THE THYROID AND PARATHYROID GLANDS 753 



looked that the effects produced by thyroidectomy and parathyroid- 

 ectomy are often very different in animals of the same kind at dif- 

 ferent ages. Speaking generally, it may be said that the influence of the 

 parathyroid is focused mainly on the nerve centers and only to a second- 

 ary degree on the metabolic functions, whereas the reverse is the case 

 with the thyroid, its main effect being on metabolism, although it prob- 

 ably also exercises a secondary effect on the nerve centers. More so 

 than in the case of any other endocrine organ, our knowledge concerning 

 the function of the thyroid has been gained by clinical experience, and 

 it is difficult to say whether the cJinical or the experimental method has 

 contributed the greater amount of information. 



The results of experimental extirpation of the thyroid vary accord- 

 ing to the age of the animal, and frequently they are by no means 

 marked, provided sufficient parathyroid tissue has been undamaged. 

 The symptoms are in general thickening and drying of the skin, with a 

 tendency to adiposity and a loss of tone of the muscle. The body tem- 

 perature is low and the sexual functions become subnormal. Nervous 

 symptoms in the direction of mental dullness and lethargy are also 

 usually present. Surgical removal of the thyroid in man produces the 

 condition known as CQchexia strumipriva. The symptoms may first of 

 all become apparent a few days after the operation, or they may remain 

 latent for years, and then develop so as' to produce the condition known 

 as myxedema. When nervous symptoms are prominent in cachexia 

 strumipriva, it is usually taken as evidence that an excessive amount 

 of parathyroid tissue has been destroyed. Kocher states that after com- 

 plete loss of the thyroid, life is impossible for more than seven years, 

 and that to prevent ultimate ill effects, at least one-fourth of the organ 

 should be left intact. 



Disease of the Thyroid 



The symptoms of diseased conditions of the thyroid may be inter- 

 preted as the consequence of increased or diminished functioning of the 

 gland. Sometimes, however, the less active gland is really increased in 

 bulk, this increase being caused by the accumulation in it of very large 

 quantities of colloid material accompanied by an attenuated condition 

 of the vesicular cells (see page 751). When the gland is atrophied at 

 birth, the condition of cretinism soon becomes developed (Fig. 194). The 

 characteristic features of cretinism are: (1) An arrest of growth, espe- 

 cially of the skeleton, accompanied by incomplete ossification of the long 

 bones and failure of the fontanelles of the skull to close properly. (2) 

 Poor development of the muscular system. (3) An unhealthy, dry, swollen 

 condition of the skin, so that it is yellowish in color, the face being pale 



