464 GENERAL CONSIDERATIONS 



lice are used as food by ants. Still other secretions contain scents which 

 attract individuals of the opposite sex, repel enemies, and identify 

 related individuals. Finally should be named the internal secretions, or 

 hormones. 



All animals produce secretions as the products of metabolism. These 

 are in many cases accumulated in the cells as droplets or granules and are 

 passed out when the cell receives the appropriate stimulus. Ferments 

 may be thus stored in an inactive form, when they are called zymogen, 

 and then be made active by some other substance; pepsinogen, for 

 example, which is formed in the cells of gastric glands, is changed to active 

 pepsin by the hydrochloric acid of the gastric juice. 



512. Internal Secretions. — Among the metabolic activities of animals 

 have been noted the production of a number of internal secretions, or 

 hormones, which concern some of the most vital activities of the organism. 

 It is presumable that these occur in a great variety of animals and are 

 most numerous in the higher forms. They have been most studied in 

 man and the higher vertebrates. 



Among the organs which produce internal secretions in man (Fig. 

 299) is the thymus gland. This gland, which is situated anteriorly in the 

 upper part of the thorax, produces a hormone which retards both 

 development and differentiation. When the gland is overdeveloped the 

 condition leads to a prolongation of immaturity and lessens the size of the 

 organism. The thyroid gland, which usually consists of two lobes, one 

 on each side of the trachea in the neck region, has an effect the opposite 

 of that of the thymus. Its activity promotes growth and differentiation, 

 resulting in large size and the early maturation of the organism. The 

 secretion of the thyroid gland also stimulates the metabolic processes in 

 the body and increases energy transformations. The secretion of the 

 adrenal organs, above the kidneys and in contact with them, increases 

 blood pressure and stimulates the body to more intense activity in cases 

 of emergency. The -pituitary body, lying at the base of the brain, is made 

 up of two parts, the secretions from both influencing growth and differ- 

 entiation. The secretion of the anterior portion increases particularly 

 the growth of connective tissues and muscle, sensitizes the brain cells, 

 stimulates the intellectual activities, and develops the memory. The 

 secretion of the posterior part of the pituitary gland increases vascular 

 tonus and magnifies the emotions. The parathyroids are glands situated 

 at the sides of the thyroid or in some cases are included in it. The secre- 

 tion of these glands increases calcium metabolism and has a regulating 

 effect upon the irritability of the cells of the body. The pancreas is not 

 only a gland secreting several digestive ferments but within it are masses 

 of cells knowm as islets of Langerhans, the secretion of which controls 

 carbohydrate metabolism in the body. The failure of these cells to 

 function leads to a disease known as diabetes. 



