proves the fundamental position of the established doctrine that a sensory nerve conducts only to a centre, never from it. Let any one follow the distribution of the Fifth Pair. Of the three trunks, into which this nerve is divided as it issues from the posterior root, two are called sensory, and the third is called "mixed," because, after its emergence from the Gasserian ganglion, it is joined by the nerve from the motor root. No fibres whatever from this anterior (motor) root join the two first trunks; and these two trunks are, therefore, considered on every ground of anatomy and experiment to be purely sensory. Now, I think it demonstrable by anatomy and experiment that these so-called sensory nerves have the distinguishing characters of motor nerves; that is to say, one of these nerves can be proved to transmit Neurility from the centre to an organ; and the other will not transmit a "sensitive impression" to its centre.
The first trunk is the ophthalmic. Among the parts it supplies there is one deserving particular notice—the lachrymal gland. This is the secreting organ, which is innervated solely from a branch of the ophthalmic, and a twig of the superior maxillary—that is to say, from the two purely "sensory" trunks. Yet that these nerves have a part to play in the mechanism of secretion is proved beyond a doubt by the great diminution of the secretion which follows division of the trunk. It is true that division of the trunk does not wholly suspend the secretion; but that is because the influence of a nerve upon the gland is only that of a stimulus. Let the part played by the nerves be never so small, the fact that some influence over the secretion is exercised by them, proves that they transmit a stimulus from the centre to the organ—they act centrifugally; which is precisely the character claimed for a motor nerve. What the nature of the influence may be which nerves exercise on glands is still a mystery; nor is it necessary for the present argument that anything more than the fact of a transmitted stimulus be admitted; but that fact is conclusive. All the argument needs is that a sensory nerve will act centrifugally; that proved, it follows that, if properly connected with a muscle, it would act upon the muscle as it acts upon the gland, viz., it would stimulate it.
Müller seems to have been on the point of adopting this view, but was held back by another consideration. "The affection of the nervus lachrymalis," he says, "under the influence of certain passions and ideas, is apparently an instance of the transmission of nervous influence in a centrifugal direction in a decidedly sensitive nerve; and this would be decisive proof that sensitive nerves can propagate nervous action in the centrifugal direction, if it were certain that the lachrymal nerve is not, like other branches of the fifth, accompanied by branches of the sympathetic. But it is probable that the lachrymal nerve receives grey fibres."[1] It is to be regretted that this great physiologist did not pursue the investigation, and assure himself of the actual facts. Had
- ↑ Müller, Physiology, I., 726.
