FUNCTIONS OF ANIMAL ORGANISMS 491 



these is the law of partial pressures, which is that in a mixture of gases 

 each gas exerts a pressure proportionate to the amount present and inde- 

 pendently of the pressure exerted by any other gas which may be mixed 

 with it. Since the oxygen pressure is higher in the air or water about the 

 animal than in the body itself, oxygen enters the organism; and since the 

 reverse is true in regard to carbon dioxide, this leaves the organism. 

 Oxygen enters the animal body, as has already been noted, in a variety 

 of ways. It may enter through the general body surface, as in the lower 

 invertebrates; through respiratory papillae, or skin gills, and also through 

 respiratory trees in the echinoderms; through gills in the higher aquatic 

 invertebrates generally; through the tracheae in the insects; and through 

 the lungs of terrestrial vertebrates. 



The fact that most of the oxygen is transported by the blood in com- 

 bination with hemoglobin as oxyhemoglobin has previously been noted 

 (Sees. 272 and 350). It is also true that the greater part of the carbon 

 dioxide is carried in combination with sodium oxide in the plasma as 

 sodium carbonate. 



It has been discovered that a large amount of carbon dioxide in the 

 blood causes the oxyhemoglobin to break down more rapidly and thus 

 liberate more oxygen in the capillaries for the use of tissues; while when 

 the oxygen reaches a maximum, as in the lungs, the sodium carbonate is 

 broken down more rapidly and carbon dioxide is set free at an increased 

 rate, which hastens its expiration into the lung alveoli. 



Under conditions where no oxygen is available many animals have 

 been found to survive for a considerable length of time, apparently 

 being adjusted to small oxygen consumption and being able to produce 

 the necessary free oxygen from their own bodies. In the absence of 

 oxygen about it an earthworm has been known to live one day; a plan- 

 arian, from one to two days; leeches, four days; and an ascaris, from 

 four to six days. 



513. Secretion.^ — The secretions of various animal bodies are very 

 numerous, are produced by a great variety of organs, and perform a 

 large number of functions, both mechanical and chemical. Some secre- 

 tions assume a solid form and furnish protection, connect structures, and 

 give support to bodies; examples are bone, cartilage, connective-tissue 

 fibers, chitin, and spongin. Others which become solid, such as silk 

 and wax, are used by the animal in various ways, as in the spinning of 

 cocoons by silkworms and the making of comb by bees. Watery secre- 

 tions such as tears serve to moisten surfaces and prevent drying; in 

 aquatic forms, as mucous skin secretions of fishes and amphibians, they 

 prevent drying and infection by the spores which produce disease; or, as 

 perspiration, they assist in temperature regulation. Some, such as 

 digestive secretions, contain enzymes, and others, such as mammary 

 secretions, provide nourishment for the young. The secretions of plant 



