NERVES OF ENCEPHALON. 



421 



(Fig. 118;) they are large, and arise from the nates of the 

 tubereula quadrigemina, and the corpora genlculata externa, 

 upon the optic thalami. From these origins, on either side, 

 is formed a soft, flat band, the tractus opticus, which winds 

 round the crus cerebri, and then converges to meet its fel- 

 low of the opposite side in the optic commissure or chiasma 

 before the sella turcica ; here it is connected by filaments 

 to the tuber cinereum, and the point is not settled whether 

 these nerves at this junction decussate or simply unite, or 

 whether both conditions exist. The latter opinion seems 

 the most tenable, that one portion of the fibres which 

 compose the optic nerves, at their commissure, cross each 

 other and go to the opposite eye, while the other portion 

 simply unites with its fellow and then passes to the eye of 

 the same side. Anterior to the junction, the nerve becomes 

 round and passes forward on F, G> 



the inside of the carotid, and 

 above the ophthalmic artery 

 to the optic foramen through 

 which it passes. On enter- 

 ing the orbit it is surrounded 

 by a process of dura mater ? 

 which divides into two por- 

 tions, the one being continu- 

 ous with the periosteum of 

 the orbit, the other with the 

 sclerotic coat of the eye. The 

 nerve then enters the back 

 part of the eye through the 

 sclerotic and choroid coats 

 to terminate in the retina. 



Third Pair } or Motores Oculorum, (Figs. 118, 101,) are 



FIG. 118 represents the second pair or nerves of sight, the Optic. 1 1 Ball 

 of the eye, the right one has the sclerotic and cervical coats removed to show 

 the retina. 2 Chiasm of optic nerves. 3 Corpora albicantia. 4 Infundibulum. 

 5 Pons Varolii. 6 Medulla oblongata. 7 Third pair motores oculi. 8 Fourth 

 pair pathetici. 9 Fifth pair trigemini. 10 Sixth pair motores externi. 11 

 Facial and auditory. 12 Eighth pair pneumogastric, spinal accessory, and 

 glosso-pharyngeal. 13 Ninth pair hypo-glossal. 



