LEWES ON THE SENSORY AND MOTOR FUNCTIONS OF NERVES. 181 



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 superieur, se 

 distribuent dans la glande lachrymale et tiennent en grande partie sous 

 leur dependance la secretion des larmes ; car celle-ci diminue conside- 

 rablement apres la section de la cinquienie paire, mais sans cesser, toute- 

 fois, completement. Ce qui a fait supposer que les filets du grand sym- 

 pathetique qui accompagnent les arteres de la glande lachrymale avaient 

 aussi une certaine influence sur la secretion."-'' 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 fila- 

 ments 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 pre- 

 sently, 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 dif- 

 ference," 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 con- 

 nexions. 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, Nevrologie, p. 250. 



