304 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



d) The eye muscles: Remove the tissue from about the eye on the same side 

 of the animal as under c) and completely expose the eyeball. In doing this, 

 first cut away the upper eyelid (or in the skate the skin over the eye), noting that 

 the inner lining of the eyelid is continuous with a thin layer {conjunctiva) which 

 adheres closely to the external surface of the eyeball. Next cut away very care- 

 fully the cartilage between the eye and the brain, which is seen as a white- 

 lobed structure in the median region, and also the cartilage in front of the eye. 

 Do not injure the brain and do not cut into the elevation dorsal to the spiracle. 

 The stout, white bands seen in this dissection are cranial nerves. In the skate 

 very little cutting is required. The large, somewhat spherical body exposed is 

 the eyeball. In the dogfishes it is imbedded in a gelatinous material which should 

 be carefully cleaned out. 



The eyeball reposes in a cavity, the orbit, to the walls of which it is attached 

 by muscular bands, the eye muscles. They are voluntary muscles derived from 

 the myotomes of the second, third, and fourth segments of the head. There are 

 six of these eye muscles which should be identified as follows. From the dorsal 

 view four of them will be seen. The one which is attached to the anterior wall 

 of the orbit is the superior oblique. The other three originate from the postero- 

 lateral angle of the orbit and are named recti muscles. The most anterior one 

 is the internal or medial rectus; its insertion on the eyeball is covered dorsally 

 by the superior oblique. The next rectus muscle is the superior rectus, more 

 dorsally situated than the others. The third, the external (or lateral) rectus, is 

 inserted on the posterior surface of the eyeball. Next, raise the eyeball dor- 

 sally and note that the conjunctiva or most superficial coat over the external 

 surface of the eyeball is continuous with the lining of the lower lid. Cut through 

 this and free the eyeball ventrally, cleaning out the gelatinous and fibrous tissue 

 which will be found here. On lifting the eyeball the remaining two eye muscles 

 will be seen. The inferior oblique originates from the anteromedial corner of 

 the orbit, the inferior rectus from the posteromedial angle of the orbit; both are 

 inserted in contact with each other on the middle of the ventral surface of the 

 eyeball. The white cords seen among the eye muscles are nerves. 



Draw the eyeball and its muscles from dorsal view, showing as many of the 

 muscles as possible. 



The eye muscles originate from the orbit and are inserted on the eyeball. 

 Their action is to turn the eyeball in various directions. As already stated they 

 are derived from three head segments and are in most vertebrates practically 

 the only muscles developed from the head myotomes. These three muscle- 

 forming myotomes are designated the first, second, and third, although they are 

 in reality the second, third, and fourth of the head myotomes, since the true 

 first segment was not noticed until later. According to the recent investiga- 

 tions of Neal, the third myotome gives rise to part of the external rectus, the 

 second to the rest of the external rectus and to the superior oblique, while the 



