Mammalian Nerve-Muscle and Rejiex Preparations. 345 



strength, while its muscle remains quiescent if the current is ascending in 

 direction, was first observed by Eckhard (3). The difference which exists 

 between the; reactions given by the nerve-muscle preparations of cold-blooded 

 and warm-blooded animals is partly explained by the extreme susceptibility 

 of the isolated tissues of the latter to nutritional and temperature changes. 

 Impairraent^due to these causes is progressive and depends on the period of 

 exposure, as is shown by the following experimental observations : — (1) The 



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Fig. 2. — jNerve-muscle preparation of cat with the nerve left exposed after its isolation. 

 The muscle is in a " tonic " contraction which is depressed by the ascending and 

 augmented by the descending current. 



thresliold value of the effective stimulus increases and the height of the 

 resultant contraction diminishes with lapse of time after isolation. (2) If the 

 motor nerve has been exposed for some time, the break contraction is often 

 produced with a smaller ascending current than is the make. (3) If the 

 nerve is still more impaired, a constant ascending current causes an apparent 

 relaxation of the muscle at make which is removed only at break (fig. 2). 

 On the other hand, a small contraction occurs at make of the descending 

 current, which is maintained during its flow. The damage to the nerve 

 appears to be the cause of continuous stimulation, which induces without 



2 D 2 



