THE REFRACTORY PERIOD AND FATIGUE 169 



the duration of the refractory period upon the supply of oxy- 

 gen as the spinal cord centers of the frog, when their irri- 

 tability has been increased to the maximum by strychnine. 1 

 Various observers, such as Loven, Buchanan, H. von Baeyer and 

 others, investigated the action current by the capillary electrom- 

 eter. As a means of studying the number of impulses in the 

 strychnine tetanus, we can upon the basis of their figures roughly 

 assume the number of impulses to equal ten per second at room 

 temperature. In short, in the freshly strychninized frog the dura- 

 tion of the refractory period is about .1 second. By means of the 

 method of artificial circulation already mentioned a deficiency of 

 oxygen can readily be brought about. It has been demonstrated 

 that the rhythmic in contrast to the continuous method of intro- 

 duction of circulatory fluid is superior in that the former repro- 

 duces more closely the natural conditions of the circulation of 

 the blood and renders the smallest capillaries more permeable. 

 In consequence I have recently constructed a small appliance for 

 artificial circulation, which accomplishes this in a manner as 

 simple as it is complete. (Figure 31.) The fluid flows from a ves- 

 sel, E, provided with an outlet tube through a thin rubber tube 

 into a glass canula, which is introduced into the general aorta of 

 the frog, F. The tube is automatically occluded by the rhythmical 

 movement of the armative of an electromagnet, D, produced by a 

 metronom, B. The pressure of the circulating fluid can be readily 

 changed at will by varying the level of the vessel and the fre- 

 quency of the pulse by the rhythm of the metronom, which makes 

 and breaks the current to the electromagnet. 2 In this way it is 

 possible to artificially replace the normal circulation with satis- 

 factory exactitude and substitute for the blood, circulating in the 

 vessels of the frog, any desired fluid. If the entire quantity of 

 blood of a frog is displaced by a continuous stream of oxygen- 

 free saline solution and a weak strychnine solution is injected 

 with a Pravaz syringe, a violent strychnine tetanus appears after 



1 Max Verworn : "Ermiidung Erschopf ung und Erholung der nervosen Centra des 

 Riickenmarks." Arch. f. Anat. u. Physiol. physiol. Abt. Suppl. 1900. The same: 

 Ermiidung und Erholung. Berliner Klin. Wochenschrift 1901. 



2 As I have not yet described this method elsewhere the above figure will suffice 

 for demonstration. 



