CHAPTEK X. 



THE NERVOUS SYSTEMS OF THE INVERTEBRATA. 



Nervous tissue consists of two distinct structural parts : 

 («) nerve-cells and (6) nerve-fibres. Tlie nerve-cells are 

 usually found in aggregates, termed ganglia or nerve-centres ; 

 and ganglia are united to ganglia by nerve-cords or bundles, 

 which consist of many delicate nerve-fibres. The latter act as 

 the conductors of nervous force ; in fact, " the characteristic 

 function of nerve-fibres is that of conducting stimuli to a 

 distance. The function of nerve-cells is different, viz-; that 

 of accumulating nervous energy, and, at fitting times, o£ dis- 

 charging this energy into the attached nerve-fibres. The 

 nervous energy, when thus discharged, acts as a stimulus to 

 the nerve-fibre ; so that if a muscle is attached to the end of 

 a fibre, it contracts on receiving this stimulus. When nerve- 

 cells are collected into ganglia, they often appear to discharge 

 their energy spontaneously; so that in all but the very 

 lowest animals, whenever we see apparently spontaneous 

 action, we infer that ganglia are probably present." There 

 is another important point, viz., the difference between 

 muscles and nerves under the influence of a stimulus. " A 

 stimulus applied to a nerveless muscle can only course through 

 the muscle by giving rise to a visible wave of contraction, 

 which spreads in all directions from the seat of disturbance 

 as from a centre. A nerve, on the other hand, conducts 

 the stimulus without sensibly moving or undergoing any 

 change of shape. Therefore muscle-fibres convey a visible 

 wave of contraction, and nerve-fibres convey an invisible, or 



