical physiologists 

 have been diligently searching to try and discover what the active 

 material in thyroid extract is which produces such marvellous 

 results; the view at present held is that the efficacy of thyroid 

 extract is mainly though not entirely due to a substance which 

 Baumann separated from the gland, and which stands almost unique 

 among physiological compounds by containing a large percentage 

 of iodine in its molecule. Thyro-iodin or lodo-thyrin, as this 

 substance has been called, is present in combination with protein 

 matter in the colloid substance. 



Intravenous injection of thyroid extract in a normal animal 

 lowers blood-pressure ; but in an animal from which the thyroid has 

 been removed it stimulates the heart and raises blood-pressure. 



In healthy animals and men, administration of thyroid produces 

 an increase in nitrogenous metabolism. 



Parathyroids. 



These are small bodies, usually four in number, situated near 

 or imbedded in the substance of the thyroid. They are made up 

 of elongated groups of polyhedral cells, bound together by connec- 

 tive tissue, and well supplied with blood-vessels. Some have 

 supposed that parathyroid is only immature thyroid tissue, but a 

 study of development shows that the parathyroids have a different 

 embryonic origin from the thyroid, and in the lower vertebrates the 

 two organs are entirely distinct. It is only in the mammals that 

 they are so closely associated anatomically. They are probably 

 associated physiologically also, and it has been by no means easy 

 to determine the role of each. Most of the facts described in the 

 preceding section on the thyroid were discovered previous to the 

 recognition of the parathyroids, and since then the view has been 

 advanced that in removing the thyroid it is really the simultaneous 

 removal of the parathyroids which causes the nervous symptoms. 

 Certainly the most prominent symptom after extirpation of the 

 parathyroids is tetany (muscular spasms and twitchings). 



The parathyroids contain no iodine, and it is doubtful if 

 they form an internal secretion. If they do not, their function 

 must be to neutralise poisonous substances formed elsewhere, and 



