I shall now conclude this chapter by giving a brief account of those general principles of the physiology of nerve and muscle with which it is necessary to be fully acquainted, in order to understand the course of the following experiments. Nerve-tissue, then, universally consists of two elementary structures, viz. very minute nerve-cells and very minute nerve-fibres. The fibres proceed to and from the cells, so in some cases serving to unite the cells with one another, and in other cases with distant parts of the animal body. Nerve-cells are usually found collected together in aggregates, which are called nerve-centres or ganglia, to and from which large bundles of nerve-fibres come and go. To explain the _function_ of nerve-tissue, it is necessary to begin by explaining what physiologists mean by the term "excitability." Suppose that a muscle has been cut from the body of a freshly killed animal; so long as it is not interfered with in any way, so long will it remain quite passive. But every time a stimulus is supplied to it, either by means of a pinch, a burn, an electrical shock, or a chemical irritant, the muscle will give a single contraction in response to every stimulation. And it is this readiness of organic tissues to respond to a suitable stimulus that physiologists designate by the term "excitability." Nerves, no less than muscles, present the property of being excitable. 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 own substance_, 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-fibres_, viz. that of conducting stimuli to a distance. The function of nerve-_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 is to be drawn between muscle and nerve. A stimulus applied to a nerveless muscle can only course through the muscle by giving rise to a visible wave of contraction,