CH. XVI.] SYSTEMS OF RELAY 189 



multipolar cells of the anterior cornu of the spinal grey matter; 

 one of these shown in the figure is labelled A.C.C. This motor-cell 

 is thus stirred up to action and sends an impulse by its axon to the 

 muscular fibres (M) it supplies. Thus excitation of the skin will 

 cause, by this spinal reflex arc, the contraction of muscles. In some 

 cases severe excitation will cause contraction of the muscles of the 

 opposite side of the body (crossed reflex) ; under such circumstances 

 the intermediary neuron (P.C.C.) sends its axon to the anterior 

 horn-cells of the opposite side. The synaptic junctions are naturally 

 the places which the impulse has the greatest difficulty in traversing ; 

 and some observers believe that at the points of contact there is a 

 kind of undifferentiated interstitial protoplasm which the impulse 

 has to get through. 



This example illustrates a most important general truth, namely, 

 that a nervous impulse does not necessarily travel along the same 

 nerve-fibre all the way, but there is what we may term a system of 

 relays. The nervous system is very often compared to a telegraphic 

 system throughout a country. The telegraph offices represent the 

 nerve-centres, the afferent nerve-fibres correspond to the wires that 

 carry the messages to the central offices, and the efferent nerve- 

 fibres are represented by the wires that convey messages from the 

 central offices to more or less distant parts of the country. This 

 illustration will serve us very well for our present purpose, provided 

 that it is always remembered that a nervous impulse travels more 

 slowly than electricity. Suppose, now, one wishes to send a message 

 from the metropolis, which will represent the brain, to a distant 

 house, say in the Highlands of Scotland. There is no wire straight 

 from London to that house, but the message ultimately reaches the 

 house; one wire takes the message to Edinburgh; another wire 

 carries it on to the telegraph station in the town nearest to the 

 house in question ; and the last part of the journey is accomplished 

 by a messenger on foot or horseback. There are at least two relays 

 on the journey. 



"We may take another illustration of this. Suppose one wishes 

 to move the arm ; the impulse starts in the nerve-cells of the brain, 

 but there are no fibres that go straight from the brain to the 

 muscles of the arm. The impulse travels down the spinal cord, by 

 what are called pyramidal fibres, which form synapses with the 

 nerve-cells of the spinal cord, and from these cells fresh nerve- 

 fibres pass on the impulse to the arm-muscles. This is shown in 

 the accompanying diagram (fig. 185). The cell of the cerebral grey 

 matter is represented by C.C., and its axon (pyramidal fibre) by 

 P.F. This passes into the white matter of the brain, and in the 

 medulla oblongata it crosses over, and then travels down the 

 opposite side of the spinal cord. It enters the grey matter in the 



