304 THE POPULAR SCIENCE MONTHLY. 



Nerves, no less than muscles, present the property of being exci- 

 table. If, together with the excised muscle, there had been removed 

 from the animal's body an attached nerve, every time any part of this 

 nerve is stimulated the attached muscle will contract as before. But 

 it must be carefully observed that there is this great difference between 

 these two cases of response on the part of the muscle — that, while in 

 the former case the muscle responded to a stimulus applied directly to 

 its oicn suhstance, in the latter case the muscle responded to a stimulus 

 applied at a distance from its own substance, which stimulus was then 

 conducted to the muscle by the nerve. And in this we perceive the 

 characteristic function of nerve-fbres, viz., that of conducting stimuli 

 to a distance. The function of neive-cells is different, viz., that of 

 accumulating nervous energy ; and, at fitting times, of discharging 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. 

 I may add that, 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. Lastly, another important 

 distinction must be borne in mind — the distinction, namely, which I 

 now draw between muscle and nerve. A stimulus applied to a nerve- 

 less muscle can only course through the muscle by giving rise to a visi- 

 ble 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 undergoing any change of shape. Now, in order 

 not to forget this distinction, I shall alwaj'S speak of muscle-fibres as 

 conveying a visible wave of contraction, and of nerve-fibres as convey- 

 ing an invisible, or molecular, wave of stitmdation. Nerve-fibres, then, 

 are functionally distinguished from muscle-fibres — and also from proto- 

 plasm — by displaying the property of conducting invisible, or molecu- 

 lar, waves of stimulation from one part of an organism to another, so 

 establishing physiological continuity between such parts, loithout the 

 necessary passage of contractile leaves. 



Such being the structure and the function of nerve-tissue in its fully 

 evolved form, I will now proceed to give the results of my researches 

 on the structure and function of nerve-tissue where this tissue is first 

 found to occur in the ascending series of animal life. The animals in 

 which it so occurs are the Medusm or jelly-fishes, which must be familiar 

 to all who frequent the seaside. These animals present the general 

 form of a mushroom. The organ which occupies the same position as 

 the stalk does in the mushroom is the mouth and stomach of the 

 medusa, and is called the polypite ; while the organ which resembles in 

 shape the dome of the mushroom consitutes the main bulk of the ani- 

 mal, and is called the swimming-bell. Both the polypite and the swim- 

 ming-bell are almost entirely composed of a thick, transparent, and non- 



