NERVOUS SYSTEMS, PERIPHERAL AND CENTRAL 475 



consequences for the central nervous system, since it shows that a disturbance 

 cannot be permanently altered in strength by passing through some region of 

 decrement. We cannot assume that it can be made in this way too small to 

 pass through the synapse, which must itself be looked upon as a place of decrement 

 (see page 426). This fact shows the importance of the actual connections of a 

 particular neurone ; in other words, the anatomy of tracts and the centres which 

 they bring into relation with one another is of essential importance. At the 

 same time, the effect of strychnine shows that, in the spinal cord, there is potential 

 communication, at the least, between a receptor and all the motor neurones. A 

 localised stimulus sets into activity the whole of the muscles of the body. 



Irreciprocal Conduction. -It seems to be a very usual property of the synaptic 

 membrane to allow impulses to pass in one direction only. Thus Gotch and 

 Horsley (1891, p. 485) found that stimulation of the central end of an efferent root 

 caused no electrical change in the spinal cord above, although that of an afferent 

 root did so. On the other hand, the discharge of a spinal centre flows, in part, 

 backwards down the other afferent, dorsal roots. Veszi (1909), by an ingenious 

 form of experiment, has shown that continued stimulation of a motor nerve 

 produces no fatigue in the reflex centres ; the excitatory process does not spread 

 inwards as far as the place where central fatigue occurs. Some further facts will 

 be found in the next chapter (page 491). 



We have seen (page 141) how permeability of a membrane to one only of the 

 ions of a salt may allow an electrical current to pass in one direction only. 

 Irreciprocal permeability is, therefore, a state experimentally realisable. 



Fatigue. We have already seen that a motor centre may be fatigued for one 

 reflex but remain unaffected for another (page 423). This state of fatigue is, 

 accordingly, situated in some synapses, not in the efferent neurone itself. Excessive 

 fatigue has been found to result in changes in the cell substance, as the experiments 

 of Dolley, referred to on page 16 above, and of other observers, show. 



REFLEX ACTION 



In the most primitive condition, an effector may be excited directly by a 

 receptor cell, or its prolongation, as in the sea anemone. But this arrangement 

 cannot be called a central nervous system. Although the neurone is the anatomical 

 unit of such systems, the reflex is the functional unit, having as its anatomical 

 basis the reflex arc. This, in its simplest possible form, consists of at least 

 two neurones in addition to the effector cell itself. The receptor neurone forms 

 a synapse with the motor neurone, whose cell body is in the central nervous mass 

 and whose axone passes out to excite an effector. Put in another way, the 

 mechanism consists of three parts, receptive, conductive (including nerve fibre 

 and central cell), and effective (the peripheral organ set in action). Even in 

 the vertebrate, the cell body of the receptor neurone, although moved up to the 

 dorsal root ganglion, as stated above, is still outside the central nervous system. 

 But there is, in any central nervous system, except the very simplest, a synapse 

 between at least two neurones. The economy, as well as the integrative efficiency, 

 resulting from the use of one motor neurone by several receptors could not 

 otherwise be obtained. This is the principle which is called by Sherrington (1906, 

 p. 55) that of the "final common path." 



The simplest kind of reflex arc is to be found in the stellar ganglion of 

 the Cephalopod, according to the work of Frohlich (1909, 1). That this ganglion 

 has central functions is shown by the fact that stimulation of a point of the 

 mantle causes contraction over a wide area if the ganglion is intact, but if 

 it is removed, contraction is limited to the spot stimulated. The application 

 of strychnine has no effect, hence the conclusion is drawn that the intermediate 

 neurone, or its synapse, on which this alkaloid acts, is absent and the arc 

 consists only of the receptor neurone forming a synapse with the motor neurone 

 in the ganglion. That is, there are two neurones and one synapse. 



According to the experiments of Jolly (1910), the "knee-jerk" that is, the 

 contraction of the extensor muscles of the knee evoked by tapping the tendon 



