CENTRAL NER VO US 8 YSTEM. 625 



pressing it between the thumb and forefinger and sliding these along the 

 nerve, a contraction of the diaphragm is caused. The part of the nerve thus 

 stimulated is soon exhausted. If, now, the same operation is repeated on a 

 portion of the nerve lying nearer the spinal cord, contraction of the diaphragm 

 again follows. This result was originally used to support the theory of a 

 nerve-fluid, and was held to demonstrate that after the nerve-tubes in the 

 portion of the trunk compressed had been emptied so that no reaction followed 

 further pressure, then if the pressure were applied still nearer the cord the 

 fluid from that part of the nerve could be driven forward and a contraction 

 of the diaphragm would result. The notion of a nerve-fluid in the sense in 

 which that term was used by the earlier physiologists has long since been 

 abandoned, but for our purpose the experiment is important as showing that 

 irritability and conductivity do not under such treatment disappear at the 

 same time, but that the fibres remain conductive after they cease to be irrita- 

 ble, as is shown by the fact that the peripheral part of the nerve, though irre- 

 sponsive, still permits the impulses aroused nearer the cord to pass through it. 



It has been also shown l that in young regenerating motor fibres it often 

 happens that while no response is to be obtained by the direct stimulation of 

 the regenerated peripheral portion, yet the stimulation of the central and fully 

 grown portion does cause a contraction of the muscles controlled by these 

 fibres. In this case the newly formed fibres can conduct an impulse which 

 gives rise to a contraction, although such an impulse cannot be aroused by 

 directly stimulating them. 



In the case of the cell-body certain conditions must be present in order 

 that an impulse sufficient to cause an evident response shall be aroused. 

 There is certainly no evidence that stimuli which for one reason or another 

 do not cause such responses are without any effect whatever. At the same 

 time all cases in which there may be marked delay in the response occur 

 where the impulse passes from one cell to another, and hence the question can 

 always be raised as to the exact point at which delay occurs. 



Number of Stimuli necessary to Elicit a Response. In an isolated 

 portion of a nerve-cell, like a nerve-fibre for instance, a single stimulus 

 is followed by a single nerve-impulse ; on the other hand, the studies which 

 have been made to determine the number of weak stimuli necessary to dis- 

 charge a series of cell-elements indicate that there is a summation of stimuli, 

 i. e. the discharge does not follow until a series of stimuli has been given. 

 These experiments have been made for the most part with reflex frogs, and 

 they indicate that with very weak stimuli that can be individualized, like 

 mechanical impacts or single induction shocks, a given reaction can be obtained 

 with remarkable regularity after a given number of stimuli, while the intervals 

 between the single shocks may be varied within comparatively wide limits 

 without modifying the number required. 2 



1 Howell and Huber: Journal of Physiology, 1892, vol. xiii. 



2 Ward: Arehiv fur Anatomie und Physiologic, 1880; Stirling: Arbeiten aits der physiologischen 

 Anstalt in Leipzig, 1874. 

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