THE NERVOUS SYSTEM 301 



duction across the space which separates the end-twigs of 

 axons from the dendrites into which their impulses pass, is 

 by means of the thorns, although the thorns are not in them- 

 selves conductors. Conduction occurs only when films of 

 cytoplasm surround the thorns. The first effect of impulses 

 is to call out the films, in the same kind of way that a cur- 

 rent of electricity converts a row of falling drops into a con- 

 tinuous stream. A succession of impulses, by adding to the 

 number of the filaments which are enveloped in cytoplasm, or 

 by increasing the amount of cytoplasm investing certain 

 groups of filaments, increases the openness of the path. Sleep is 

 a condition in which all paths are open. Hence no impulses 

 are effective. Wakefulness, alertness, depends upon the 

 closing of all paths save those which are actually in use. We 

 may go further. The power of concentrating attention is the 

 power of limiting the spread of nerve-impulses in the brain. 

 Alcohol opens extra paths ; the concentrated effort which was 

 making progress with a problem becomes more diffuse. The 

 first effect appears in greater brilliance of thought, gained at 

 some sacrifice of cogency. Unexpected analogies are dis- 

 covered. Imagination takes a wider range. But as the dose 

 is increased, a condition akin to sleep is set up. Nerve-im- 

 pulses become ineffective because, many paths being open, they 

 do not attain a sufficient intensity in any set of paths. These 

 few illustrations are given for the sake of showing the need of a 

 theory of the opening and closing of paths. It is not sug- 

 gested that they favour the particular hypothesis here set forth 

 as to the structural arrangement which provides the paths and 

 regulates their accessibility. 



Recent discoveries in the finer structure of the central nervous 

 system have provided many problems which at present appear 

 insoluble. One of the discoveries most difficult to make use of 

 in constructing theory is the existence of extracellular or peri- 

 cellular nets, which have the appearance of extraordinarily 

 delicate cases of wire-netting immediately surrounding the 

 nerve-cells. It is somewhat remarkable that the spacing of 

 the nets is often very similar to, if not identical with, the 

 spacing of thorns. While some anatomists look upon the nets 

 as nervous, others regard them as pertaining to the con- 

 nective tissue of the nervous system. At present it is not 



