43 6 TEXT-BOOK OF PHYSIOLOGY 



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. 



FIG. 201. TRACING SHOWING THE CHEYNE-STOKES FORM OF RESPIRATION. (Hill.} 



THE EFFECT OF THE RESPIRATORY MOVEMENTS ON THE FLOW OF 



BLOOD THROUGH THE INTR A -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 pulmonic 

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

 tion sustain a positive pressure, the intra-thoracic pressure (see page 305), 

 about six millimeters of mercury less than that in the lungs, or, in other words, 

 a pressure negative to that of the atmosphere by six millimeters. As a result 

 the blood in the systemic vessels under atmospheric pressure will flow 

 steadily toward the intra-thoracic veins, the venae cavae, and the right 

 side of the heart, i.e., from a point of high to a point of low pressure. During 

 inspiration there is a decrease in the intra-thoracic pressure, the decrease 

 being proportional to the extent of the inspiration. With this decrease 

 of pressure, the intra-thoracic veins expand and their internal pressure falls. 

 As the systemic or extra-thoracic veins are subjected to atmospheric pressure, 

 the blood in these vessels is forced, by reason of the difference of pressure 

 between these two regions, to flow more rapidly and freely into the intra- 

 thoracic veins and right side of the heart. The right heart being more 

 generously 'filled with blood will discharge a larger volume with each con- 

 traction into the pulmonic artery. 



Coincident with these effects a similar effect is produced in the arterioles 

 and capillaries of the pulmonic alveoli. Situated between the two elastic 

 layers of the alveolar wall, embedded in a meshwork of connective tissue, 

 the pressure to which they are subjected at the end of an expiration will 

 also be a few millimeters less than the intra-pulmonic pressure; and at the 

 end of an inspiration it will be considerably less. With the inspiration there- 

 fore there will occur a dilatation of these vessels, and hence a larger flow of 

 blood through them and into the pulmonic veins. The left heart, being 



