THE CIRCULATION OF THE BLOOD 93 



clamp to stop the flow of the solution from the reservoir and do not 

 open the clamp after this except to clear the cannula of a clot ; and 

 this cannot be done, of course, while the cannula is in the artery. 

 Ligate carotid artery at the upper end of the incision. Clamp the 

 lower portion of the carotid with the seraphin forceps, place the 

 finger under the artery, make a longitudinal incision in the middle 

 of the exposed portion of the carotid. Insert the point of the cannula 

 into the lumen of the artery, tie the cannula in place, and remove the 

 seraphin forceps. 



4. Observations. As soon as the seraphin forceps have been 

 removed the blood will rush into the cannula and tube for a dis- 

 tance of 4 to 8 cm., the mercury will rise in the distal limb of the 

 manometer to a corresponding degree. 



(1) Measure this rise in the distal limb of the manometer. What 

 is the blood pressure in centimetres of mercury per unit area? 



(2) Note that the mercury rises and falls in the manometer with 

 a rhythmical motion. Attach the manometer tambour or adjust the 

 float and watch the movements of the tracing point. Feel the pulse 

 of the animal and note whether the movements of the tracing point 

 correspond to the heart beats. 



(3) Bring the kymograph into position, adjust the tracing point 

 of the blood-pressure apparatus, also the chronograph, and take a 

 tracing. What is the rate of heart beat? 



(4) Are the respiratory movements evident in the tracing? If so, 

 what is the influence of inspiration upon blood pressure? What is 

 the influence of expiration? Account for the influence of respira- 

 tory movements upon blood pressure. 



(5) What causes the blood pressure to rise during inspiration? 

 Modification in blood pressure must be due either to the rate or 

 strength of the heart beat or to the condition of peripheral resistance. 



(6) If a line were drawn through the lowest point of the individual 

 cardiac waves, this waving line would represent the influence of 

 respiratory movements upon blood pressure. If the lowest point 

 of these respiratory waves were joined by a line, would this line 

 be a straight one or would it be a long, undulating curve? If such 

 a curve is observed, it may be recognized as the Traube-Hering 

 curve. This curve represents a gradual rise and fall of the blood 

 pressure under the influence of changing peripheral resistance, 

 which in turn is controlled by the vasomotor nerve centres. 



VIII. THE SPHYGMOMANOMETER AND PULSE PRESSURE. 



Various clinical instruments have been devised for the purpose 

 of determining blood pressure in the human subject in health and 

 in disease. The most satisfactory of these devices involves the use 



