4 o8 VETERINAKY PHYSIOLOGY 



which regulates the rate of the metabolic processes in the 

 body by increasing them when such an increase is desirable. 

 When the thyroid is not developed, the growth and develop- 

 ment of the individual are partially arrested, and the condition 

 of cretinism is produced. Atrophy of the structure in adult 

 life causes a train of symptoms somewhat resembling those 

 produced by its removal, and constituting the disease, myxcedema. 

 It has been suggested that a condition of increased activity of 

 the action of the heart, usually accompanied by prominence of 

 the eyes and swelling in the region of the thyroid exophthalmic 

 goitre may be due either to increased activity of the structure, 

 or to deficient action of the parathyroids. 



4. Parathyroids. Two to four small nodules are found in 

 close relationship to each lobe of the thyroid, often lying in 

 its substance, and these are formed of columns of cells with 

 capillary blood vessels between them (fig. 164). More or less 

 successful attempts have been made in different animals to 

 remove them without the thyroid, or the thyroid without 

 them, and the general result of these experiments is that the 

 nervous symptoms which follow ordinary thyroidectomy the 

 tremors, etc. seem to be due to the loss of the parathyroids, 

 while the metabolic changes are probably due to want of the 

 internal secretion of the thyroid. After removal of the thyroid 

 some observers have found that the parathyroids undergo a 

 change and become like the thyroid. 



5. Ovaries and Testes. It is well known that removal of 

 these organs causes characteristic changes in the animal: a 

 tendency to the deposition of fat being produced, the activity 

 of the central nervous system being somewhat modified, the 

 voice in the male losing its masculine character, and the 

 thymus persisting, in the male at least, for a considerably 

 longer period than in the normal. That it is not the tubules 

 of the testes but some product of the interstitial cells which 

 act in this way is demonstrated by the fact that ligature of the 

 vasa deferentia which causes atrophy of the tubules does not 

 prevent the development of sexual characters. Several years 

 ago Brown-Se"quard maintained that the general effects of 

 atrophy of these organs might be obviated by the adminis- 

 tration of testicular substance ; and more recently, as a result 

 of clinical experience, the administration of extracts of the 



