EXCRETION 103 



a complex reddish compound containing iron and carried 

 in the red corpuscles. Under ordinary atmospheric pres- 

 sure this substance forms with oxygen a feeble compound 

 that under reduced pressure, as in an air pump, again 

 breaks up into hemoglobin and free oxygen. In the lungs 

 therefore, oxygen combines with the hemoglobin and in 

 this combined state is transported to the tissue capillaries. 

 Here the surrounding lymph is practically without oxy- 

 gen, the pressure or tension is nearly zero, and as in the 

 air-pump the oxygen separates from the hemoglobin, dif- 

 fuses through the capillary wall into the lymph and from 

 thence into the cells. 



In some of the lower animals, such as the clams, snails, 

 crabs and their relatives, compounds of copper act like 

 hemoglobin in the respiratory process. There is also 

 some evidence that substances carrying zinc and man- 

 ganese may perform a similar function in certain species, 

 but the subject is one of great complexity and is very 

 imperfectly understood. 



In the removal of carbon dioxid from the body the case 

 is quite similar to that of oxygen. Upon diffusing from 

 the cell this gas enters the lymph, diffuses through the 

 capillary wall, always travelling into regions of low ten- 

 sion, and in the blood forms a loose union with the plasma 

 that releases it under the diminished carbon dioxid pres- 

 sure of the lungs through whose walls it makes its escape. 



Excretion. — ^The oxidation of cell materials results, 

 as has been described, in the formation of worthless 

 products known as excretions. In addition to carbon di- 

 oxid gas, these include mineral salts, water, and a con- 

 siderable number of simple nitrogen compounds. In 

 the amoeba, paramoecium, and many related unicellular 

 animals, these substances are believed to accumulate in 

 one or two definite spots in the body along with water 

 absorbed from the outside. At frequent intervals this 

 contractile vacuole, as it is termed, empties the droplet 

 to the exterior and thus functions as a simple excretory 



