828 THE SPINAL CORD. 



instances the assigning of the excitation to its particular group or groups 

 of adequate stimuli has still to be done. Meanwhile it is well to 

 keep in mind analogy with plant reactions, for instance, the 

 reaction of vertical climbing shown by many insects (Coccinella) and by 

 echinoderms (Cucumaria) with the "negative geotropism" of plasmodia 

 of Myxomycetes. The extent to which, even in absence of connection 

 with the highest parts of the nervous system, the lower reactions are 

 recognisant of differences between different adequate stimuli, is illustrated 

 by the rejection of bits of paper soaked in sea-water by the regenerated 

 tentacles of Cerianthus, while bits of crab-flesh are accepted and carried 

 toward the artificial mouth. 1 The chelate limbs of Limulus and 

 Astacus? after ablation of the supra-oesophageal ganglion, and in the 

 former when the limb is isolated from the sub-oesophageal ganglion as 

 well, react differently according as a piece of wood or of food is given to 

 them ; in the latter case, the reflex movement carries the morsel to the 

 mouth, in the former the object is let fall. Again, the water-beetle 

 (Dytiscus), after removal of the cephalic ganglia, remains motionless so 

 long as it rests on a dry surface, but when placed in water executes 

 the usual swimming motions with even excess of energy and rapidity. 3 



Summation of stimuli — A single induction shock, the discharge of 

 a Leyden jar, a single prick, etc., can excite a reflex action. Such brief 

 " single " stimuli must, however, in order to produce reaction, be violent, 

 indeed, so violent as to injure the sensifacient tissue too much for re- 

 petition. Even when thus strong, they are less efficient than less severe 

 stimuli applied not singly but repeatedly. Stimuli far too weak to singly 

 evoke reflex reaction, are easily able to do so if repeated. With separate 

 electrical or mechanical stimuli, the reflex tends to occur after a 

 given number of stimuli have been applied, although the time-intervals 

 between the individual stimuli may be varied within wide limits, — e.g. 

 ranging between -05 sec. and - 4 sec. 4 There seems to result an addition 

 of the effects of the separately inefficient stimuli up to a point at which 

 they ultimately produce the effect, — e.g. the reflex discharge. This 

 phenomenon is akin to the " latent addition " of subminimal stimuli by 

 muscle and peripheral nerve. It has been suggested 5 that perhaps 

 no single stimulus whatever is in reality efficient to excite a reflex 

 reaction, those instances in which a single stimulus appears to do so 

 being explicable by the prolonged and repetitive stimuli set up by the 

 accompanying injury done the sensifacient tissue. When applied to the 

 spinal cord severed from the bulbar vasomotor centre, single induction 

 shocks have no effect upon arterial pressure, or have only a minimal effect, 

 even when the shocks are of extreme intensity. 6 On the other hand, 

 shocks of moderate intensity produce marked vasomotor effects when 

 applied to the cord at a frequency of twenty per second. A repetition 

 at the rate of twice or thrice a second makes them effective. 



Intensity of the stimulus — The threshold stimulus for exciting 

 contraction of a muscle from the afferent spinal root is higher than 



1 Loeb, "Untersuch. z. pliysiol. Morphol. d. Thiere," Wurzburg, 1891. 



2 Bcthe, Arch. f. d. ges. Physiol., Bonn, 1897, Bd. lxviii. 



:! Carpenter, "Retlex Action in the Nervous System of Invertebrata, " London, 1839. 



4 J. Ward, Arch./. Physiol., Leipzig, 1880. 



5 W. Stirling, Arb. a. d. physiol. Anst. zu Leipzig, 1874, S. 223 ; Du Bois-Reymond, 

 " Auslosung v. Reflexbeweg.," Berlin, 1880 ; S. Exner, " Entwurf einer physiol. Erklarung 

 psyeliisch. Erscheinimgen," Wien, 1894, Bd. ii. Abth. 1, S. 49. 



6 Kronecker and Nicolaides, Arch. f. Physiol., Leipzig, 1883, S. 27-42. 



