334 VEGETABLE DRUGS 



will, after exhibition of digitalis, cause diastolic arrest of the heart. In 

 regard to the vessels, experiments conducted on the terrapin exhibit the 

 fact that when the vessels are deprived of their nerve supply, the heart 

 excised, and an artificial circulation substituted, even then vascular con- 

 traction and retardation of flow will occur under the influence of digitalin 

 added to the factitious blood. The resultant of the various actions of 

 medicinal doses of digitalis is increased work of the heart, so that more 

 blood is pumped throughout the body in any given unit of time. We may 

 sum up the action of digitalis on the circulation as follows : 



1. It increases the force of cardiac systole. 



2. It causes more blood to be expelled during each systole. 



3. It slows the heart, chiefly through prolonging diastole. 



4. It acts in a certain degree to cause vaso-constriction and in- 

 creased blood pressure, but this is often not in evidence with therapeutic 

 doses by the mouth. 



5. It alters the distribution and rate of flow of blood in various 

 parts of the body through its stimulating effect upon the heart. 



Respiration. — The respiratory centres are only influenced by toxic 

 doses, being first stimulated and then depressed by digitalis : the respira- 

 tion is first rapid and deep, and later weak and imperfect. 



Nervous System and Muscles. — These are not influenced by thera- 

 peutic doses of digitalis. Toxic quantities cause loss of reflex action, 

 muscular weakness, vomiting, and convulsions in the frog. The first two 

 phenomena are due to primary stimulation of the inhibitory reflex centres 

 of Setschenow in the medulla, followed by general paralysis of the spinal 

 cord, and direct depression of the motor nerves and muscles; while the 

 convulsions are also caused by stimulation of the medulla. 



Temperature. — The temperature is unaffected by medicinal doses. 

 Toxic doses reduce temperature. Fever is lowered by large doses of 

 digitalis, but it is rarely safe to use the drug as an antipyretic. More- 

 over, digitalis is sometimes inoperative as a heart stimulant in fever, 

 because the functional activity of the vagus centres and peripheral termi- 

 nations is depressed and insensitive to the action of the drug. 



Kidneys. — Metabolism and Elimination. — The influence of digitalis 

 on the amount of urinary secretion is variable. It may exert a slight 

 stimulating effect upon the renal secreting cells, (albumin and blood in 

 urine in poisoning). The consensus of opinion among the best authori- 

 ties is, however, that digitalis exerts little direct action upon the renal 

 epithelium or blood vessels but that it acts as a diuretic chiefly in over- 

 coming a poor circulation (low blood pressure and venous stasis, or 

 obstructed venous flow from the kidneys) in heart weakness. The effect 

 of digitalis on tissue waste is uncertain and the inode of its elimination 

 is unknown. Experiments relative to the composition of the urine are 

 conflicting. The smooth muscle of the uterus is said to be stimulated to 

 contraction by digitalis. 



Cumulative Action. — Digitalis and strychnine are said to be cumula- 

 tive in their action. Evidence is stronger in the case of the former drug 

 than in that of the latter. By cumulative action is meant sudden transi- 



Digitized by Microsoft® 



