180 ORIGINAL ARTICLES. 



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 sen- 

 sory, 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 de- 

 monstrable 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 secre- 

 tion ; 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. 



Miiller seems to have been on the point of adopting this view, bnt was 

 held back by another consideration. ' ' The affection of the nerviis lachry- 

 malis," 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."* It is to be regretted that this great physiologist did not 

 pursue the investigation, and assure himself of the actual facts. Had 



* Miiller, Physiology, I., 726. 



