he done so, I believe he would have seen that no grey fibres mingle with this lachrymal branch. I have sought in vain for any connexion between the sympathetic and this branch; and Hirschfeld states that it is only the filaments of the sympathetic which accompany the artery in the gland, to which the secretion may be due, after division of the fifth. "La branche lachrymale du nerf ophthahnique de Willis, et un filet lachrymale de la branche orbitaire du nerf maxillair supérieur, se distribuent dans la glande lachrymale et tiennent en grande partie sous leur dépendance la sécrétion des larmes; car celle-ci diminue considérablement après la section de la cinquième paire, mais sans cesser, toutefois, complétement. Ce qui a fait supposer que les filets du grand sympathétique qui accompagnent les artères de la glande lachrymale avaient aussi une certaine influence sur la sécrétion."[1] Observe that not only has the presence of the grey fibres in the lachrymal nerve to be demonstrated as a fact, but I think their presence might be admitted without damage to my argument; for an examination of the connexion which does exist between the sympathetic and the fifth pair, will show that division of the fifth would not interfere with the action of the sympathetic filaments joining it from the carotid. Granting, therefore, that one part of the nervous stimulus reaches the gland through the sympathetic, we have still the greater part reaching it through the lachrymal nerve. In other words, a sensory nerve acts centrifugally.
The second point to which I referred, in the functions of the fifth, is the "insensibility" of the nasal branch; but this must be noticed presently, in connexion with the analogous "insensibility" of the motor nerves.
If there is any difference between sensory and motor nerves, it is not a difference of kind, but of use. Each nerve is capable of serving either function, provided it be properly distributed. If nerves are distributed through the substance of muscles, they will be motor—if distributed through glands, they will be secretory—if distributed to the surfaces, they will be sensory. There will probably be little objection raised to this statement. But we must go farther, and ask whether the skin-nerve is ever motor, and whether the muscle-nerve is ever sensory? To answer this, we must first settle one or two points of physiology and anatomy. A nerve is sensory because it stimulates the Sensibility of its Centre, and not because its termination is in the skin. It is not the nerve which is sensitive, bat the centre. Stimuli, which reach the nerve through the skin, affect the centre. It is to the centre, therefore, that we must look. So much for physiology; now for anatomy. "There is no difference," says Dr. Todd, "between a motor and sensory nerve as regards structure. We can attribute the difference of endowment of the fibres to no other cause but to the nature of their peripheral and central connexions. The same nervous force is propagated by the fibres of each kind; but whether that force is to excite motion or sensation, must depend on
- ↑ Hirschfeld, Névrologie, p. 250.
