RESPIRATION AND RELEASE OF ENERGY 117 



the water to pass out but settle back immediately after, so as to prevent 

 water from entering there. The action is repeated, and a pulsating 

 current of water is kept up. Lobsters have a fanlike structure at one 

 edge of the gill chamber, and by its movement a continuous stream of 

 water is kept flowing over the gills. 



Land animals have various devices acting to the same end. Insects 

 expand their chitinous exoskeleton by muscular movement, and air 

 rushes in: the skeleton collapses, and the air is forced out. Valves at the 

 entrances of the tracheae determine which ones shall receive air. In 

 general the air chambers or passages have, of themselves, no power of 

 either expansion or contraction ; they are manipulated by something else. 

 The lungs in man are expanded at all times, to fill the cavity of the 

 thorax, merely by the air pressure within them. If the chest expands, 

 more air is forced in from the outside to equalize the pressure. In 

 inspiration, the volume of the chest is increased by two means: (1) 

 raising the ribs, and (2) lowering the diaphragm. The ribs are movably 

 joined to the vertebral column, from which they slope downward both 

 laterally and forward. The muscles between the ribs contract, so that 

 all ribs are lifted, the lowest ones most of all. Since the ribs slope down- 

 ward, elevating them pushes them outward (sidewise and to the front), 

 thus enlarging the chest in both directions. The diaphragm, a muscular 

 sheet across the bottom of the thorax, is convex like an inverted bowl. 

 When its muscles contract, the diaphragm is flattened, thus further 

 increasing the size of the chest cavity. Air pressure in the lungs is thus 

 reduced, hence air is forced in to restore an equilibrium. In expiration, 

 the rib muscles relax, and the ribs drop, largely by their own weight. 

 Both width and depth of the thorax are thus decreased. When the 

 muscles of the diaphragm relax, tension of the muscles of the abdominal 

 wall presses the viscera up against it and the diaphragm rises. With 

 the accompanying decrease in the size of the thorax, air is forced out 

 of the lungs. 



All such movements designed to ensure a continuous supply of oxj^gen, 

 whether in air or water, are termed breathing movements. To supply 

 the right amount of air, these movements must vary in vigor as the 

 animal's activities change. In man, the rate of breathing is controlled 

 by a nerve center in the medulla, posterior division of the brain. The 

 action of this center depends on the amount of carbon dioxide in the 

 blood. If muscular activity increases, much more carbon dioxide enters 

 the blood from the tissues; this extra quantity stimulates the respiratory 

 center in the medulla, and breathing becomes more rapid. Panting is 

 an extreme response to such stimulation. If the breath is voluntarily 

 "held" for a short time, carbon dioxide accumulates in the blood to 

 such an extent that restoration of breathing is forced. No will power 



