150 HUMAN PHYSIOLOGY 



blood, by changing them into harmless products, or by 

 forming and giving up to the body substances which influence 

 either the metabolism or act upon the nervous or muscular 

 system. This process is called "internal secretion," since 

 these organs throw their products into the blood. 



Among the organs which have an internal secretion are 

 all the blood glands, the thyroid and suprarenal glands, the 

 liver, the pancreas, and perhaps the testes. 



Among the blood glands are also classed the spleen and thymus 

 gland, whose function is not so much a secretion as the formation 

 and destruction of blood corpuscles (see pages 88 and 89). 



I. The thyroid gland. The thyroid gland contains in its 

 connective tissue many completely closed vesicles. The 

 walls of these vesicles are formed by a single layer of 

 cuboidal cells and the vesicles are filled with a " colloid " 

 substance. The thyroid gland must be regarded as a true 

 ductless gland, the cuboidal cells being the secreting gland- 

 cells; the colloidal contents of the vesicles, the secretion. 

 The contents of the vesicles are emptied into the lymph 

 spaces between the vesicles and are thus carried to the 

 blood. 



After the thyroid gland has been excised for goitre, a 

 series of severe disturbances has been observed which sooner 

 or later end in death (cachexia strumipriva). Diseases of 

 the nervous system (diminution of psychical functions, 

 idiocy, and motor and sensory paralysis or spasms), degen- 

 eration of the liver and kidneys, disturbances in metabolism 

 and in the regulation of body heat set in. These phenomena 

 are also present during disease of the thyroid gland, when 

 cedamentous swelling of the skin and idiocy are especially 

 marked (myxcedema). In dogs, extirpation of the thyroid 

 gland produces death in a few days, preceded by strong 

 spasms and severe disturbance in nutrition. In rabbits, 

 extirpation of the thyroid is generally not fatal, but leads to 

 changes in metabolism, myxcedemous swelling of the skin, 

 scaly eruptions, and shedding of the hair. 



The injurious effects of extirpation or disease do not appear 



