7io A MANUAL OF PHYSIOLOGY 



and even apparently * purposive ' in their action. This also is less 

 true of movements caused by stimulation of naked nerve-trunks than 

 of movements caused by stimulation of sensory surfaces. If a piece 

 of skin in a brainless frog is severed from the rest, but left in connec- 

 tion with its nerves, excitation of the latter will produce simple and 

 comparatively aimless contractions, while pinching of the skin or 

 painting it with dilute acid may cause extensive movements evidently 

 aimed at the removal of the irritation. When a drop of dilute acid 

 is applied to the flank of a ' reflex ' frog, it will attempt to wipe it off 

 with the foot which is situated most conveniently for the purpose. 

 If this foot be held, it will use the other. 



Under ordinary conditions we must suppose that the resistance to 

 the passage of impulses is greater for certain paths than for others, 

 that it is easier, e.g., in a brainless frog for an impulse travelling up 

 a posterior root to reach the anterior root-fibres of the same segment 

 on the same side than to cross the middle line and tap the opposite 

 efferent tract, or to extend longitudinally along the cord and flow over 

 into efferent tracts coming off at a higher or lower level. The action 

 of strychnia must be to diminish the resistance in the whole of the 

 spinal cord, so that an impulse, instead of being confined to a fairly 

 definite path, spreads indiscriminately from neuron to neuron in the 

 grey matter itself, and also by the short paths of the antero-lateral 

 ground bundle (p. 677) from the grey matter of one segment to that 

 of another. 



Peripheral Reflex Centres. The question whether any reflex 

 centres exist outside of the spinal cord and brain, and especially in 

 the sympathetic ganglia, has been the subject of a lengthy contro- 

 versy. That the spinal ganglia cannot act as reflex centres is generally 

 acknowledged, and it is not difficult to see why, 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 con- 

 nection 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 medullated fibres of 

 small calibre is found in a spinal nerve-trunk immediately distal to the 

 junction of the roots than in both roots taken together, this appears to 

 be due to the passage into the nerve (from the grey ramus communi- 

 cans) of medullated fibres which end in the bloodvessels or other 

 tissue of the ganglion (Dale). Here it is evident that there is no 

 possibility of a complete reflex arc. Indeed, it is not certain that the 

 normal afferent impulses pass through the bodies of the spinal ganglion 

 cells at all. For (i) a negative variation can be observed in the posterior 



