300 THE BODY AT WORK 



dendrite seems to be irreconcilable with the hypothesis that 

 they are disposed in the lines of conduction. 



In common with those of various other types of neurone, 

 the dendrites of spinal motor cells are beset with " thorns." 

 These projections are not rugosities or serrations, but short, 

 delicate threads which stand out at right angles from the 

 dendrites (cf. Fig. 1). About a dozen years ago, the writer 

 made a careful investigation of these structures ; at a time 

 when most anatomists regarded them as artifacts. He found 

 that their claim to be regarded as parts of the neurone is as 

 good as that of its axon or its dendrites ; although never seen 

 on certain types of cell, the thorns, of cells which carry them, are 

 perfectly definite in arrangement and spacing. In some kinds 

 of cell they are more numerous, in others less. Neuro-fibrillae, 

 as we now know them, had not been discovered at the date 

 when this investigation was undertaken; but on various grounds 

 the conclusion was arrived at that thorns are the cell-ends 

 of fibrils which pass from the end-twigs of arborizing axons 

 into dendrites. Upon this conclusion was based an hypothesis 

 of conduction which is here submitted, not because there is not 

 much to be said against it or, at any rate, many a hiatus 

 in knowledge to be filled but because it happens to be the 

 writer's own. The chrome-silver and methylene-blue methods 

 which reveal the existence of thorns do not stain neuro- 

 fibrillae. They colour the soft protoplasm in which fibrils are 

 embedded. By modifying the chrome-silver method in every 

 way which still allows a result to be obtained, it was found that 

 thorns sometimes appear as comparatively long slender fila- 

 ments, at others as shorter filaments ending in minute knobs, 

 or as filaments bearing two or three dots ; or finally no filaments 

 are visible, but the dots are in the position which they would 

 occupy if fibrils were present, but not stained. From this it 

 was argued that the soft protoplasm which during life surrounds 

 the filament as a continuous film, either falls back towards 

 the cell after death or is made to shrink into the cell by re- 

 agents. This accounts for the appearance of rod and knob. 

 What is supposed to happen may bo illustrated by dipping a wire 

 in treacle. At first, when the wire is withdrawn, it is surrounded 

 with a film. Then the film gathers into droplets. It was sug- 

 gested that the entrance of impulses into dendrites, their con- 



