THE EYE. 197 



THE EYE. 



[For an osteological description of the Orbit, see page 29.] 

 What is the Visual Apparatus ? It comprises the eyeballs and their 

 appendages, which collect the luminous impressions, and the optic nerves, 

 which convey these impressions to the brain. 



Describe the Eyeball. It is a spherical organ, situated in the anterior 

 part of the orbital cavity, on a cushion of connective tissue and fat, where it is 

 retained by its muscles, the optic nerve, the conjunctiva, etc., and protected in 

 front by the eyelids and eyebrows. Projecting from its anterior surface is the 

 segment of a smaller, transparent sphere, the Cornea. The eyeball has the 

 following 



Diameters, Antero-posterior, about .95 inch, Transverse, .92 inch, 



Vertical, .90 inch, in the adult. 



Anterior Pole,\s> the geometric centre of the cornea. 

 Posterior Pole, is the geometric centre of the fundus. 

 Optic Axis, is an imaginary straight line connecting the poles. 

 Visual Axis, an imaginary straight line from the yellow spot to the object, 

 through the nodal point. It cuts the cornea above the optic axis, at an 

 angle therewith of 3 to 7, the Visual Angle. 

 Nodal Point, the centre of curvature of the refracting surfaces. 

 Equatorial Plane, an imaginary plane through the centre of the eyeball, 

 perpendicular to the axis, dividing the ball or globe into the anterior and 

 posterior hemispheres. The Equator is the line where this plane cuts the' 

 surface of the globe. 



Meridional Planes, are imaginary planes coinciding with the axis of the 

 eyeball. Meridians are the lines where these planes intersect the surface 

 of the globe. 



Describe the Tunica Vaginalis Oculi. It is a fibrous capsule, sur- 

 rounding the eyeball and the intra -orbital portion of the optic nerve ; arising 

 from the optic foramen and being lost anteriorly on the sclerotic. It consists 

 of two layers (parietal and visceral) enclosing a lymph-space which communi- 

 cates with the lymph- space between the sclerotic and choroid coats. The 

 tendons of the ocular muscles pierce it. The 



Capsule of Bonnet, is the portion of tunica vaginalis posterior to the point 

 where the tendons pass through it. This part is loosely attached to the 

 eyeball, which rotates in it. 



Capsule of Tenon, is the portion of the tunica vaginalis anterior to the 

 passage of the tendons. [The whole capsule or Tunica Vaginalis is by 

 many anatomists named the Capsule of Tenon.] 



