406 ESSENTIALS OF PHYSIOLOGY. 



changes, e.g. obesity, and failure of sexual development ; in the adult 

 degenerative changes take place in the sexual organs. 



In man two abnormal conditions have been found in association 

 with hypertrophy of the pituitary body. In the first place, hyper- 

 trophy has been observed in unusually tall people, and, secondly, over- 

 growth or tumours of the gland are found in the disease known as 

 acromegaly . This occurs in adults, and is characterised by progressive 

 enlargement of the bones of the face and extremities. 



Both these changes are probably the result of increased functional 

 activity of the anterior lobe only, since young animals to whose food 

 the anterior lobe is added grow more rapidly than control animals 

 which receive no such addition ; extracts of the posterior lobe or pars 

 intermedia have no influence on growth. 



We may conclude, therefore, that the anterior lobe is concerned 

 with the process of growth, hypertrophy leading to overgrowth of the 

 skeleton, and partial removal to failure of development of the body as 

 a whole and of the sexual glands. 



Extracts of the posterior lobe, when injected into an animal, have a 

 direct action on plain muscle all over the body ; they cause constriction 

 of the arterioles and a rise of blood pressure, contraction of the 

 muscular coats of the digestive tract and of the bronchioles, and 

 contraction of the uterus. 



The extracts also produce an increased flow of urine, which was at 

 first attributed to the presence of a substance having a specific effect 

 upon the renal cells ; it is probable, however, that the diuretic effect 

 is merely an indirect result of the more rapid flow of blood through the 

 kidney which follows the injection of the extract. 



Extracts of the posterior lobe increase the secretion of milk, and 

 after the injection a larger amount of milk is formed by the animal 

 in the course of twenty-four hours. The active principle of the extract 

 has not been isolated, but it is not destroyed by boiling, and is compara- 

 tively stable. It is formed entirely by the pars intermedia, extracts of 

 the posterior lobe proper being quite inert. 



The secretion of the pars intermedia is believed by some writers to 

 pass into the cerebro-spinal fluid, but the extent to which the effects 

 produced by extracts of the posterior lobe of the gland are normally 

 brought about in the body is unknown. 



THE THYROID AND PARATHYROID GLANDS. 



The thyroid gland consists of two lobes, one on each side of the 

 trachea, united by an isthmus, and is composed of closed spherical 

 vesicles containing a viscid colloid material ; the walls of the vesicles 



