132 NERVOUS SYSTEM. 



third nerves are divided in the cranial cavity in a living 

 pigeon, the pupils become fully dilated, and do not contract 

 on the admission of intense light; and, when the same 

 nerves are pinched in the living or dead bird, the pupils are 

 contracted for an instant on each injury of the nerves. The 

 same results follow division or irritation of the optic nerves 

 under similar conditions ; but when the third nerves have 

 been divided, no change in the pupil ensues on irritating 

 the entire or divided optic nerves. 1 



The above experiments are accepted by nearly all physio- 

 logical writers ; and the assumption is that the third nerves 

 animate the muscular fibres that contract the pupil, the con- 

 traction produced by irritation of the optic nerves being re- 

 flex in its character. Later observers, however, have carried 

 their experiments somewhat further. Longet divided the 

 motor oculi and the optic nerve upon the right side. He 

 found that irritation of the central end of the divided op- 

 tic nerve produced no movement of the pupil of the side 

 upon which the motor oculi had been divided, but caused 

 contraction of the iris upon the other side. This, taken in 

 connection with the fact that, in amaurosis affecting one eye, 

 the iris on the affected side will not contract under the stim- 

 ulus of light applied to the same eye, but will act when the 

 uninjured eye is exposed to the light, further illustrates the 

 reflex action which takes place through these nerves. 3 



The reflex action by which the iris is contracted is not 

 instantaneous, like most of the analogous phenomena ob- 

 served in the cerebro-spinal system, and its operations are 

 rather characteristic of the sympathetic system and the non- 

 striated muscular tissue. It has been found, also, by Ber- 

 nard, in experiments upon rabbits, that the pupil is not 

 immediately dilated after division of the third nerve. The 

 method employed by Bernard, introducing a hook into the 



1 MAYO, Anatomical and Physiological Commentaries, Number ii., London! 

 1823, p. 4. 



8 LONGET, Traite de physiologic, Paris, 1869, tome iii., p. 656. 



