144 PHYSIOLOGY OF FARM ANIMALS [CH. 



The Parathyroids. Normally there are two parathjrroid 

 glands on each side; sometimes they are embedded in the 

 th.>T:"oid tissue but more generally they are situated just outside. 

 Accessory parathj^roids are not uncommon. Unlike the thyroids 

 proper the parathyroids are composed of solid masses of rounded 

 cells and contain no vesicles. It is said however that after 

 thyroidectomy the parathyroids may develop a vesicular structure 

 and even take the place of the thyroids proper. Extirpation of 

 the parathyroids without the thyroids induces tonic muscular 

 contractions or tetany, and the injection of parathyroid extract 

 relieves the condition. It is suggested that many of the effects 

 of thyroid removal may in reality be due to the extirpation of the 

 parathyroids since it is not easy to remove the former organs 

 without destroying the latter ones. 



1 



•rr9\ 



ii ".V 



4.' » . » * 



1 • « j«^ ' » 



,- ♦'• ' 



^,_** a,'f~ '. ,,'. ,.' ,, ^.^,y ccnn Ctii^ 



Fig. 70. Section through parathyroid. Jild. v. 

 blood vessel (after Vincent and Jolly), coll. 

 colloid, conn. tin. connective tissue, epith.c. 

 epithelial cell. ra^. vesicle. 



The functions of the thyroids and parathyroids are still only 

 imperfectly understood. They appear to exercise a profound 

 influence on the nervous system and on the nutrition of the whole 

 body, bvit how much of this influence is due to the thyroids and 

 how much to the parathyroids is uncertain. Moreover the age 

 of the animal is a factor in the results produced, and these are 

 not the same for all species of animals. 



The disease known as goitre is an enlargement of the thyroid 

 and the condition produced may be one of either hypothj^roidism 

 or hyperthyroidism. In the case of endemic goitre, notwith- 

 standing the enlargement of the gland the symptoms are those of 

 hypothyroidism. In the case of exophthalmic goitre or Grave's 

 disease, the condition is apparently the result of hyper-secretion, 



