574 THE HUMAN BODY. 



The Spinal Cord as a Centre. The^pinaLcord, forming 

 (except the slender sympathetic) the only direct communi- 

 cation between the brain an^niQ&LQlih^nerves of_the Body, 

 was considered by the older physiologists as merely a huge 

 nerve-trunk, into which the various spinal nerves were 

 collected on their way to the encephalon. It does, it is 

 true, contain the paths for the conductionjjj^all those, im^. 

 ich., originating in the cerebrum, give_risc to. 



\^kuitary__jnoYements of the trunk andjimbsj also for all 

 the_centrally traveling, impulses which give rise _tq-&cn&i- 

 tions ascribed to^ those parts; and jt is also the path for cor- 



s for 



_ 



jexample, those which, originating in thcrespiratory centre,, 

 tiajel to the phrenic and intercostal neryj^ 



If, however, the cord were merely collected and con- 

 tinued nerve-roots it ought to increase considerably in bulk 

 as it approached the skull, and this it does not do in any- 

 thing like the required proportion; a histological examina- 

 tion, too, shows that few, if any, of the entering fibres pro- 

 ceed directly to the brain; they pass to the central core of 

 gray substance, containing nerve-cells and different from 

 anything in a true nerve-trunk, (in the gray core some 

 fibres f anterior roots') mid in noils of T.hftn.nfpTi'n|' horns, and 

 others (posterior roots) in that fine Jiiwork which pervades 

 the^ whole gray substance (p. 179); the nerve-cells by their 

 branches are also in continuity with this network and so, 

 ultimately, with all the nerve-fibres entering the cord. 

 From the network fibres arise at different levels, and pass 

 out into the white columns; some place two regions of the 

 gray matter in intimate connection, while others go on 

 to the brain. In order to understand physiological facts 

 we must assume, first, that a nervous impulse entering the 

 gray network at any point may, under certain conditions, 

 travel all through it, and give rise to efferent impulses 

 emerging at any level; and, on the other hand, that there 

 are certain lines or paths of easiest propagation between 

 different points in this network, which, the impulses keep 

 to under ordinary conditions. 



Reflex Actions. When a frog is decapitated it lies down 



