162 ELEMENTS OF MAMMALIAN ANATOMY. 
Over them course numerous thin-walled capillaries, so that 
the carbon dioxid of the blood passes out into the air as 
the oxygen passes into the blood. Expiration is the process 
Fic. 85. PHorocrapH of Human Heart anp LuNcs FROM VENTRAL 
ASPECT. 
a, aorta; t, trachea. 
of forcing the air out of the lungs which have been filled 
by an inspiration chiefly effected by depressing the dia- 
phragm and elevating the ribs. The two processes together 
are spoken of as a respiration. 
PRACTICAL QUESTIONS AND SUGGESTIONS. 
1. Of what four elements are the soft parts of the body largely 
composed ? 
