70 ELECTRO-PHYSIOLOGY CHAP. 



the other hypothesis, however, covers the case of reflex move- 

 ments, the peculiarities of which are remarkable even in the 

 lower vertebrates. The movements involved are always co- 

 ordinated, i.e. they originate in the activity of a definite number 

 of definitely grouped muscles. This is most obvious in reflex 

 movements in the narrower sense, i.e. in those movements pro- 

 duced in striated skeletal muscle, after exciting a sensory nerve 

 through a central organ. 



If the end of a toe is pinched in a decapitated frog, the 

 leg is withdrawn, and then remains quiescent. This is a typical 

 reflex, via the spinal cord. The excitation, acting on the sensory 

 nerves of the skin, travels centripetally from the periphery to 

 the spinal cord, and gives rise to an impulse in the contrary 

 direction from cord to certain muscles of the leg. In this, and 

 all similar cases, we must assume a definite irradiation of 

 the excitation in the central organ, for the number of motor 

 fibres excited is obviously in considerable excess of the number 

 of primarily excited sensory fibres. Any touch, or least contact 

 of the skin, with the fine point of a needle, suffices to throw a 

 great number of muscles into simultaneous contraction, and as in 

 the sensitive mimosa we concluded from the diffuse reaction con- 

 sequent on local excitation, for the propagation of the stimulus 

 along certain paths, so in this case we must assume that each 

 sensory fibre is functionally connected with many motor fibres 

 within the central organ all possibility of transmission ceasing 

 as soon as the latter is destroyed. The law of isolated conduction, 

 which is universally valid in the region of the peripheral nervous 

 system, does not therefore hold good for reflex processes. From 

 the facts already discussed we may affirm that if it were possible 

 to excite a single primitive fibre of a motor nerve, the muscle- 

 fibres which are supplied by this fibre would alone contract, and 

 the same is true of sensory nerve-fibres until they enter the 

 central organ. In reflex movements it is otherwise ; the excita- 

 tion is here conveyed by one or a few sensory fibres via the 

 central organ to a plurality of motor elements. It may be 

 objected that a rigid anatomical relation between certain centri- 

 petal and certain centrifugal fibres still underlies this irradiation 

 of excitation. This view is, however, unfounded. The strength 

 of the peripheral stimulus is the most important factor in the 

 diffusion of irradiation. In a headless frog, if the sensory 



