CHAPTER VIII.

RESPIRATION 123

_The Objects of Respiration--The Lungs--The Air-Passages--The Movements of Respiration--Expiration and Inspiration--The Frequency of Respiration--Capacity of the Lungs--The Air we Breathe--Changes in the Air from Respiration--Changes in the Blood--Interchange of Gases in the Lungs--Comparison between Arterial and {7} Venous Blood--Respiratory Labor--Impurities of the Air--Dust--Carbonic Acid--Effects of Impure Air--Nature's Provision for Purifying the Air--Ventilation--Animal Heat_.