226 THE CIRCULATION OF THE BLOOD 



back through the ramus vertebralis to join those cervical nerves that enter 

 into the brachial plexus, figure 210. 



The nerves for the blood-vessels of the lower limbs arise from the tenth 

 dorsal to the second lumbar nerves. These pass to the ganglionic chain, 

 and gray rami are given off which join the lumbar plexus and run with the 

 divisions of that nerve complex to their distribution in the skin and muscles. 

 Vaso-constrictors and vaso-dilators have a common course to the lower limbs. 



The Vaso- constrictor Nerves for the Veins. Mall has proven 

 that vaso-constrictors are present for the portal vein. These fibers are 

 present in the splanchnic nerves. Other evidences have been observed 

 which render the view probable that vaso-motors for the veins in general 

 exist. Hough, for example, in an extended study of the capillary pressure 

 found many variations which were readily explained only on the assump- 

 tion of veno-motor activity, see figure 201. 



LABORATORY EXPERIMENTS ON THE CIRCULATION. 



1. The Rate of the Human Heart-beat. Determine the rate of 

 the heart-beat per minute by counting the radial pulse, using a watch for 

 the time. Make the determination after sitting quietly in a chair for five 

 minutes. Take the average of at least ten determinations for your own case. 

 Determine the heart-rate under the same conditions for as many different 

 persons as you can. Tabulate these determinations in a table which shows 

 age, sex, weight, and height of the different individuals, and compute a 

 general average for your entire set. 



Note the effect on the averages obtained above after the person lies down 

 for five minutes, after standing quietly for the same time, and after five minutes' 

 brisk walk. Tabulate as directed. 



Count the heart-rate immediately after two minutes' fast running, allow- 

 ing the person immediately to sit in a chair. Count the rate by two minutes 

 until there is a complete return to the normal, as determined above. Tabu- 

 late these results and compare the figures obtained from several different 

 individuals. 



Count your own heart-rate at intervals during one entire day, giving 

 special attention to the rate just before and just after meals, but in every 

 case make the count after sitting quietly for five minutes. A marked diurnal 

 variation will usually appear. Determine these rates on several individuals, 

 and tabulate as before. 



2. Human Cardiogram. Apply a Burdon-Sanderson cardiograph 

 to the thorax over the point between the fifth and sixth ribs of the left 

 side, at which point the cardiac impulse is felt most distinctly. Connect 

 the cardiograph with a recording tambour, Marey's form, adjust the tension 

 of the cardiograph and the pressure of the air within the system, and take a 



