44 2 Anatomy Applied to Medicine and Surge* y. 



the chiasma then total blindness results. Should the in- 

 jury be confined to the course of the optic path behind the 

 decussation, then, there will result loss of vision in the 

 temporal half of the retina on the side involved, and, in 

 addition, loss of vision in the nasal half of the other eye. 



Third Nerve. This nerve arises from nuclei in the 

 region of the floor of the aqueduct of Sylvius, and, after 

 passing through the crus, it enters the cavernous sinus on 

 its way to the orbit, where it supplies all the muscles of 

 the eye, with the exception of the superior oblique and the 

 external rectus. It sends filaments to the lenticular gan- 

 glion and through the ciliary nerves branches of this 

 ganglion it supplies the ciliary muscle, which controls 

 the lens or focussing portion of the eye. It supplies, also, 

 the constricting fibres of the iris. Injury, therefore, to its 

 nucleus, causes ( 1 ) ptosis or drooping of the upper eye- 

 lid (paralysis of the levator palpebrae). This drooping of 

 the upper lid is not due to contraction of the orbicularis 

 palpebrarum, since, when the lesion is at the nucleus of 

 origin of the third nerve, the orbicularis is also afrected. 

 Apparently the facial nerve supplies this muscle, but, in re- 

 ality, the facial acts merely as a carrier of the fibres, since 

 these fibres arise from the nucleus of the third nerve and 

 pass down in the pons to join the facial at the genu nervi 

 facialis. Further, it will be noticed that the motor oculi 

 supplies not only muscles inside the orbit but also muscles 

 outside of it and protecting it. (2) Divergent strabis- 

 mus, due to the action of the unopposed sixth nerve; (3) 

 dilatation of the pupil, because of the unopposed action 

 of the dilating fibres of the iris, governed by the sympa- 

 thetic ; (4) loss of acommodation from paralysis of the 

 ciliary muscle, which, therefore, cannot retract the capsule 

 sufficiently so as to alter the convexity of the lens a ne- 

 cessity in the accommodation for perfect vision. Worms, 



