THE CIRCULATION OF THE BLOOD 



tions in mind, repeat the above experiment, using stimulating currents of 

 increasing intensity until complete cardiac inhibition is produced. Perform 

 experiments showing the influence of the time of the stimulus on the inhibi- 

 tion, 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 the stimuli will usually 

 produce inhibitions and not acceleration. Occasionally with very weak 

 preparations direct acceleration may be produced. To get the pure inhibi- 

 tory or pure accelerator effects one must dissect back to the origin of the vagus 

 before it is joined by the sympathetic fibers; or to the sympathetic trunk 

 between the third spinal nerve and the point where it joins the vagus trunk. 

 In the study of the conditions in the above experiments one should note 

 the rate per minute and the amplitude in the normal, the period just before 

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

 same at different times after the stimulation until constant results are ob- 

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

 influence of each of the various factors recommended in the experiment. 



11. 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 sympathetic can very readily be isolated, and accelerator 

 fibers have been described for it. In the experience of the laboratory of 

 the author no experiments have yet demonstrated unquestionable cases of 

 cardiac acceleration. The vagus produces inhibitions which differ from 

 the effects in the frog in that complete inhibitions of the ventricle are followed 

 by contractions that are apparently at once maximal. In the frog the con- 

 tractions when they reappear are at first slight, but gradually increase in 

 amplitude until they have their former value. 



12. The Arterial Blood Pressure in a Mammal. The arterial 

 blood pressure may be measured on the anesthetized cat, dog, or rabbit. 

 Simple blood pressure was originally measured by Bale's method of connect- 

 ing the artery with a vertical tube and allowing the blood to flow freely into 

 the tube until a column was raised to the height which balanced the pressure 

 in the vessel. This simple method is decidedly the best for the beginner, 

 since it does not necessitate the use of very complicated apparatus. At the 

 same time it gives practice in anesthesia and in operations of vivisection, 

 and therefore serves as a good preparation for the more complicated ex- 

 periments which follow. 



The necessary apparatus should be prepared first, as follows: A vertical 

 tube supported on a stand with a scale graduated in the metric system, as- 

 sorted cannulae of approximately the size of the carotid artery of the animal 

 to be operated on, linen thread ligatures, dissecting set in good condition, 

 an animal-holder with strings or straps firmly to fasten the anesthetized 

 animal, a chloroform -e f >er mixture for dogs (or other anesthesia according 



