42 SPECIAL SENSES. 



ceedingly probable by pathological cases like those just cited. 1 

 In the case reported by Dr. Keen, the left side of the brain 

 was injured, as the paralysis occurred in the right leg and 

 arm. 



With the exception of the few filaments derived from 

 what have been described as the gray roots, the fibres of the 

 optic tracts and the optic nerves are of the medullated vari- 

 ety, and present no differences in structure from the ordinary 

 cerebro-spinal nerves. 



The optic commissure is covered with a fibrous membrane, 

 and is consequently more resisting than the optic tracts. 

 From its anterior and outer border, arise the optic nerves, 

 which take a curved direction to the eyes. The nerves are 

 rounded, and are enclosed in a double fibrous sheath derived 

 from the dura mater and the arachnoid. They pass into the 

 orbit upon the two sides by the optic foramina, and penetrate 

 the sclerotic at the posterior, inferior, and internal portion of 

 the globe. As the nerves enter the globe, they lose their 

 coverings from the dura mater and arachnoid. The sheath 

 derived from the dura mater is adherent to the periosteum 

 of the orbit at the foramen opticum, and when it reaches the 

 globe it fuses with the sclerotic coat. Just before the nerves 

 penetrate the globe, they each present a well-marked con- 

 striction. At the point of penetration, there is a thin but 

 strong membrane, presenting numerous perforations for the 

 passage of the nervous filaments. This membrane, the lami- 

 na cribrosa, is in part derived from the sclerotic, and in part, 

 from the coverings of the individual nerve-fibres, which lose 

 their investing membranes at this point. In the interior of 

 each eye, there is a little, mammillated eminence, formed 

 by the united fibres of the nerve. The retina, with which 

 the optic nerve is connected, will be described as one of the 

 coats of the eye. 



1 KEEN AND THOMSON, Gunshot- Wound of the Brain, followed by Fungus 

 Cerebri, and Recovery with Hemiopsia, Extracted from the Photographic Review 

 of Medicine and Surgery, February, 1871. 



