388 HUMAN PHYSIOLOGY 



vaso-dilator. When the thyroid is not developed, the growth 

 and development 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 accom- 

 panied 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. 152). 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, &c. 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. 



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. Several years ago Brown- 

 Sequard maintained that by the administration of testicular 

 substance the general effects of atrophy of these organs might 

 be obviated ; and more recently, as a result of clinical experi- 

 ence, the administration of extracts of the ovaries has been 

 described as relieving certain of the nervous symptoms 

 which supervene on their removal or atrophy. It has further 

 been found that ovarian substance when given to dogs, 

 whether male or female, causes an increase in the rate of 

 proteid metabolism, although no similar action is found 

 with testicular substance. There is thus evidence that the 

 ovaries, like the thyroid, form an internal secretion having 



