844 



ORGANS OF SPEC FA L SENSE 



The greatest interval between two neighl)Ouring tendons is tliat between the 

 superior and internal recti (about 12 mm.); the least is between the superior and 

 external (7 mm.). The form of the lines of insertion of the different tendons 

 varies considerably, the inferior being almost straight, the superior and external 

 convex forwards, the internal further removed from the corneal border below than 

 above. 



The insertions of the obliques are at more than double the average distance of 

 the insertions of the recti from the corneal border. That of the superior oblique is 

 found on the superior surface of the sclerotic, about sixteen millimetres from the 

 corneal edge, in the form of a line sloping from before backwards and inwards. 

 The inferior oblique has a long fleshy insertion lying betAveen the external rectus 

 and the optic nerve entrance; the posterior end of the insertion, which is also the 

 highest, is only about six millimetres from the optic nerve, and from this point 

 it slopes forwards, outwards, and slightly downwards. 



Several small nerves and two arteries may be seen running forwards and ulti- 

 mately perforating the sclerotic not far from the entrance of the optic nerve. The 

 two arteries are the long posterior ciliary; they both perforate the globe in the 

 horizontal meridian, one on the outer, the other on the inner side. The short 

 ciliary arteries are too small to be seen in an ordinary examination. The nerves 



Fig. 474.— Diagrammatic View of the Insertions of the Ocular Muscles. 



(After Merkel.) 



are the long and short ciliary. Nearer the equator, the large venous trunks emerge; 

 they can be traced for some distance in front of their exit as dark lines, running 

 antero-posteriorly beneath the sclerotic. The optic nerve is seen in section, sur- 

 rounded loosely by a thick outer slieath; in the centre of the nerve-section a small 

 red spot indicates the position of the central retinal blood-vessels. 



For ordinary dissections eyes of the sheep, pig, or Ijullock should be obtained. Divide an 

 eyeball into fore and hind halves by cutting through it in the equatorial plane. 



1. Posterior hemisphere seen from in front. This is mucli the sjiuk' view that tlie oi)lithal- 

 moscope affords us. Unless tlie eye be very fresh, however, the retina will have lost its tians- 

 pareney, and will now present the appearance of a thin whitish nienihrane, detached in liilds 

 from the underlying coats, but still adherent at the ojitic disc. The vitreous jelly lying within 

 the retinal cup may he torn away. In the human eye the retina next the {)osterior pole is 

 stained yellow (macula lutea). On turning the retina over, a little jtigment may he seen adher- 

 ing to its outer surface here and there. Cut through the retina close to the ojitic disc all round 

 and remove it : note how easily it is torn. We now see a dark brown surface, consisting of the 

 retinal pigment layer, adhenMit to the inner surface of the choioid. Brush off the retinal pig- 

 ment under water. The choroid thus exposed can for the most part be fairly easily torn away 

 from the thick sclerotic, as a lymph-space exists between them, but the attachment is firm 

 around the oi)tic nerve entrance, and also where the arteries and nerves join the choroid alter 

 penetrating the sclerotic. The choroid is darkly i)igmented of a brown colour, with markings ou 

 its surfaces corresponding to the distribution of its large veins. The inner surface of the sclerotic 



