ENDOCRINE GLANDS AND THEIR FUNCTIONS 203 



the most definite knowledge and which is commonly associated with 

 this portion of the organ. Pituitrin is known to stimulate the mus- 

 cles of the arterial system, increasing or decreasing the blood pres- 

 sure according to the amount of the hormone released in the blood. 

 It is also a stimulant for the musculature of the uterus and the 

 intestinal muscles. 



It is concerned also with the regulation and disposal of carbo- 

 hydrates in the body. The body is able to use an increased quantity 

 of sugar when the secretion of pituitrin is reduced; and, on the 

 other hand, when the quantity of the hormone is more than normal, 

 the body needs less sugar; carbohydrates not actually needed are 

 stored as fat, resulting often in abnormally fat people, extreme cases 

 of which are seen in the circus. The posterior lobe of the pituitary 

 probably does not affect the development of the bones, but the func- 

 tion just discussed is quite definite. 



The posterior lobe of the pituitary is concerned also with the 

 regulation of the secretion by the kidneys. A diseased condition, 

 known as diabetes insipidus, in which the patient voids large quan- 

 tities of urine, is treated by injection of the postpituitary hormone. 

 It appears that the hormone probably enables tissues to utilize and 

 store larger amounts of water than is possible in its absence. 



Still another effect of the secretion is found in the case of certain 

 amphibians and reptiles ; that is, its effect on the pigmentation of 

 the skin of these animals. The removal of the pituitary gland of 

 a frog results, among other things, in the bleaching of the animal 

 and the inability of the frog to alter the color scheme of its skin 

 to agree with the surroundings. When in a strong light or on a 

 light background the retinas of the frog's eyes are stimulated by 

 light rays, and some of the impulses reach the pituitary 's posterior 

 lobe, resulting in a suppression of its secretion, and consequently 

 a lightening of the frog's skin. When the light is decreased the 

 pituitary increases its secretion and the frog has a darker pigmenta- 

 tion. These reactions probably do not occur so directly in the higher 

 groups of vertebrates. Removal of the pituitary gland tends to 

 cause atrophy of the other endocrine glands. 



The Thymus Gland 



The thymus, a small glandular structure located in the chest be- 

 tween the upper part of the sternum and the pericardium, is a tem- 

 porary organ, which normally atrophies in human beings by the time 



