101 



of 1 niol. of calcium to 50 of sodium sufficed to entirely inhibit 

 rectal contractions while constituting an optimum proportion 

 for the maintenance of intestinal peristalsis. In such cases a 

 further increase of calcium, for example to the proportion of 

 4 mols. of calcium chloride to 50 of sodium chloride, led also 

 to the inhibition of the intestine, and the entire preparation 

 was immobilized. In other instances the optimal proportion 

 of calcium for intestinal peristalsis was much higher, even as 

 high as 4 to 50. In every case, however, without any excep- 

 tion, the optimal concentration of calcium for intestinal con- 

 tractions was far above the optimum for rectal contractions, 

 and generally sufficient to inhibit them altogether. Conversely, 

 a pro])ortion low enough to be optimal for the rectum and 

 rectal glands was always below the optimum for the intestine, 

 and generally so low as to inhibit contractions of the intestine 

 altogether; in fact, in the great majority of instances, pure 

 sodium chloride solution appeared to very nearly represent the 

 optimum for the rectal contractions. 



We have in the fly's intestine, therefore, a clear-cut 

 instance of a phenomenon to which attention has but rarely 

 been drawn, namely, the existence of a definite ojHimum ratio 

 of calcium to sodium above or below which the muscular move- 

 ments are inhibited. It has long been recogfnized that £1 



Ca 

 reduction of the^ ratio increases the irritability of skeletal 



muscles in the vertebrata, but it has been supposed that this 



increase of irritability is merely the greater, the greater the 



reduction of the ratio. In the auricles and sinus of the heart 



the irritability is sufficient to permit rhythmic contractions 



even in the presence of the calcium salts in blood or in Ringer's 



or Locke's solutions, but the isolated ventricular strip 



Ca 

 requires a reduction of the-vv- ratio below the level obtaining 



in the blood before spontaneous rhythm is possible. DijBferences 



Ca 



in the "threshold value" of the^ ratio in different parts of 



the same muscular organ have therefore been recognized. 

 Below this threshold, differing in different muscular tissues, 

 spontaneous rhythm is possible; above this threshold it is not. 

 But the existence of a lower as well as an upper threshold of 

 the ratio, between which lies an optimum, has, so far as I am 

 aware, only been hitherto observed by Bancroft, in the gal- 

 vanotropic response of Fardmoec'inm (5). It is possible that 

 such a double threshold exists in other muscular tissues than 

 the fly's intestine, but the example which this tissue affords is 

 of so striking a nature as to immediately compel attention, 

 while the absence of comment upon such phenomena in other 



