324 THE BODY AT WORK 



In various situations in the central nervous system a certain 

 type of cell is found for which, in the present state of knowledge, 

 it is impossible to account. We mention these cells lest it 

 should be inferred, from what has been said above, that all 

 neurones can be fitted into a simple scheme of conducting arcs. 

 In the spinal ganglia there are neurones whose axons divide to 

 form " baskets " around other ganglion-cells. In the cere- 

 bellum there are similar cells, the axons of which divide into 

 branches, which break up to encase Purkinje cells. Cells of 

 the same kind are found in a few other situations. In some 

 cases the end-branches which enter into the formation of 

 the baskets are few in number, and thick and clumsy. They 

 grasp the body of the cell which they surround, with gouty 

 fingers, as it were. In other cases the basket is a tangle of fine 

 threads. It is difficult to see what role cells of this kind can 

 play in conduction. From the olfactory and optic centres 

 nerve-fibres extend outwards to the olfactory bulb and retina. 

 Here again is an arrangement which does not fit in with 

 any scheme. We might multiply examples. But enough 

 has been said, perhaps, to convey the impression which we 

 wish to leave, that, although experiment abundantly proves 

 that the nervous system consists of an association of sensori- 

 motor conducting arcs, and although anatomical investigation 

 demonstrates the existence of chains of neurones which take 

 part in the formation of such arcs, it is impossible to reduce 

 the system to schemata or to prepare diagrams in which all 

 structural elements are, even hypothetically, fitted into place. 



It may be convenient at this point to call attention to the 

 differences which distinguish the sympathetic system the 

 ganglia and nerves of the viscera and bloodvessels from the 

 system devoted to bringing sense-organs into connection with 

 the skeletal musculature which we have chiefly considered 

 hitherto. The fibres of the posterior root of a spinal nerve 

 which convey impulses from the skin and muscular sense- 

 organs, and the fibres of its anterior root which convey impulses 

 to skeletal muscles, have a similar diameter of about 15 p. 

 In addition to these, the roots contain fibres which carry im- 

 pulses from and take them to the viscera. Those which 

 bring impulses from the viscera vary greatly in thickness, 

 some being as large as the other sensory nerves of the posterior 



