498 TEXT-BOOK OF PHYSIOLOGY 



the metabolism of the neuro-muscular systems at least. The myxedema and 

 the failure of the mental powers are attributed to the loss or degeneration of 

 the gland and hence its internal secretion, and cretinism to the arrest of its 

 development. 



THE PARATHYROIDS 



The parathyroids are small bodies, usually four in number, two on each 

 side. They are divided into superior and inferior. The superior are situ- 

 ated internally and on the posterior surface in close relation to, and frequently 

 imbedded in, the substance of the thyroid; the inferior are situated externally, 

 sometimes in contact with, and at other times removed a variable distance 

 from the thyroid (Fig. 219). Microscopically the parathyroids consist of 

 thick cords of epithelial cells separated by septa of fine connective tissue 

 and surrounded by capillary blood-vessels. 



Effects of Parathyroid Removal. The surgical removal of the para- 

 thyroids is followed in the course of from two to five days by the death of the 

 animal preceded in most instances by a series of symptoms which are em- 

 braced under the general term "tetany." These symptoms are fibrillary 

 contractions of muscles, tremors, spasmodic contractures and paralyses of 



groups of muscles and not infrequently 

 convulsive seizures and coma. During the 

 convulsion there is an acceleration of the 

 heart-beat, and increase in the respiratory 

 movements which frequently become dysp- 

 neic in character. There is also a loss 

 of appetite, nausea, mucous vomiting, and 

 diarrhea. Death may occur during a 

 convulsion or from coma. (Morat and 

 Doyon.) The administration of calcium 

 causes the tetany to disappear and relieves 

 the animal of these various symptoms, from 

 which it is inferred that the parathyroids 

 influence or regulate calcium metabolism. 



These results for the most part occur 

 only when all the parathyroids are removed. 

 It is asserted that even if one gland is re- 

 tained the animal does not die. The above 

 described symptoms may manifest them- 

 selves, however, but they are slight in* 

 degree. Under these circumstances there 

 is a diminished tolerance for sugar as shown 

 by the appearance of sugar in the urine 

 FIG. 219. The position of the para- when the normal amount is ingested, 

 thyroid glmfe.-(Zuckerkandl.) Vincent and Jolly have recently pub- 



lished the results of a series of experiments 



which seem to negative to some extent the preceding statements. These 

 experimenters state that while it is true, that, as a rule, the removal of both 

 thyroids and parathyroids in the carnivora is a fatal operation, there are 

 nevertheless many exceptions; and in the mammalia generally, e.g., cats, 

 dogs, foxes, guinea-pigs, rats, and monkeys, the exception becomes the rule 



