THE RABBIT. 35 



some 13 per cent, of it is removed. Besides, why should 

 the term " purification " be restricted to one of the processes 

 by which waste is removed ? The removal of nitrogenous 

 waste (urea, etc.) is no less needful than that of carbon 

 dioxide, and from that point of view the purest blood in the 

 body is that in the renal veins. The best terms to denote the 

 difference between the blood travelling from the lungs and 

 that travelling to the lungs are " oxygenated " for the 

 former, and "deoxygenated" for the latter, since the taking 

 in of oxygen to the blood is a function of the lung shared by 

 no other organ (in the rabbit). Moreover, it is the relative 

 amount of oxygen present which causes the difference in 

 colour between " arterial " and " venous blood." 



2. Essential and Accessory Processes in Respiration. 



In the process of respiration we must distinguish essential 

 and accessory processes. The essential process is the ex- 

 change of gases between the blood and the medium in which 

 the animal lives air in the case of the rabbit. This is a 

 simple physical process, which takes place when the two 

 fluids are separated by only a thin membrane. It must be 

 clearly understood that no chemical processes go on in the 

 lung : there is no oxidation : the oxidation by which carbon 

 dioxide is formed goes on in the living tissues from which 

 the blood has come. 



The accessory or mechanical processes are those by which 

 new supplies of air and of blood are brought to the re- 

 spiratory organ. Part of this mechanism is of course that 

 of the blood-circulation. The other part, concerned with 

 the replacement of the air in the lungs, needs further 

 description. 



3. Larynx and Trachea. The glottis leads into a 

 long pipe lying ventral to the oesophagus and extending 

 through the neck to the thorax. This is the trachea or 

 windpipe, and the most anterior part is enlarged and 

 modified in structure to form the larynx, the organ of voice 

 of small importance in the rabbit. The wall of the trachea 

 is strengthened and kept from collapsing by a series of 

 cartilaginous rings, which are incomplete on the dorsal eide 



