190 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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

 portion ol* 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 under such treatment irritability and conductivity do not dis- 

 appear at the same time, but that the fibres remain conductive after they 

 cease to be irritable. 



It has been shown also that 1 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. 



Summation of Stimuli in Nerve-cells. — In an axone a single stimulus 

 if 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 afferent cell-elements, when stimulated by way of the afferent nerves, 

 indicate that there may be a summation of stimuli — i. c, the discharge does 

 not follow until a scries of stimuli has been given. 2 



Whether, however, the delay in the response is due to the failure of the 

 cytoplasm of the receiving cell to discharge until repeated impulses have 

 reached it, or whether the modification of the cell which causes the delay is 

 a process taking place at the point where the impulse passes over from the 

 branches of one cell to those of another, is not directly determined by the 

 experiments. The indirect evidence is, however, entirely in favor of the 

 view that the delay which is notable in the arousal of a reflex response 

 occurs at the point where the impulse passes from one cell to another. 



C. The Nutrition of the Nerve-cell. 



The metabolic processes within the nerve-cell are continuous, and the 

 chemical changes there taking place involve not only those prerequisite to 

 the enlargement of the cell during growth, but also those leading to the 

 formation of such substances as by their katabolism cause the nerve-impulse. 

 The passage of the nerve-impulses probably alters the osmotic powers of the 

 cell-wall toward the surrounding plasma, and this of course is fundamental 

 to the nutritive exchange. It follows, therefore, that the passage of nerve- 

 impulses is one factor determining the nutrition of these cells. 



Histologically we look upon the cell-bodies as the part in which the most 



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



1 Ward: Archiv f. Anatomie u. Physiologie, 1880 ; Stirling: Arbeiten aus den physiologischen 

 Anstalt in Leipzig, 1874. 



