METABOLISM, NUTRITION AND DIETETICS 559 



much more likely to escape removal with the thyroid in the case 

 of herbivorous than in the case of carnivorous animals. 



But even in one and the same species considerable variations 

 may exist. It is easy to see, then, that in removing the thyroid 

 the parathyroids would sometimes be completely removed as 

 well, while at other times all or some of the parathyroid tissue 

 would be spared. Add to this that sporadic masses of thyroid 

 tissue (accessory thyroids), often existing as far down as the 

 root of the aorta (always, indeed, in certain animals e.g., the 

 dog), must necessarily be spared in the most complete thy- 

 roidectomy, and it will cease to excite surprise that the symptoms 

 and pathological changes described after that operation should 

 have been so various and so contradictory. We know now that 

 the parathyroids are perfectly distinct organs from the thyroid 

 in histological structure, in function, and in the consequences 

 of their removal. Nor do they show any compensatory hyper- 

 trophy when the thyroid alone is excised, or any changes which 

 would indicate a definite relation to, still less an active participa- 

 tion in, the pathological processes occurring in the thyroid in 

 goitre. The parathyroids contain no iodine, while iodine is a 

 characteristic constituent of the thyroid. 



Parathyroidectomy. Total extirpation of the parathyroids is 

 followed by a train of acute symptoms, ending fatally, as a rule, 

 in from one to ten days. The typical nervous symptoms follow- 

 ing the operation have been described as those of ' tetany/ and the 

 tetany which used to be included among the consequences of 

 removal of the thyroid is now known to be due to the simul- 

 taneous excision of the parathyroids (Kocher). A cat, after the 

 combined operation, is perfectly well on the first day. On the 

 second day a curious shaking of the paws is seen, tremors of 

 central origin soon appear, and increase in severity until at length 

 they culminate in general spasmodic attacks. Even when the 

 animal is at rest the fore-legs tend to be flexed, while the hind- 

 legs are extended, and this attitude is exaggerated in the con- 

 vulsions. In the later stages unconsciousness is associated with 

 the onset of the convulsions. Similar results follow excision of 

 the parathyroids alone in dogs. Although the tetany is the most 

 striking symptom, it is only one token of a profound general dis- 

 turbance of nutrition. The pulse-rate and the rate of respiration 

 are markedly increased. There is profuse salivation, with dilata- 

 tion of the stomach and duodenum, and fever. The exact signifi- 

 cance of these symptoms is unknown. The administration of 

 calcium completely relieves them, and by its use death may be 

 long or perhaps indefinitely postponed ( W. G. MacCallum) . The 

 mode of action of the calcium has not been made clear as yet. It 

 does not seem to be so efficacious in rabbits as in dogs ( Arthus) . 



