42 SECRETION 



strong emotions, epileptic attacks, etc., indicating a control 

 of the central nervous system. The connection between the 

 gland and the uterus, which stand in close relation, is mainly 

 through the blood. 



That the secretion of milk may be continuous is not known 

 with certainty, but it is probable that as it accumulates in 

 the sacculated ducts of the gland the tension finally inhibits 

 further secretion. The emptying of the ducts is the normal 

 stimulus, either directly or reflexly, for a renewed activity of 

 the gland. Otherwise the cells undergo retrogressive changes, 

 but they never become as they were before the first pregnancy. 



Thyroid Gland. The thyroid is more or less closely asso- 

 ciated with four bodies, the parathyroids, which are totally 

 different in structure and in function. Many of the experi- 

 mental discrepancies in the past were probably due to failure 

 to recognize this difference. In man the custom of removing 

 the thyroid in cases of goitre often led to unconscious extirpa- 

 tion of all or some of the parathyroids, leading respectively 

 to rapid death or to a condition called cachexia strumipriva. 

 In the latter case the symptoms resemble those of myxedema, 

 and are characterized by hyperplasia. The newly formed 

 connective tissue contains much mucin. The skin is dry 

 and the hair falls out. The expression becomes stupid; there 

 is a decided failure in mental power; and the movements are 

 clumsy and tremulous. When the thyroids are affected early 

 in life a peculiar condition of idiocy, known as cretinism, 

 results. 



The symptoms of pure thyroidectomy differ in different 

 animals, being more severe in the young, and more severe 

 in carnivora than in herbivora. Emaciation, muscular weak- 

 ness, and subnormal temperature are marked. All of these 

 symptoms are obviated if a portion of the thyroid is left in 

 place, or if a portion be grafted into the abdominal cavity or 

 under the skin. Administration of extracts of the gland or 

 even feeding the fresh gland has a similar effect. The active 

 principle in producing this effect is supposed to be a compound 

 rich in iodine, called iodothyrin. The amount of iodine in the 

 thyroid is variable. It is absolutely essential to the normal 

 activity of the gland. The physiological and therapeutic 

 activity of thyroid substance varies directly with the iodine 

 in organic combination. 



