RESPONSIVENESS IN VERTEBRATES 



Larynx 



Thyroid 



Trachea 



Secretion 

 Capillary 



A B 



Fig. 4.24. The thyroid gland. A, the gland of man as seen from the front, showing its position 

 with reference to the larynx and trachea. B, diagram of a section of the gland, showing the 

 follicle cells which produce the secretion and the capillaries which distribute it. 



isolated in 1916 and synthesized in 1927. Thyroxine regulates the basal 

 metabolic rate, since it controls the level of cellular oxidation. Current 

 interpretation is that the iodine-containing secretion affects the permeability 

 of the mitochondrial membrane and thus in some wav modifies the enzyme 

 team of the mitochondrion (p. 19). Hyperthyroidism, or oversecretion, 

 results in a high rate of cellular oxidation, reflected in rapid heart beat and 

 excitability. Abnormal function of the thyroid gland may bring about its 

 enlargement, forming a goitre. Hypothyroidism, or undersecretion, in 

 children results in stunted physical and mental growth; in extreme cases, the 

 individual is a cretin. In regions where the soil is deficient in iodine, hypo- 

 thyroidism is endemic. In many parts of the United States the use of iodized 

 table salt is a dietary requirement. The thyrotropic hormone of the adeno- 

 hypophysis stimulates the production of hormone by the thyroid. If the 

 amount of thyroxin in the blood is adequate, thyrotropic hormone is not 

 released. 



The protein secretion of the parathyroid glands is related to the control 

 of calcium in the blood. It will be recalled that vitamin D also contributes 

 to this control (p. 31). Calcium is necessary for the differentiation of bone 

 and for the clotting of blood and is required for the normal sensitivity of 

 nerve endings and muscle. Hypofunction of the parathyroids, followed by a 

 lowering of the calcium ion content of the intercellular fluid, results in hyper- 

 sensitivity leading to convulsive contractions of muscles. When the amount 

 of calcium in the blood plasma falls below a certain level, the parathyroid 

 hormone is released. Its effect is on the kidney tubule where it decreases 

 the amount of phosphate reabsorbed. This results in a reduction of the 

 amount of phosphate in the blood which, in turn, leads to withdrawal of 

 calcium from bone to build up the calcium level of the blood. 



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