912 THE CENTRAL NERVOUS SYSTEM 



there is already a beaten track between the posterior root-fibres 

 and the cells of the anterior horn, this overflow may be established 

 immediately after section of the cord, and may of itself lead to an 

 exaggeration of the reflexes. In animals like the dog a longer time 

 may be necessary before the unaccustomed route from the end 

 arborizations of the afferent axons and their collaterals to the 

 dendrites or the bodies of the motor cells becomes natural and easy ; 

 in man a still longer interval may be required. Moore and Oertel 

 have made a careful comparative study of reflex action after com- 

 plete section of the cord in the cervical or upper dorsal region, and 

 conclude that the spinal reflexes in the higher animals are far more 

 dependent on the upper portions of the central nervous system than 

 in the frog. 



Spinal Shock. The phenomena of spinal shock and its varying 

 severity in different animals may be accounted for by the rupture of 

 the paths normally used in the reflexes. The theory that the shock is 

 due to an inhibition set up by the mechanical injury is untenable. For 

 shock affects only the portion of the central nervous system distal (or 

 aboral) to the lesion. When a dog is allowed to live after transection 

 of the cord in the lower cervical region till shock has been recovered 

 from, a second transection distal to the first is followed by only slight 

 and very transient depression of the reflex power, although the direct 

 effect of the second injury ought, of course, to be as great as that of the 

 first. Finally, according to Sherrington, the condition of the spinal 

 reflex arcs in shock differs from the condition caused by inhibition, and 

 resembles rather a general spinal fatigue in which conduction along the 

 arc and especially across the synapses is difficult and uncertain. This 

 condition is supposed to be due to the loss of a ' tonic ' influence of 

 higher centres, assumed to be necessary for the maintenance of the 

 normal conductivity of the arc. These cranial centres, if they exist, or, 

 at least, the most efficient of them, must be assumed to be situated 

 distal to the cerebral cortex, probably in the pons or mid-brain. For 

 section just behind the pons causes much more severe shock than 

 removal of the cerebral hemispheres. 



Peripheral Reflex Centres. The question whether any reflex centres 

 exist outside of the spinal cord and brain, and especially in the sympa- 

 thetic ganglia, has been the subject of a lengthy controversy. That 

 the spinal ganglia cannot act as reflex centres is generally acknowledged, 

 and it is not difficult to see that, for anatomical reasons, this must be so. 

 A reflex arc must, so far as we know, in all highly -organized animals 

 include at least two neurons. There is no proof that an afferent 

 impulse can ascend an axon to a cell-body and there excite an efferent 

 impulse, which, descending the same axon in a separate set of fibrils, 

 gives rise to a reflex contraction, or a reflex secretion. Now, the cells 

 of a spinal ganglion represent the original neuroblasts from which the 

 posterior root-fibres grew out as processes towards the cord on the one 

 side and the periphery on the other. A sensory fibre passing into the 

 ganglion makes connection with a cell by a T-shaped junction and 

 passes on its course again. No afferent fibres run from the nerve-trunk 

 into the ganglion, to end in arborizations around the ganglion cells, 

 and no efferent fibres arise from nerve-cells in the ganglion to pass out 

 into the trunk. For although a slightly greater number of medullatcd 

 fibres of small calibre is found in a spinal nerve-trunk immediately 



