358 THE HUMAN BODY. 



color and very vascular, richly supplied with nerves, and is 

 subdivided by connective tissue into cavities or alveoli, the 

 largest of which are just visible to the unaided eye. Each 

 alveolus is lined by a single layer of cuboidal cells, and filled 

 by a glairy fluid which appears to contain mucin. 



The very abundant blood-supply of the thyroid suggests 

 that it is the seat of important metabolic or chemical changes, 

 and observation and experiment confirm this. Extensive 

 disease of the thyroid leads to great changes in the general 

 nutrition of the Body, ending in the condition named 

 myxodosma; muciginous liquid collects in the connective tis- 

 sues, nervous and muscular activity are much impaired, 

 tremors and convulsions occur, and finally a semi idiotic con- 

 dition (cretinism) comes on and is followed by death if all the 

 gland be diseased. Quite similar symptoms follow the com- 

 plete removal of the thyroid body from animals, or from man 

 for tumors; but if even a small part of healthy gland-tissue 

 be left behind the symptoms do not occur. Moreover, if a 

 portion of living thyroid from one animal be grafted beneath 

 the skin of another, the thyroid of the latter can be com- 

 pletely removed without influencing the general health. It 

 would seem then that the gland is the place of formation of 

 some substance essential to the healthy working of the Body, 

 but that under ordinary conditions of life the whole organ 

 is not required to produce the necessary minimum of this 

 substance. This view is strengthened by the fact that in 

 patients with thyroid disease and in animals deprived of the 

 organ the symptoms of myxodoema may be relieved or removed 

 by adding raw thyroid tissue to the food, or by subcutaneous 

 injection of the expressed juice of a fresh gland. When in- 

 jected into a healthy animal extract of thyroid causes arterial 

 dilatation, and a lowering of blood pressure. 



The Thymus. This is a temporary organ of unknown 

 function. It has its greatest size in proportion to the whole 

 weight of the Body a short time before birth. After birth 

 it grows in absolute weight for some time, but then begins 

 to dwindle away and has usually completely disappeared by 

 the twelfth or fourteenth year. It lies in front of the wind- 

 pipe in the lower part of the neck and the upper part of the 

 thorax, and is the "neck" sweetbread of the butcher as dis- 

 tinguished from the true sweetbread or pancreas. The 



