442 



ORGAN SYSTEMS OF MAN 



that the offspring has ceased to rely on the 

 mammary secretion as its principal source 

 of food. Once lactation ends altogether the 

 menstrual cycle begins again. 



The thyroid 



The thyroid, together with the remaining 

 glands, are purely endocrine in function. 

 The thyroid has various shapes in different 

 vertebrates but in man is bilobed and lies 

 on either side and under the larynx (Fig. 

 16-29). The two lobes are connected by a 

 narrow strip of tissue, called the isthmus, 

 passing across the trachea. The presence 

 of the gland can be determined by merely 

 feeling it with the fingers. 



This gland was seen, naturally, by early 

 anatomists, and its importance suspected 

 because they noted that in certain individ- 

 uals it became enlarged, seeming even to 

 cause their death. At the beginning of the 

 Christian Era the Greek physicians pre- 

 scribed the drinking of sea water as a cure 

 for goiter (the term used for the swollen 

 gland). Later, others gave their patients 

 products of the sea, such as dried seaweed 

 leaves, which undoubtedly gave some relief 

 because all sea products are rich in iodine, 

 the important ingredient in the production 

 of the thyroid hormone, thyroxin. The exact 

 function of the thyroid was not known until 

 replacement experiments in 1885 demon- 

 strated that the gland did produce a hor- 

 mone. This was isolated in pure form in 

 1916 and synthesized in 1927 by Haring- 

 ton and Barger, two English investigators. 

 When this substance is administered to an 

 animal deprived of its thyroid, the animal 

 remains perfectly normal in every respect. 

 If it is denied such treatment, stark meta- 

 bohc changes occur which, if prolonged, 

 may terminate the life of the animal. What 

 is the specific function of this gland? 



It is generally agreed that the thyroid 

 gland secretes thyroxin, which controls the 

 level of basal metabolism, that is, the rate 

 of burning foods and formation of nitrog- 

 enous wastes, as well as the degree of 



irritability. Thyroxin must be produced at 

 a uniform rate in order that these important 

 processes proceed at what is spoken of as 

 a normal level. If more or less is produced, 

 these processes accordingly increase or de- 

 crease in speed with accompanying symp- 

 toms that are very definite and easily 

 recognized. A diseased thyroid merely pro- 

 duces too little or too much of its secretion. 

 Underactivity. When the gland fails to 

 produce the proper amount of thyroxin the 

 effects are somewhat more pronounced in 

 a young animal than in an adult. For ex- 

 ample, if the thyroids are removed from 

 tadpoles or pups the animals will not ma- 

 ture properly. The tadpoles will not meta- 

 Inorphose into frogs and the pup will not 

 mature into an adult dog. Likewise in 

 human beings, if a child has a deficient 

 thyroid he becomes a cretin. Such a child 

 is small and badly formed, with pudgy, 

 puffy skin and swollen tongue, and his men- 

 tal development is at a rather complete 

 standstill. If given thyroxin in the early 

 stages of the disease, the child responds re- 

 markably well and can grow into a normal 

 adult. Obviously, if a human cretin is al- 

 lowed to live for twenty years without treat- 

 ment, thyroxin will do him little good be- 

 cause his body tissues have completed their 

 development and can be changed but little. 

 If the thyroid becomes atrophied for 

 some reason and fails to produce an ade- 

 quate supply of thyroxin in the adult, a 

 familiar disease known as myxedema re- 

 sults. The obvious symptoms of the disease 

 are general loss in vigor, reduction in men- 

 tal activity, increase in weight, and a thick- 

 ening of the skin to give it a puffy appear- 

 ance. Less obvious symptoms are a drop in 

 basal metabolism, the improper burning 

 of food, sometimes as much as 40 per cent 

 below normal, a slowing of the heart rate, 

 and a lessening in the sex drive. It seems 

 that the entire machinery of the body 

 slows down. The administration of proper 

 amounts of thyroxin or thyroid extracts 

 restores the rate of metabolism to its nor- 



