102 



muscular tissues bears witness either to tlieir rarity or to the 

 technical difficulty of making them evident. 



Magnesium chloride has a still more strongly inhibitory 

 action upon rectal and intestinal movements than calcium 

 chloride, and, so far as I have been able to observe its action, 

 is wholly inhibitory and never stimulatory, thus agreeing with 

 its action upon the mammalian intestine, as established by 

 Meltzer and Auer (6). The conti;ast between the action of 

 magnesium chloride and that of calbluni chloride is displayed 

 in the following protocol : — 



Table 2. 





Experiment 



No. 51. 







Solution. 



Post. 



Rect. 



Rect. 

 Gl. 



Ant. Rect. 

 and Valve. 



Dist. 

 Int. 



NaCl 



... + 



+ 



+ 



- 



50 N.aCl+1 MgOL 



... + 



— 



— 



— 



NaCl 



... + 



+ 



+ 



— 



50 NaCl+l CaCl, 



— 



— 



— 



+ 



50 N.aCl+1 MgCi, 



..■. + 



- 



- 



- 



A phenomenon which appears to be so unique invites an 

 alternative explanation. It might be imagined, for example, 

 that the contractions of the rectum inhibit those of the intes- 

 tines and vice versa. This, indeed, was the explanation which 

 first occurred to the author, but it is negatived by the fact that 

 preparations are repeatedly obtained, especially when immersed 

 in the mixture 50NaCl to ICaCl,, in which active contraction 

 of both the rectal and intestinal segments are simultaneously 

 and synchronously occurring. In such preparations the rectal 

 contractions can be inhibited by raising the proportion of 

 calcium to sodium to a point which still permits active 

 peristalsis of the intestine, while the intestinal contractions 

 may be inhibited and the rectal contractions left unimpaired 

 by immersing the preparation in pure sodium chloride solution . 



As will be seen from succeeding experiments, the effects 

 of various drugs upon the intestine combine to support the 

 view that there exists in the fly's intestine (including the 

 rectum) a gradient of ^ thresholds, increasing from below 



upwards, analogous to the gradients of metabolic activity, 

 conduction and rhythmic activity which have been shown by 

 Alvarez and his co-workers to exist in the mammalian intestine 

 (7, 8, 9), and the question necessarily suggests itself whether 

 this is not, after all, the fundamental gradient of which the 

 others are the indirect expression. Certainly this view would 

 explain the remarkable effect of the reduction of the-^ ratio by 



