CHEMISTRY OF RESPIRATION. 387 



rhythmicality i. e., the regularity with which expiration follows 

 inspiration, then a pause, and again an inspiration followed by an 

 expiration. It is true that this regularity is not as marked in the 

 aged and in children as in others, but in a condition of health it is 

 not markedly departed from. In certain forms of disease, how- 

 ever, the respiratory movements are very irregular. One such is 

 Cheyne-Stokes respiration (Fig. 213), which may occur in fatty 

 degeneration of the heart, uremia, some brain diseases, etc. It is 



FIG. 213. Cheyne-Stokes respiration (after Waller). 



characterized by a beginning shallowness of respiration, the respi- 

 rations gradually becoming deeper and deeper, then a return to 

 shallowness, and finally a complete cessation of respiratory move- 

 ments. This pause lasts for half a minute or more, when the 

 shallow movements begin as before, followed by deeper and again 

 by shallower respirations and then by a pause, etc. This grouping 

 of the respirations is shown in the above curve of this kind of 

 breathing. 



In all reflex acts not only must there be nerve-centers which 

 receive the impulses coming from without and those which gen- 

 erate and emit impulses, but there must be afferent nerves to carry 

 the impulses to the centers and efferent nerves to carry the outgoing 

 impulses. The main afferent respiratory nerve is the vagus or 

 pneumogastric, and it has been demonstrated that this nerve con- 

 tains two kinds of nerve-fibers one which carries the impulses to 

 the inspiratory and the other to the expiratory center, so that 

 division of one nerve slows and deepens respiration to some degree, 

 much more when both nerves are divided. If the end still in 

 communication with the nerve-centers, the central end, is stimu- 

 lated powerfully with electricity, the movements of inspiration 

 become greater, and the diaphragm not only contracts i. e., de- 

 scends but remains in the position of contraction. If, on the 

 other hand, only a weak stimulus is applied, the expiratory move- 

 ments are increased, and the diaphragm remains in the position it is 

 in at the end of expiration. It has further been demonstrated that 

 whenever air is pumped into the lungs so as to distend them, the 

 contraction of the diaphragm diminishes, and when fully distended 

 the diaphragm is in the expiratory position i. e., as it is at the 

 end of an expiration. This distention of the lungs constitutes 

 positive ventilation. On the other hand, if air is pumped out of 



