and counters a fairly uniform assortment and storing part of his wares out 

 of sight. 



Sometimes surplus materials accumulate in special cells or tissues, in a 

 relatively insoluble state. When there is an abundance of calcium, for 

 example, the excess is deposited in small spike-shaped structures, or spicules, 

 inside the long bones. When the intake of calcium is meager, these spicules 

 disappear, being apparently dissolved and redistributed. Fats and proteins, 

 like calcium, are also stored by being segregated in special regions. 



Such segregation of "reserve" material is in some ways like the storage of 

 reserve carbohydrates in underground parts of plants; that is, it seems to 

 be regulated by osmosis and by the action of enzymes. Some of these en- 

 zymes condense soluble substances into colloids or insoluble forms, and 

 some "digest" the reserves into crystalloid forms. In more complex animals, 

 however, the storage of reserves (as well as their later release into the cir- 

 culation) is largely regulated by the nervous system and the "ductless 

 glands" (see pages 302-304). The nerves and glands are set working, how- 

 ever, by chemical changes in the blood. 



Overflow Another way in which the materials in the blood are kept 

 constant is through excretion, or overflow. Waste substances that get into 

 the blood from the active tissues are normally removed by the lungs, the 

 kidneys, and the sweat glands (see pages 216-218). If such substances be- 

 came too concentrated in the blood or lymph, they would be reabsorbed by 

 the cells and there act as poisons. But an excess of sugar, salt, vitamin C, 

 and other substances may be discharged through the kidneys. An excessive 

 intake of water is compensated by an increased flow of urine or by increased 

 sweating: the blood does not become perceptibly diluted. Similarly, exces- 

 sive amounts of carbon dioxide in the blood are quickly removed by the 

 increased ventilation of the lungs and an overflow of carbon dioxide into 

 the lung sacs. 



Under normal conditions only wastes are excreted. Needed reserves may 

 be excreted during certain diseased conditions, however. In diabetes, for 

 example, valuable sugar overflows through the kidneys and is lost in the 

 urine. In other conditions the calcium reserve is lost. 



Hunger and Intake Maintaining the stability of the blood requires 

 not merely removing excesses, but also ensuring suitable intake. Chemical 

 changes in the blood due to deficiencies in nutrients or in water act upon 

 the nerves and upon ductless glands. Feelings of "hunger" or of "thirst" 

 arise in higher organisms, and these "feelings" influence the further conduct 

 of the organisms — specifically with respect to food or drink. Having an 

 appetite or being thirsty does not, of course, ensure getting what the organ- 

 ism needs. But these conditions are parts of the adaptive behavior of or- 

 ganisms, and they are related to the constancy of the blood. 



195 



