234 INTERNAL SECRETION 



from the 3rd and 4th branchial clefts respectively (see 

 Fig. 30). 



Thyroid Deficiency 



In the adult, degeneration of the thyroid causes the 

 condition known as myxoedema. This is characterised by 

 a dry and thickened skin which pits on pressure, loss of 

 hair, subnormal temperature, low blood-pressure, muscular 

 weakness and hypotonus, and mental dullness. There is 

 a general diminution of the metabolic processes evidenced 

 by a lessened oxygen intake and nitrogen excretion. 

 There is an increase in sugar tolerance and a tendency to 

 deposit fat. Regeneration of tissue after injury is impaired. 



In children, to the above signs • are added failure of 

 growth and of mental and sexual development. This is 

 the condition known as cretinism. 



In short, deficiency of the thyroid leads to a slowing 

 down of all the bodily functions. 



In animals analogous changes can be induced by removal 

 of the thyroid, the parathyroids being left intact. 



Excess of Thyroid 



The activity of the thyroid is increased during pregnancy 

 and lactation, during puberty and menstruation, in the 

 sexual act and other emotional states. 



Exophthalmic goitre is a pathological enlargement of 

 the thyroid associated with increased activity. It is 

 characterised by a rapid pulse, high blood-pressure, mus- 

 cular tremors, protrusion of the eyeballs, and an excitable, 

 nervous state. There is a general quickening of the 

 metabolic processes and a loss of body fat. Sugar-tolerance 

 is diminished. Histologically the gland shows evidence of 

 active secretion — irregularity of the vesicle walls indicatijig 

 an increase of surface from which secretion can occur— 

 the columnar form of cell and absence of colloid. 



