268 THE CIRCULATION OF THE BLOOD 



sufficient time after stimulation for a return to the normal. It will be 

 better to take one good tracing, showing all the facts of the experiment, 

 than several partial tracings, none of which are complete. 



With these suggestions in mind, (c) repeat the above experiment, 

 using stimulating currents of increasing intensity until complete cardiac 

 inhibition is produced, (d) Perform experiments showing the influence 

 of the duration of the stimulation on the inhibition; i.e., stimuli of i 

 second, 2 seconds, 10 seconds, and 30 seconds. 



In the frog the vagus, or inhibitory, and sympathetic, or accelerator, 

 fibers, are found in one trunk, the vago -sympathetic, but stimuli will usually 

 produce inhibitions and not acceleration. Occasionally with very weak 

 preparations direct acceleration may be produced. To get the pure 

 inhibitory or pure accelerator effects one must dissect back to and (e) 

 stimulate the trunk of the vagus before it is joined by the sympathetic 

 fibers; or to the sympathetic trunk and (/), stimulate between the third 

 spinal nerve and the point where it joins the vagus trunk. Pure accelera- 

 tor effects may be demonstrated by (g), stimulating the vago-sympathetic 

 after applying i c.c. of o.i per cent, atropine to the heart to poison the 

 vagus endings, (ti) perfusion of i c.c. of o.oi per cent, epinephrin will 

 chemically stimulate the accelerator endings in the presence of normal 

 vagus endings. 



In the study of the above experiments one should note the rates per 

 minute and the amplitude of the normal period just before stimulation r 

 the rate and amplitude during the period of stimulation, and the same 

 at different times after the stimulation until constant results are obtained. 

 A tabulation of these results will usually enable one to judge the influence 

 of each of the various factors recommended in the experiment. 



12. Influence of the Cardiac Nerves on the Terrapin's Heart. 

 Instead of the frog one may use the terrapin in the above experiment. 

 In this animal the vagus and sympathetic in the neck- can very readily be 

 isolated. It is usually quite impossible to demonstrate any cardiac accel- 

 eration. But the vagus produces inhibitions which differ from the effects 

 in the frog in that during the recovery from complete inhibitions the 

 ventricular contractions are apparently at once maximal, see figures 180 

 and 181. In the frog the ventricular contractions when they reappear 

 are at first slight, but gradually increase in amplitude until they have 

 their former value. The student should explain the significance of these 

 observations. 



13. Arterial Blood-pressure in Man. The arterial blood-pressure in 

 man can be measured indirectly by measuring the pressure which it 

 takes around the arm completely to close the artery. Some form of the 

 Riva-Rocci type of apparatus, preferably the Tyco or Faught, should be 

 used. Two points fundamental to physiology and to clinical diagnosis can 

 be determined, the systolic or maximum pressure in the artery, and the 

 diastolic or minimum pressure. 



