THE NERVOUS SYSTEM. 555 



conceived, the nerve-centres are composed of neurons, while the nerve- 

 trunks are made up of the neuraxons with their various terminals. (See 

 p. 91 et scq.) There are other ganglia besides these, distributed elsewhere 

 and not within the cranium and spinal column, but these are, for the 

 sake of convenience, considered apart, under the head of the sympathetic 

 system, as they present some differences to the more central ganglia. 



The cerebro-spinal centres then are distinguished from mere nerve- 

 trunks by the possession of nerve-cells; these are, as we have seen in a 

 former chapter (p. 99 et seq.}, of different kinds; they very possibly 

 differ in function. It is, however, to the possession of ganglion-cells 

 that the increase of the functions of nerve-centres over that of nerve- 

 trunks is credited. Before turning to the discussion of the functions 

 of the spinal cord it will be as well to devote a little time therefore to 

 the question of the functions of the nerve-centres in general. The 

 ganglia of the sympathetic system also contain nerve-cells, but to these 

 it is supposed a different use is to be assigned, and what is said as to the 

 functions of nerve-ganglia in this place is only to be applied to those 

 of the cerebro-spinal centres. 



FUNCTIONS OF NERVE-CENTRES. 



Reflex action. One of the chief functions of nerve-cells appears 

 to be the power of sending out impulses to the periphery along efferent 

 nerves in response to impulses reaching them through afferent nerves. 

 This power is sometimes called the conversion of an afferent into an 

 efferent impulse. If may be supposed that an impulse passing to a 

 nerve-cell may produce such, a change in its metabolism that a discharge 

 of energy ensues. This discharge is in some way passed down an efferent 

 nerve as stimulus, and effects some change motor, secretory, or nutri- 

 tive, at the peripheral extremity of the latter the difference in effect 

 depending on the kind of peripheral-nerve termination. The reflex action 

 may be limited in its effect, or it may be extensive. Reflex movements, oc- 

 curring quite independently of sensation, are generally called excito~moior ; 

 those which are guided or acompanied by sensation, but not to the extent 

 of a distinct perception, or intellectual process, are termed sensor i-motor. 



(a) For the manifestation of every reflex action, these things are 

 necessary: (1), one or more perfect afferent fibres, to convey an impres- 

 sion; (2), a nervous centre for its reception, and by which it may be re- 

 flected; (3), one or more efferent nerve-fibres, along which the impres- 

 sion may be conducted to (4) the muscular or other tissue by which the 

 effect is manifested. All this means, in simpler statement,- that for the 

 production of a reflex action there must be two perfect neurons, a sen- 

 sory or afferent and a motor or efferent. This arrangement is shown in 



