112 THE ANATOMY OF THE HONEY BEE. 



VIII. THE RESPIBATORY SYSTEM. 



The lives of all animals depend upon a constant distribution of 

 free oxygen gas throughout their bodies. This oxygen, continually 

 inhaled and exhaled, is not used in the formation of tissues, it does 

 not become a part of the living protoplasm of the animal — it is the 

 physiological scavenger that eats up certain waste products of me- 

 tabolism which are deadly to the system unless constantly removed or 

 changed into less harmful compounds. The action of oxygen upon 

 these waste substances within the body is comparable with ordinary 

 combustion in that it results in the formation of carbon dioxid gas 

 and water and in the generation of heat. Since the air, which is com- 

 posed of both oxygen and nitrogen, is the source of the oxygen supply, 

 the ordinary breathing processes involve an inhalation also of nitro- 

 gen gas, and the tissues become permeated with it as well as with 

 oxygen. The nitrogen of the air, however, is not known to serve any 

 physiological purpose in the body, its presence being simply unavoid- 

 ably incidental to the inhalation of oxygen. While oxygen and nitro- 

 gen are two most important food elements, the tissues -of animals can 

 not make use of either in the gaseous condition, but must be supplied 

 with substances containing these elements in combination with others 

 in the form of solid and liquid food stuflfs taken into the alimentary 

 canal. Hence, air is not a food, and the respiratory system is to be 

 regarded as chiefly excretory in function. 



The means by which different animals receive oxygen into 

 their systems are various. All aquatic breathers of course use 

 that which is naturally dissolved in water. Many of the lower ani- 

 mals absorb air directly through their skins and into their tissues, 

 while the carbon dioxid escapes the same way. Others that live in 

 the water and whose bodies are covered by an impervious skin or 

 shell have thin-walled, hollow, branching appendages, called gills, 

 through which the blood circulates freely and through whose walls 

 the necessary exchange of gases takes place. Land animals very 

 commonly have some sort of an invagination from the exterior which 

 allows the air to enter thin-walled tubes or cavities and be absorbed 

 into the blood. Land vertebrates have a tube opening from the back 

 of the mouth whose inner end branches profusely and forms a pair of 

 organs called the lungs, through which the blood circulates freely in 

 delicate tubes that allow the transfer of gases. Insects, finally, have 

 a system of internal air tubes, called trachece, opening to the exterior 

 by a number of small orifices, called spiracles, situated along the sides 

 of the thorax and abdomen, which give off branches that ramify 

 minutely to all parts of the organism, thus virtually making a lung 

 of the entire body. The tracheae are thin tubes made of flat epithelial 



