200 TUP INTFRNAL nNVIRONMFNT OF Tlir nODY Part III 



the blood depends upon the proper amount of a hormone produced by the 

 parathyroid glands (Chap. \5). Potassium acts in opposition to calcium and 

 it is the balance between the two rather than the exact amount of either 

 one that is essential. In blood plasma there are about 20 milligrams of po- 

 tassium per 100 cubic millimeters of plasma but over 800 milligrams of 

 sodium; in muscle cells this proportion is reversed. If blood potassium rises 

 a little, muscle is stimulated; if it rises too much, the muscle is paralyzed. Its 

 amount is regulated partly by a hormone from the cortex of the adrenal gland. 

 Nervoii.s Control of Body Heat. The main control of body temperature is 

 in the hypothalamus, an ancient part of the floor of the brain. When this is 

 destroyed, the muscles are paralyzed and their ability to liberate heat is lost; 

 when it is stimulated, the muscles are activated and the body temperature 

 rises (Fig. 12.3). 



Human Blood Cells 



This description of blood cells is based chiefly upon human blood with 

 references to other vertebrates (Fig. 12.4). 



There are two main kinds of blood cells, red and white ones (Fig. 12.5). 

 Erythrocytes, red cells or corpuscles are those whose cytoplasm is permeated 

 with nonliving hemoglobin. Mammalian red cells lose their nuclei as they 



Fig. 12.3. Nervous control of body heat. Rabbit with left ear in normal con- 

 dition; the blood vessels are kept in a state of partial constriction by vasocon- 

 strictor nerves. The vasoconstrictor nerves to the right ear have been cut; the 

 vessels are dilated and the ear is unnaturally hot. (From Best and Taylor: The 

 Livinii Body. ed. 3. Copyrighted by Henry Holt and Co. Reprinted with their 

 permission. ) 



