91. That direct stimulation of the nerve suffices to move the muscles, is familiar to all experimenters. There is no centre, or ganglion, in the amputated leg of the frog, which nevertheless contracts whenever the sciatic nerve is stimulated. And after the nerve has been exhausted, and refuses to respond to any stimulus, the muscle itself may be directly stimulated. Inasmuch as the movement depends on the contractility of the muscles, a stimulation through centre, through motor-nerve, or through muscle, will be followed by contraction. Let us take a clear case of reflex action. The pupil of the eye contracts when a beam of light falls on it, and dilates when the beam is shut off. The path of the neural process is normally this: the light stimulates the optic nerve, which in turn stimulates the corpora quadrigemina; (here the nerves which move the eye are experimentally proved to be stimulated;) and it is through these that the pupil is caused to contract. If the optic nerve be divided, no such reflex takes place--proving that the contraction does not, at least normally, come from the ciliary ganglion. But now it is matter of observation that the pupil will contract and dilate under the stimuli of light and darkness, when there is no such reflex pathway open. Removal of the eye from the body obliterates this path, cuts the eye off from all connection with the centre. Brown Séquard removed both eyes from a frog, placed one in a dark box, and left the other exposed to the light: the pupil of the former was found dilated, that of the latter contracted. On reversing the experiment, and placing the eye with contracted pupil in the dark box, he found it there dilate, while the dilated pupil exposed to the light contracted.[117] In frogs with very irritable tissues, I have found not only the pupil contracting, after the whole cranial cavity has been emptied, but even the eyelid close, on irritating the conjunctiva[118]--yet this is one of the typical reflex actions! I am disposed to think that even the action of swallowing may be faintly excited by stimulation of the pharynx of a brainless frog; but I have not observations sufficiently precise to enable me to speak confidently. Goltz has, however, shown that after removal of brain and spinal cord and heart, there is spontaneous and active movement in œsophagus and stomach.[119] This will no doubt be referred to the agency of the ganglionic plexus; but similar movements have been observed by Engelmann in the ureter, and in isolated fragments of the ureter in which not a ganglionic cell was present.[120]