COMPARATIVE ANATOMY 73 



f. The inferior rectus inserts on the ventral side of the eye. 

 It may be seen by cutting the other muscles at their insertions. 

 It is innervated by the third nerve. 



On the median side of the eyeball observe the large optic 

 nerve. Also, in the posterior corner of the orbit among the 

 rectus muscles, find the optic pedicel^ a cartilaginous stalk for 

 supporting the eyeball. 



Anatomy of the Eyeball. — Remove the eyeball from 

 the orbit, cutting any tissues that may hold it. 



The outermost covering of the eyeball is the sclerotic coat, 

 composed of very tough connective tissue. The muscles of 

 the eye are attached to it. A transparent continuation of the 

 sclerotic coat, the cornea, extends over the front of the eye. 

 The pupil is the opening that may be seen through the 

 cornea. 



Remove the dorsal wall of the eyeball and observe the struc- 

 tures within. The large cavity in the posterior part is filled 

 during life with a fluid, the vitreous humor. The pigmented 

 layer just within the sclerotic coat is the choroid coat, and the thin 

 white membrane within this is the retina, the sensory part of 

 the eye. 



The lens is held in place by a suspensory ligament attached 

 to the choroid coat. The iris, a pigmented extension of the 

 choroid coat, surrounds the pupil in front of the lens. The iris 

 controls the amount of light entering the eye by regulating 

 the size of the pupil. The cavity between iris and cornea is 

 filled during life with a fluid, the aqueous humor. The aqueous 

 humor and vitreous humor aid the lens in bending the light 

 rays so that they focus on the retina. 



The structure of the eye is fundamentally the same in all 

 vertebrates. Examine the model of the human eye for compari- 

 son with the eye of the shark. 



Middle Ear of an Amphibian 



In the frog the ear consists of two regions, an inner ear and a 

 middle ear. In a demonstration dissection of the middle ear 

 observe the columella, a single bone extending from the tympa- 

 num across the cavity of the middle ear. What is the evolu- 

 tionary origin of this bone .^ The cavity of the middle ear and 



