208 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



which are capable of assuming the functions of the thyroids to a greater or 

 less extent when these latter are removed or injured. 



Accessory Thyroids. In addition to the parathyroids a variable number of 

 accessory thyroids have been described by different observers, occurring in the 

 neck or even as far down as the heart. These bodies possess the structure of 

 the thyroid, and presumably have the same function. After removal of the 

 thyroids they may suffice to prevent a fatal result. 



Functions of the Thyroids. Very great interest has been excited within 

 recent years with regard to the functions of the thyroids. In 1856 Schiff 

 showed that in dogs complete extirpation of the two thyroids is followed by the 

 death of the animal ; and within the last few years similar results have been 

 obtained by numerous observers. Death is preceded by a number of character- 

 istic symptoms, such as muscular tremors, which may pass into spasms and con- 

 vulsions, cachexia, emaciation and a more or less marked condition of apathy. 

 The muscular phenomena seem to proceed from the central nervous system, 

 since section of the motor nerves protects the muscles from the irritation. The 

 metabolic changes may also be due primarily to an alteration in the condition 

 of the cord and brain. Similar results have been obtained in cats. Among the 

 herbivorous animals it was at first stated that removal of the thyroids does not 

 cause death ; but so far as the rabbit is concerned Gley l has shown that if care 

 be taken to remove the parathyroids also, death is as certain and more rapid 

 than in the case of the carnivora ; and a similar result has been obtained upon 

 rats by Christiani. It is still asserted, however, that in sheep, horses, and birds 

 the glands may be removed without serious injury to the animal. Cases have 

 been reported also in which dogs have recovered after complete thyroidectomy, 

 but these cases are rare and may be explained probably by the presence of acces- 

 sory thyroids which remain after the operation. It has been observed, too, that 

 the operation is more rapidly and certainly fatal in young animals than in old 

 ones. In the monkey as well as in man the evil results following the removal 

 of the glands develop more slowly than in the lower animals, and give rise to 

 a series of symptoms resembling those of myxredema in man. Among these 

 symptoms may be mentioned a pronounced anaemia, diminution of muscular 

 strength, failure of the mental powers, abnormal dryness of the skin, loss of 

 hairs, and a peculiar swelling of the subcutaneous connective tissue. Physiol- 

 ogists have shown that in the case of dogs the fatal results following thyroid- 

 ectomy may be mitigated or entirely obviated by grafting a portion of the 

 gland under the skin or in the peritoneal cavity. If the piece grafted is suffi- 

 ciently large the animal recovers apparently completely from the operation. 

 So also in removing the thyroids, if a small portion of the gland, or the para- 

 thyroids, be left undisturbed the fatal symptoms do not develop. In human 

 beings suffering from myxoedema as the result of loss of function of the thy- 

 roids it has been abundantly shown that injections of -thyroid extracts, or 

 feeding the fresh gland, restores the individual to an approximately normal 

 condition. 



1 Archives de Physiologic normale et Pathologique, 1892, p. 135. 



