WHY. 



1 My flesh and ifiy heart fainteth ; but God is the strength of my heart, and my 

 portion for ever." PSALM LXXIII, 



888. What becomes of tlie arterial Hood thus formed? 



It is sent back from the lungs to the right side of the heart, 

 from which it is sent into the great trunk of the aorta, and from 

 thence it passes into smaller blood-vessels, until it finds its way tc 

 e-'-ery part of the system. 



Fig. 61. THE OR&AKS OP RESPIRATION. 



A. The heart. 

 BB. The lungs. 



C. The aorta, and on either side of the aorta the vessels which convoy tlio 

 venous blood to the lungs to be oxygenised, and the corresponding vessels which 

 return it to the heart, after it has undergone that operation. (For aorta see 

 Fig. 50.) 



D. The trachea,, or large air passage, through which the air passes into tho 

 spongy texture of the lungs, when we breathe. 



E E. Arteries and veins, being the trunks of the vessels that supply tho 

 head, &c. 



889. Why does the chest expand when we breathe ? 



Because the lungs consist of millions of hollow tubes, and 

 cells, which, having been emptied by throwing off carbonic acid 

 gas and nitrogen, become compressed, and the atmospheric nil 



