PRESIDENTIAL ADDRESS. 713 



of the nerve cells. Accordiug to the neuron theory the fibres of different 

 nerve cells end more or less blindly, and, at any rate in vertebrates, do not 

 demonstrably unite at their termini within the central mass; hence gaps exist 

 at the junction unbridged by the dirtereutiated structural continuum. But since 

 the nervous impulse can pass from one set to the other, a physiological continuum 

 undoubtedly exists ; it is necessary, therefore, to assume either that the electro- 

 lytic change in one neuron can by mere contiguity in "space arouse a similar 

 change in a neighbouring neuron process, or that a difl'erentiated connection 

 actually exists, but of such structural delicacy that it cannot be microscopically 

 demonstrated. Recently several physiologists have stated their belief in such 

 continuity ; one of these, E. Pfluger, bases his view upon the admitted intra- 

 cellular nature of peripheral nerve endings in muscles, glands, epithelial cells, and 

 electrical organs. Arguing from analogy, he infers that the central nerve endings 

 of one neiu'on probably pierce and enter the cell processes of another neuron.' Such 

 a coimection can be actually seen, as a pericellular plexus, in the ganglia of 

 Crustacea, and has been occasionally described as observed in higher animals.' 

 Whether the central termini of neuron processes are in reality joined by extremely 

 fine fibrillar filaments or whether they end blindly in mere juxtaposition, it is 

 undoubted that the functional synapsis presents peculiar features. The chief 

 peculiarities of synaptic activities as distinct from the activities of the nerve fibres 

 are the following: Marked retardation iu the maximum rate of propagation; 

 irreciprocity of conduction, which is favoui'ed in the natural or homodromous 

 direction, whilst in the unnatural or heterodromous direction it is obstructed or 

 completely blocked; susceptibility to fatigue; special susceptibility to stimula- 

 tion and impairment by definite chemical substances, by strychnine, absinthe, 

 anaesthetics, &c. ; the presence of a resistance which diminishes rapidly when 

 subjected to the assault of a series of entering or centripetal nervous impulses even 

 when each member of the series is aloue quite powerless to force a passage. All 

 these peculiarities are more or less demonstrable in all nerve endings, peripheral 

 as well as central, and are presumably, therefore, related to the character of 

 the propagation which occurs in the finely-divided non-medullated twigs or 

 * arborisations ' into which the nerve fibres break up in such endings, and possibly 

 to some further 'receptive' substance lying beyond the endings. The retarded 

 propagation, showing itself by an apparent delay, occurs in the motor nerve 

 endings of muscles and in the multitudinous nerve endings of electrical organs, as 

 well as in the central nervous system. Garten's researches on non-medullated nerves 

 suggest that it may be connected with such slowed development of the electrolytic 

 redistribution and of its accompanying electromotive alterations as is demon- 

 strable in these structures.- Irreciprocity of conduction occurs where nerve endings 

 are continued into muscle substance, since the activity process passes from nerve 

 to muscle, but not the reverse way. In 1896 Engelmann succeeded by means of 

 a double muscle-bath in so modifying one end of a muscle fibre that the wave of 

 contraction, whilst it travelled freely along the muscle fibre from the unmodified 

 to the modified portion, would not do so the reverse way.^ The particular modifica- 

 tion which produced this abnormal result is an interesting one ; it is the develop- 

 ment of an abnormally sluggish type of mobility, the whole activity of the 

 modified region being greatly prolonged by means of veratria. This suggests that 

 diflerence iu the duration of the active process on the two sides of a central 

 nervous synapsis would, if present, be one factor in producing the well-known central 

 irreciprocity. The susceptibility to fatigue may be associated with this augmented 

 difficulty of propagation, and it undoubtedly occurs to a marked extent in muscular 

 nerve endings ; for, according to the investigations of Joteyko, it may be more 

 pronounced in this peripheral ending than it is even in the spinal cord.* Evea 



' E. Pfluger, ' Ueber den elementaren Ban des Nervensystems,' Archiv f. die Ges. 

 Physiol, cxii., 1906. 



-' Garten, Beitrdge zur Physiologie der niarklosen Nerven, Jena, 1903. 



' Engelmann, ' Versuche iiber irreciproke Reizleilung in Muskelfasern,' Archiv 

 f. die Ges. Physiol., Ixii. 1896, p. 400. 



* Joteyko, Travavx de VInstitnt Solvay, Bruxelles, iii, 2, 1900. 



