THE CIRCULATION OF THE BLOOD 103 



XV. THE ACTION OF DIGITALIS UPON THE HEART. 



1. Material. Tincture digitalis; sulphate of morphine; sodic 

 chloride; chloroform; two dogs; one frog; sodic carbonate (one-half 

 saturated solution). 



2. Preparation. Make solution of morphine, 0.6 grm. to 10 c.c. 

 Pith frog. Morphinize dogs, using 0.03 grm., and chloroform them 

 previous to operation. Set up induction coil so as to obtain tetanizing 

 current, having contact key in primary circuit. Prepare kymograph 

 for tracing. 



3. Experiments and Observations. (1) Fasten a dog firmly to 

 the dog board and lightly anaesthetize. Expose the vagus. Count 

 the pulse. Using shielded electrodes and separating secondary from 

 primary coil, find a current just weak enough not to affect heart 

 when applied to vagus. Now inject subcutaneously 0.3 c.c. tincture 

 digitalis per kilo animal. After waiting at least twenty minutes, in 

 the mean time using no anaesthetic except a repetition of the morphine 

 if necessary, and keeping the wound closed after moistening with 

 saline solution, stimulate the vagus with same current that before 

 the exhibition of digitalis was unable to affect the heart. 



(a) What is the function of the cardiac fibres of the vagus? 



(b) What result is produced by the stimulation of these fibres 

 in the normal animal? 



(c) Does digitalis increase or decrease the influence of the vagus? 

 (Maximum effect occurs after two hours.) 



(d) With the stimulus applied to the vagus fibres, the cardiac 

 fibres carrying impulses centrifugally, could this altered excitability 

 be due to central action of the digitalis? 



(2) After morphinizing dog, fasten firmly to dog board and lightly 

 anaesthetize; expose carotid artery. 



Insert the cannula of the manometer tambour apparatus into 

 the artery. There must be no air-bubbles in the apparatus at any 

 point. 



The anaesthetic should be discontinued as soon as the cannula is 

 inserted into the artery. Take normal tracing and read pressure as 

 indicated in the manometer. Now give the dog, hypodermically, 

 0.3 c.c. tincture digitalis for each kilo of weight. 



(a) Watch effect on elevation of mercury meniscus, making 

 tracings at short intervals. 



(b) What factors enter into arterial pressure? 



(c) How does a "high-pressure" tracing differ from a "low- 

 pressure" tracing? 



(d) What effect has digitalis on arterial pressure? 



(3) Having firmly fastened a pithed frog to frog board with web 

 stretched over a hole in the board, focus the microscope upon a 

 certain arteriole in the field, and measure its diameter with an eye- 



