Respiratory System. 171 



CHAPTER XXIII. 



RESPIRATORY SYSTEM. 



All vital activities are carried on at the expense of animal tissue, 

 some of which is broken down with every movement, accompanied with 

 the production of heat. Food is the raw material from which new tis- 

 sue is formed, and it must be constantly supplied to every animal body 

 in order to keep it running. The force with which an animal moves or 

 which in the shape of heat keeps up the working temperature of his 

 body, is the same force which was consumed in putting together the 

 minerals of which his food was formed. This force was originally the 

 sunlight which, acting through the chlorophyl of plant leaves, extracted 

 carbon from the carbonic dioxide of the atmosphere and packed it away 

 in the tissues of plants. When in the chemistry of the vital processes the 

 carbon is sufficiently reduced to regain its affinity for oxygen, the two 

 are again united to form again carbonic dioxide, and the same amount 

 of force which the sun originally expended in separating the two, is now 

 given up to move and heat the animal. 



This union in the animal of oxygen and carbon is, therefore, abso. 

 lutely essential to its continued activity and even its life. The carbon is 

 furnished by the tissues, the blood and the food, in process of diges- 

 tion, and the oxygen is furnished by the atmosphere; It is essential, 

 therefore, that the air should be brought into contact with the interior 

 parts of the body. This contact is called respiration, and it is accom- 

 plished in every living organism, high or low. 



In the lowest organisms, such as the amo3ba, in which there is no dif- 

 ferentiation of parts, the whole body respires, each particle of the sur- 

 face being in contact with the ox3 r gen of the air. In the more highly 

 developed, some special parts are differentiated and adapted to the spe- 

 cial function of admitting the air to contact with the working parts of the 

 body. Most insects have tubes called tracheae, which extend inward 

 from the surface, to convey the air. Animals that are aquatic must in 

 general depend for their respiration on the small amount of air which is 

 enclosed in the water. This air is brought into contact with the blood 

 by means of cilia in some cases, but generally by gills of one form or 

 another. In some marine worms the gills are in little tufts on each 

 side, one pair attached to each segment composing the articulated body. 

 In others the gills are reduced in number and made more effective, and 

 in the higher worms and fishes are placed near the head. In all cases 

 they are a middle ground, a meeting point for the oxygen of the air and 

 the carbon of the blood, and the principal blood vessels are made to run 



