THE DUCTLESS GLANDS. 407 



consist of a single layer of cubical epithelial cells. The colloid material 

 contains iodine in organic combination as a substance, iodothyrine, and 

 the vesicles are bound together by connective tissue in which lie 

 numerous blood-vessels and nerves. 



The parathyroid glands are four in number and lie close to, or 

 embedded in, the thyroid gland ; they are quite small and consist of 

 columns of granular cells, some of which are pigmented. 



Attention was called to the importance of the thyroid gland first 

 by the observation that the gland is atrophied in the disease known as 

 jtti/,*:oedema, and later by the serious and even fatal effects of its 

 complete removal in man ; this may be followed either by acute nervous 

 symptoms accompanied by muscular spasms (tetany), or by chronic 

 changes resembling myxcedema. The symptoms of myxoedema are 

 obesity, dryness and thickening of the skin, falling out of the hair, 

 slowness of mental processes and of speech : indeed, all the metabolic 

 processes in the body become more sluggish, and the respiratory 

 exchange is diminished. 



Deficiency or absence of the gland at birth gives rise to the disease 

 known as cretinism, in which growth, both mental and physical, is 

 extremely retarded ; a cretin aged fifteen to eighteen years may 

 resemble a child of two or three years of age in its size and mental 

 development. The symptoms, both of cretinism and of myxcedema, are 

 due to the absence of some substance normally formed by the thyroid 

 gland, from which it passes into the lymphatics and so into the blood 

 stream. This is shown by the fact that extracts of the gland, or the 

 gland itself, when given by the mouth, lead to complete recovery in 

 myxcedema and to very marked improvement in cretinism. 



The action of the gland seems to depend partly or wholly upon the 

 presence of iodothyrine, since the activity of the extracts is greater 

 when they contain much iodine. It is evident that the thyroid gland 

 exerts an important influence on the metabolism of the body, including 

 the nervous system. This is further shown by the observation that in 

 myxoedema much larger quantities of sugar can be taken by the 

 mouth without producing alimentary gtycosuria than is the case in 

 normal persons. 



The effects of removal of the gland in animals are of two kinds. 

 Frequently, especially in carnivora, its removal is rapidly followed by 

 acute symptoms, of which the most striking are spasms of the skeletal 

 muscles known as tetany, and in young animals death may occur in a 

 few days. The acute stage may be followed or may be replaced by 

 chronic disturbance of nutrition, and in monkeys typical myxcedema 

 similar to that seen in rnan has been observed. Opinion is divided as 



