132 THE INDIVIDUAL ORGANISM 



of Langerhans, produce the hormone insulin. This substance (in cooperation with 

 one of the pituitary hormones) regulates the sugar metabolism of the body. In- 

 sufficient production of insulin causes diabetes, in which the sugar content of the 

 blood rises and sugar is excreted in the urine, the power of the body to oxidize 

 sugar is reduced, and many of the amino acids derived from protein foods are 

 transformed into glucose. An excess of insulin causes a fall in the level of blood 

 sugar, which if sufficiently pronounced may cause insulin shock. Insulin must be 

 liberated into the blood stream continuously to maintain proper sugar balance, 

 and it is required in amounts nearly proportional to the carbohydrate intake of 

 the body. 



Pineal body. This is a small mass of tissue near the base of the brain, which 

 embryologically corresponds with the third median eye of some of the lower 

 vertebrates. In man it develops as a glandular organ until about the seventh year, 

 after which it gradually transforms into a non-glandular fibrous structure; the 

 change is completed some time after puberty. There is reason to believe that some 

 hormone affecting rate of development and time of attainment of sexual maturity 

 is produced by the pineal body, but the hormone has not been isolated and its 

 effects are still obscure. 



Thymus. In infancy and childhood this is a large gland that surrounds the 

 trachea at its lower end. It nearly or completely disappears after puberty. Its 

 chemical composition is unique in that it contains a larger proportion of nucleo- 

 protein than does any other animal structure. Although it may be one of the 

 endocrine glands, this has not been established, and there is some evidence that 

 it may be a storage place for certain food materials useful in growth processes and 

 especially required while sexual maturity is being attained. Experimental animals 

 from which the thymus has been removed in infancy can sometimes be reared to 

 full and apparently normal development if an adequate growth-promoting diet 

 is furnished. Daily injection of thymus extract into rats through nine generations 

 of inbreeding was followed, after the third generation, by precocity in growth and 

 development, which became more marked in each succeeding generation. Com- 

 pared with the first generation and with controls, the following changes occurred 

 from the first to the ninth generation: increase in birth weight from 5.1 to 6 

 grams; teeth present at birth instead of erupting at 8 to 9 days; eyes open at 

 l}i instead of 12 to 14 days after birth; testes descending at 2 to 3 instead of at 

 15 to 29 days after birth; females pregnant at 22 instead of at 70 days after birth, 

 and producing young in 11 instead of in 12 days. These results, though amply 

 confirmed, are not easily interpreted. 



Digestive system. In addition to secretin, already discussed, the digestive 

 system produces a number of other hormones. The stomach mucosa produces a 

 hormonal substance which is liberated into the blood under the stimulus of meat 

 extracts and peptones and causes the gastric glands to secrete. The name gastrin 

 has been proposed for this substance, but it may prove to be histamine, a well- 

 known product of protein oxidation which, produced by the dying tissues of 

 wounds, is at least partly responsible for the phenomenon of surgical shock and 

 is also thought to play a role in allergic reactions. 



The intestine produces four imperfectly known hormones besides the well- 

 understood secretin. One stimulates the gall bladder to contract and discharge 



