208 THE VITAL PROCESSES 



remove also a considerable quantity of water. This is of 

 course in the gaseous form, being known as water vapor. 



Ductless Glands and Internal Secretion. Midway in function be- 

 tween the glands that secrete useful liquids and those that remove 

 waste materials from the blood is a class of bodies, found at various 

 places, known as the ductless glands. They are so .named from their 

 having the general form of glands and from the fact that they have 

 no external openings or ducts. They prepare special materials which 

 are passed into the blood and which are supposed to exert some bene- 

 ficial effect either upon the blood or upon the tissues through which 

 the blood circulates. The most important of 'the ductless glands are 

 the thyroid gland, located in the neck ; the suprarenal bodies, situated 

 one just over each kidney ; and the thymus gland, a temporary gland 

 in the upper part of the chest. The spleen and the lymphatic glands 

 (page 68) are also classed with the ductless glands. The liver, the 

 pancreas, and (according to some authorities) the kidneys, in addition 

 to their external secretions, produce materials that pass into the blood. 

 They perform in this way a function like that of the ductless glands. 

 The work of glands in preparing substances that enter the blood is 

 known as internal secretion. 



Quantity of Excretory Products. If the weight of the 

 normal body be taken at intervals, after growth has been 

 attained, there will be found to be practically no gain or 

 loss from time to time. This shows that materials are 

 leaving the body as fast as they enter and that the tissues 

 are being torn down as fast as they are built up. It also 

 shows that substances do not remain in the body perma- 

 nently, but only so long perhaps as is necessary for them 

 to give up their energy, or serve some additional purpose 

 in the ever changing protoplasm. The excretory organs 

 then remove from the body a quantity of material that is 

 equal in weight to the materials absorbed by the organs of 

 digestion and respiration. This is estimated for the aver- 

 age individual to be about five pounds daily. The passage 

 of waste from the body is summarized in Table III. 



