426 TEXT-BOOK OF PHYSIOLOGY. 



tory and vaso-motor centers from the accumulation of the carbon dioxid. 

 With the exhaustion of the nerve-centers, there is a general relaxation of the 

 skeletal muscles, the cardiac muscle, a fall of the blood-pressure, and dilata- 

 tion of the pupils. 



The Cheyne-Stokes Respiration. A modification of the respiratory 

 movements characterized by periods of rest alternating with periods of 

 activity was described in 1818 and in 1854 by the two writers whose names it 

 bears. The periods of rest vary in duration from twenty to thirty seconds; 

 the periods of activity from thirty to sixty seconds and may include from 

 twenty to thirty respiratory movements. 



Each period of rest of the respiratory mechanism is closed by the appear- 

 ance of a slight shallow respiratory movement, which is immediately fol- 

 lowed by a second, slightly deeper, and this in turn by a third, a fourth, a 

 fifth, and so on, each becoming deeper than the preceding until a certain 

 maximum is reached, after which, each succeeding movement gradually 



FIG. 202. TRACING SHOWING THE CHEYNE-STOKES FORM OF RESPIRATION. (Hill.) 



diminishes in depth until finally the movement becomes imperceptible and a 

 new period of rest supervenes. A graphic representation of the Cheyne- 

 Stokes type of respiration is shown in Fig. 202. This type of respiration is 

 frequently an accompaniment of certain pathologic conditions, e.g., uremic 

 states, cerebral hemorrhage, heart diseases, arteriosclerosis, etc., though no 

 satisfactory explanation of it has yet been presented. A similar though far 

 less marked periodicity in the respiratory movements is frequently observed 

 during sleep, especially in children. A periodicity can also be developed 

 by dividing transversely the medulla oblongata just above the calamus 

 scriptorius, which either injures the respiratory center or removes from it 

 some cerebral influence. 



THE EFFECT OF THE RESPIRATORY MOVEMENTS ON THE FLOW OF 

 BLOOD THROUGH THE INTRA-THORACIC VESSELS, AND ON 

 THE ARTERIAL PRESSURE. 



i. On the Intra-thoracic Vessels. The forces which cause the air 

 to flow into and out of the lungs will at the same time and in a similar way 

 cause the blood of the extra-thoracic veins to flow into, through, and 

 out of the intra-thoracic vessels. From the tendency of the pulmonary 

 elastic tissue to recoil, the blood-vessels in the thorax at the end of an expira- 



