CHEMICAL CHANGES IN PLANTS AND ANIMALS. 427 



Respiration. The most important changes in respired air are the 

 changes in the quantities of oxygen and carbon dioxide. Pure air, 

 after being dried, contains, by volume, of oxygen 20.8 per cent., of 

 nitrogen 79.2 per cent., and a quantity of carbon dioxide (0.04 per 

 cent.) so small that it need not be considered. When 100 volumes of 

 air have been breathed once, it gains a little more than four parts of 

 carbon dioxide and loses a little more than five parts of oxygen ; so 

 that the composition of 100 volumes of inspired air, when expired, is, 

 after being dried, oxygen 15.4 parts, nitrogen 79.2 parts, and carbon 

 dioxide 4.3 parts by volume. 



Much the greater portion of the oxygen lost from respired air 

 enters into combination with the haemoglobin ; a small portion is 

 absorbed by the blood-serum. The immediate source of the carbon 

 dioxide is the blood, in which it exists partly in simple solution and 

 partly in a loose combination with some unknown body. 



The blood is the common carrier of the body : from the alimentary 

 canal it receives ultimately all the food material ; from the lungs it 

 receives oxygen ; these it carries to the tissues for their sustenance ; 

 from the tissues it receives the products of destructive metamorphosis, 

 and carries them to their proper organs of elimination. 



The bright-red color of the arterial blood is due to oxyhsemoglobin. 

 A large portion of this oxygen absorbed by the haemoglobin is given 

 up to the tissues as the blood passes through the capillaries, and we 

 have then the reduced haemoglobin to which is due the dark color of 

 the venous blood. 



In some way, not understood, the blood-plasma takes up the carbon 

 dioxide from the tissues and carries it to the lung. It has been shown 

 that the dark color of the venous blood is not due to the presence of 

 carbon dioxide, but to a decrease of the oxygen. 



In suspension in the plasma are found the food materials on their 

 way to different portions of the body. A small percentage of pep- 

 tones is found, but the quantity is so insignificant in proportion to 

 the total amount absorbed, that it is extremely probable that they are 

 converted into the more common forms of albumin. 



"Waste products of animal life. The changes which the food 

 suffers after having been absorbed by the animal system are ex- 

 tremely complicated, and far from being thoroughly understood. 

 Numerous products and organs are formed and nourished from 

 and by the blood ; among them muscular, nerve, and brain sub- 

 stance, excretions and secretions, such as milk, saliva, bile, gastric 



