COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 305 



first myotome produces the other four muscles. This account differs slightly 

 from that given in the textbooks. 



e) The structure of the eyeball: Cut through the eye muscles at the insertions 

 and remove the eyeball. As already noted the outermost coat covering the front 

 of the eyeball is the conjunctiva which is deflected onto the inner surface of the 

 eyelids. Note the free edge of the conjunctiva clinging to the eyeball where 

 the eyelids were cut. The conjunctiva is the epidermis of the skin and not one 

 of the true coats of the eye. The outermost coat of the eyeball is the sclera or 

 sclerotic coat, a very tough membrane composed of connective tissue. The front 

 part of the sclera is transparent and is named the cornea; the conjunctiva is 

 inseparably fused to the outer surface of the cornea. Through the transparent 

 cornea can be seen an opening, the pupil. Cut off the dorsal side of the eyeball 

 so that you can look within the cavity. Place the larger piece under water. 

 The large spherical body in the interior is the crystalline lens. Note that internal 

 to the sclera is a black coat, the chorioid coat, and internal to this a soft, often 

 collapsed, greenish layer, the retina. Follow the chorioid coat to the front of 

 the eye and note that there it is separated from the cornea forming a black curtain, 

 the iris, in the center of which is an opening, the pupil. The iris divides the 

 cavity of the eyeball into an external cavity, the anterior chamber of the eye 

 between the iris and the cornea, and an internal chamber, the cavity of the vitre- 

 ous humor, between the lens and the retina. The anterior chamber contains a 

 fluid, the aquaeous humor; the cavity of the vitreous humor contains a gelatinous 

 material, the vitreous humor or vitreous body, collapsed in the preserved specimen. 

 The lens in life is attached to the margins of the pupil and also to the margins of 

 the retina; and the small space between these two points of attachment of the 

 lens forms the posterior chamber of the eye. 



Draw the section, showing the structures of the eye. 



The retina is the nervous part of the eye containing the sensory cells (rods 

 and cones) which are stimulated by light. The lens and the two humors focus 

 the light upon the retina. The focus is changed in fishes by moving the lens 

 back and forth. The pupil regulates the amount of light admitted. The coats 

 of the eye serve for protection and to darken the interior. 



In the orbit after removal of the eyeball note the origins of the six eye muscles, 

 the optic pedicel, a cartilaginous stalk situated among the rectus muscles and 

 helping support the eyeball, and the optic nerve, a stout white stalk located in 

 front of the rectus muscles. The stout white band in the floor of the orbit is 

 the infraorbital nerve. 



f) The internal ear: The ear in fishes consists only of the internal ear or 

 membranous labyrinth. This is imbedded in the otic region of the skull. In the 

 dogfishes and skate it is situated between the spiracle and the mid-dorsal line. 

 A pronounced elevation of the chondrocranium is present at this place. In the 

 median line between the two elevations will be found a pair of small holes in 



