THE DEPRESSOR EFFECT 175 



the extra filling of the left auricle by the increased pressure of 

 the air emptying the blood from the lung capillaries. The in- 

 creased pressure in the thorax retards the flow of venous blood into 

 the right auricle ; less, therefore, is sent into the pulmonary artery by 

 the ventricle and the pressure in this artery falls. From this fall in 

 pressure, and because of the obstruction to the flow through the pul- 

 monary capillaries due to the pressure of the air upon them, less blood 

 is delivered into the left auricle, hence less into the left ventricle, and, 

 therefore, a fall in aortic blood pressure. 



By measuring the mean height of the tracing in fig. 135, above the 

 zero abscissa line it is seen that the mean blood pressure is 140 mm. of 

 mercury. 



2. Stimulation of the depressor nerve. — The nerve is laid on a 

 pair of shielded electrodes, and is stimulated while a record is being 

 taken. The experiment is repeated two or three times, varying the 

 strength of stimulation. Fig. 137 gives a typical result. It is seen 

 that after a short latent period of about two seconds the blood pres- 

 sure gradually fell from a mean pressure of 140 mm. Hg to one of 

 124 mm. Hg, but that the character of the tracing was not in any 

 way altered. The heart waves and respiratory waves are still present, 

 and remain as before. When stimulation ceased the pressure once 

 more began to rise after a latent period of about two seconds, and 

 quickly regained its original mean value. 



The depressor nerve is an afferent nerve which transmits im- 

 pulses from the heart, mainly from the endocardium to the vaso- 

 motor centre in the medulla. If the nerve be divided and its peripheral 

 end stimulated no result is obtained. Stimulation of its central end 

 produces the result already described. It does not act upon the 

 cardiac centre, but only upon the vaso-motor centre, whose action it 

 so modifies as to cause dilatation of the peripheral blood vessels and 

 therefore a fall in blood pressure. Normally it must act as a kind of 

 safety-valve to the heart under conditions when it becomes difficult 

 for the heart to empty itself against a too high pressure, due either 

 to a higher pressure in the aorta or an overfilling of the ventricle. 

 The increased resistance thus caused stimulates the depressor, the 

 blood pressure falls by dilatation of vessels, ehiefly of the splanchnic 

 area, and the heart is now able to empty itself without any special stz-ess. 



3. Stimulation of the sciatic nerve. — While recording the blood 

 pressure the central end of the divided sciatic is stimulated, varying 

 the stimulus until a typical effect is produced. The sciatic is chosen 

 simply as a convenient nerve containing afferent fibres, for precisely 

 the same effect is produced with any similar nerve. The result shows 

 a rise of pressure, starting soon after the stimulation commenced, and 



