THE RESPIRATORY SYSTEM 



107 



of the trachea into its bronchi, together with the bronchial ramifi- 

 cations, forms the trunk and main branch portions of a rather 

 complex system of tubes (Fig. 60), of which the terminal air-spaces 

 are the final and functional parts. The branching of the blood- 

 vessels interlaced with the air passages is equally complex (Fig. 59, 

 63) and these vessels, besides providing for the aeration of the blood, 



Fig. 59. Corrosion preparation of the lungs of the rabbit, dorsal view. 

 The trachea, the bronchi, and most of the more anterior bronchioles have 

 been filled with a pale mass, the veins with a dark mass, and the arteries with 

 one of intermediate shade, the tissues then having been dissolved away: 

 a, aorta; t, trachea; vcd, right superior vena cava; vci, inferior vena cava; 

 vcs, left superior vena cava; vp, right pulmonary vein. 



are said to form a blood-depot which assists in the regulation^of 

 the relative output of the two sides^ of the heart. 



Breathing 



What is commonly described as respiration, or the act of 

 breathing, is a mechanical, muscular process accessory to the 



