ORGAN OF VISION. 



227 



saigne has examined the chemical Fi *- 107 - 



composition of the optic nerve and 

 retina; and concludes, from his ex- 

 periments, that the retina is formed 

 of the same elements as the cere- 

 bral and nervous substance; differ- 

 ing only in the proportion of con- 

 stituents. 



It is a question that has often been 

 agitated, whether the optic nerves, 

 at their junction on the sella turcica, 

 simply lie alongside each other, or 

 decussate so that the root of the 

 nerve of the left eye is on the right 

 side, and that of the right on the left. 

 Anatomical investigations have, 

 hitherto, left the question unsettled; 

 and pathology appears^ to have fur- 

 nished proofs on both sides. Thus, AT 



r . , L , , , , ' Optic Nerves, with the origin of seven other 



where the right eye has been lost for Pairs of Nerves. 



a Considerable time, the Optic nerve 1, I. Globe of the eye ; the one on the left hand 

 rif flio onvno airta Vine hpon fminrl in * is perfect, but that on the right has the sclerotic 



ol the same side nas Deen round in a an ^ choro ' id coats rem0 ved m order to show the 



State Of atrophy through itS Whole ret ina. 2. Chiasm of the optic nerves. 3. Cor- 



-.- L ', , pora albicantia. 4. Infundibulum. 5. Pons Va- 



extent. In Other Cases Of the rolU. 6. Medulla oblongata. ~. Third pair. 8. 



i i , r . , _/ .1 Fourth pair. 9. Fifth pair. 10. Sixth pair. 11. 



kind, the posterior portion OI the Seventh pair. 12. Eighth pair. 13. Ninth pair, 



left nerve has been found in this 



condition. 1 Fishes have the nerve arising from one side of the brain, 

 and passing to the eye of the other side ; hence crossing, but not 

 uniting. On the other hand, Vesalius 2 gives a plate of a case in which 

 he found the optic nerves passing to the eye of the same side from which 

 they originate, without touching at all; and yet without disturbance of 

 vision. It is not necessary, however, to adduce the numerous cases that 

 have been published in favour of the one view or the other. It is impos- 

 sible to sift those that are entitled to implicit confidence from those that 

 are not. It may merely be remarked that certain observations of Val- 

 salva, Cheselden, 3 and Petit 4 appear to show, that where the brain is 

 injured, it is the eye of the opposite side that is affected; and, in cases 

 of hemiplegia or paralysis of one side of the body, we certainly have 

 many instances for testing the accuracy of this opinion. Sommering 5 

 whose correctness as an observing anatomist has never been disputed 

 affirms, that he had an opportunity of examining seven blind persons, 

 in all of whom the atrophy of the nerve was on the side or root oppo- 

 site to the eye affected. 6 



1 Rudolphi, Grundriss der Physiologic, B. ii, Abth. 1, s. 203, Berlin, 1823. 



2 De Corp. Human. Fabric., lib. iv. c. 4. 



3 Anatomy of the Human Body, 13th edit., Lond., 1792. 



4 Memoir, de 1'Acad., 1723 and 1728. 



6 Bltimenbach, Med. Bibl., ii. 2, s. 368; and De Decussatione Nervorum Opticorum, 

 Mogunt., 1786. 



A case elucidative of this point in Lallemand, Sur Les Pertes Seminales, or in Dr. Wood's 

 Translation in Dunglison's American Med. Library for 1839, p. 30. 



