1054 



PHYSIOLOGY 



begins to fall, and the fall continues right into the beginning of the 

 next inspiration, so that the highest point of the curve occurs at the 

 beginning of expiration and the lowest point at the beginning of 

 inspiration. During the fall which accompanies expiration the heart- 

 beats, as shown in the diagram (Fig. 422), become less frequent, and 

 an obvious explanation of the fall of pressure would be to ascribe it 

 to a reflex inhibition of the heart. On dividing both vagi, this difference 

 in the pulse-rate during inspiration and expiration disappears, bat the 

 main features of the blood-pressure curve remain the same ; so that 

 we must look for some mechanical explanation of the respiratory 

 undulations. 



We have already seen that under normal conditions the lungs are 



in a state of over-distension, 



Arterial 

 Hloodpre, 



Expiration 



Jtespiraloy traciiuf. 



FIG. 422. Diagram 



and that in consequence of 

 this condition they are con- 

 stantly tending to collapse, 

 and are therefore exerting a 

 pull on the chest wall. As 

 soon as we admit air into the 

 pleural cavity by perforating 



of blood-pressure curve, the chest Wall the lungs 

 effects of the respiratory movements co llapse. The force with which 

 pressure and pulse-rate. (The effects 



the lungs 



sho 



onbl 



are purposely exaggerated.) 



tend to collapse 

 amounts to 6 mm. Hg. at the 



end of a quiet expiration, so we say that in the pleural cavity there is 

 normally a negative pressure of 6 mm. Hg. As the chest expands in 

 inspiration it drags the lungs still more open. As these become more 

 distended their pull on the chest wall becomes greater, and hence the 

 negative pressure in the pleura may be increased during forcible in- 

 spiration to 30 mm. Hg. It must be remembered that the heart and 

 great veins and arteries are in the thorax separated from the pleural 

 cavity only by a thin yielding membrane, so that they are practically 

 exposed to any pressure, positive or negative, which may exist in the 

 pleural cavity. Hence even at the end of expiration the heart and large 

 vessels are subjected to a negative pressure of 6 mm. Hg. Outside the 

 thorax all the vessels are exposed to a positive pressure, conditioned 

 in the neck by the elasticity of the tissues and in the abdomen by the 

 contractions of the diaphragm and abdominal muscles. 



Blood, like any other fluid, will always flow from a point of 

 higher to a point of lower pressure. There must thus be a constant 

 aspiration of blood from peripheral parts into the thorax. This aspi- 

 ratory force will not influence arteries and veins alike. The arteries, 

 having thick, comparatively non -distensible walls, will be very little 

 affected by the negative pressure obtaining in the thoracic cavity, 



