THE PLACE OF BIOLOGY. 33 



supplied by the right side of the heart to the lungs. 

 In this form the oxygen is then carried on in the arterial- 

 ised blood, and is distributed by the action of the 

 left ventricle to the capillary blood-vessels all over 

 the body. In these delicate vessels the partial pressure 

 of oxygen is lowered owing to the passage of oxygen 

 outwards into the living tissues ; consequently the 

 oxyhemoglobin is partly dissociated, so that the red 

 corpuscles deliver up the extra charge of oxygen which 

 they had received in the lungs ; and the blood returns 

 through the veins to the heart and lungs, where it re- 

 ceives a fresh charge of oxygen and repeats the cycle. 



The carbon dioxide of the expired air is also carried 

 by the blood in easily dissociable chemical combination. 

 It is given up to the lung air by diffusion through the 

 alveolar walls ; and the arterial blood returns to the 

 systemic capillaries, where it takes up a fresh charge, 

 owing to the higher partial pressure of carbon dioxide 

 in and around these capillaries. Both the final dis- 

 appearance of free oxygen, and the production of carbon 

 dioxide, appear to occur within the living cells of the 

 tissues. 



The muscular movements associated with respira- 

 tion are initiated by nerve impulses controlled from a 

 " centre " situated in the medulla oblongata. The 

 rhythmic activity of this centre is influenced by afferent 

 stimuli of different kinds, and by stimuli proceeding from 

 other centres ; but all the main control of the centre 

 has been found to depend on the chemical state of the 

 blood supplied to the centre from the heart. Of this 

 latter control we must now consider the nature. We 



know from the most casual observation that the breath- 



3 



