GENERAL ZOOLOGY 



perform their regulatory functions. Also, like the vitamins, some are protein 

 in nature and others are lipids. Both hormones and vitamins are involved in 

 basic metabolic controls so that dysfunction may be revealed in clinical 

 syndromes. Neither can be stored to any appreciable extent in the body; they 

 must be continuously supplied. But here the similarities end. Animals can 

 synthesize for themselves the compounds we know as endocrines but are 

 dependent on food sources for their vitamins. 



The hypophysis, or pituitary gland, is composed of three parts, the anterior 

 adenohypophysis, the posterior neurohypophysis, and an intermediate lobe. 

 The adenohypophysis is exceedingly important in endocrine regulation because 

 its secretions stimulate the growth, differentiation, and normal function of 

 several other endocrine glands; the thyroid, adrenal cortex, and the male and 

 female reproductive organs respond, respectively, to the thyrotropic, adreno- 

 corticotropic (ACTH), and three gonadotropic hormones. Overall body 

 growth is controlled by the adenohypophysis, presumably by the so-called 

 growth hormone. Oversecretion in youth leads to continued growth of long 

 bones and a condition known as gigantism; undersecretion leads to achondro- 

 plastic dwarfism. Acromegaly occurs in adults when resumption of bone 

 growth is stimulated as a consequence of hypophyseal oversecretion. Chemical 

 analysis of extracts of the adenohypophysis has yielded several proteins and 

 polypeptides which appear to be the active principles to which the respective 

 target organs are responsive. The action of the hormones that regulate other 

 endocrine glands will be discussed in connection with the target organs. The 

 secretions of the adenohypophysis are known to be regulatory in protein and 

 carbohydrate metabolism, but the exact mechanisms are not so well elucidated 

 as in the case of the vitamins. Regulation of the release of the adenohypo- 

 physeal hormones is incompletely understood. 



The neurohypophysis secretes two hormones, the antidiuretic hormone and the 

 oxytocic principle, both of which are polypeptides. The antidiuretic hormone 

 promotes the resorption of water from the kidney tubule (p. 83) and thus 

 regulates water balance. If water intake is restricted, the secretion of the 

 antidiuretic hormone is increased, and the urine maximally concentrated. 

 The regulation of release of the antidiuretic hormone is mediated by centers 

 in the hypothalamus. The oxytocic principle induces contraction of the non- 

 striated muscle in the uterus during late pregnancy; it is widely used to 

 promote parturition. A function for this substance in the normal physiology 

 of the animal has not been established. 



The intermediate lobe of the hypophysis produces an internal secretion 

 known as intermedin. This brings about expansion of the pigment cells in 

 the skin of fishes and amphibia. No function can be ascribed to it in higher 

 vertebrates. It is, you might say, a vestigial hormone. 



The thyroid gland is essentially a device for the accumulation of iodine, 

 very small quantities of which are present in food in the form of iodides 

 (Fig. 4.24). Thyroxine is the iodine-containing amino acid which is the active 

 principle of the complex protein secreted by the thyroid gland. It was 



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