superior oblique 



superior rectus 



nternol rectus' 

 inferior oblique 



optic pedicel 



LAMPREY 



B 



external rectus 

 optic pedicel 



SHARK 



C TELEOST 



levator polpebroe 



quadratus 



superior rectus bursalis and 



,..««,:«, «ui: 1 retractor buibi 



superior oblique • ' 



nternal rectus 



D LIZARD 



inferior oblique 

 depressor palpebrae 

 E BIRD 

 Figure 14-4. Eye muscles of vertebrates and their innervotion. 



external rectus 

 ^inferior rectus 



F CAT 



around behind the eye and through the lower hd to the 

 nictitating membrane. Contraction of this muscle pulls the 

 nictitating membrane across the surface of the eye from 

 front to back. The inferior rectus has been modified into 

 two parts; a superficial depressor palpebrae, which pulls 

 down the lower eyelid, and the inferior rectus proper. 

 Again there is a problem of terminology: should one use 

 pars depressor palpebrae and pars rectus for these two 

 divisions or identify them as distinct muscles? The superior 

 rectus also may be subdivided into a levator palpebrae and 

 rectus proper. 



The crocodilian eye musculature is like that of the bird 

 but is not as highly modified. 



In the mammal there is a marked peculiarity in the form 

 of the superior oblique. It arises from the eye-socket wall 

 above the origin of the rectus muscles, which as usual arise 

 around the optic foramen. The superior oblique passes for- 

 ward and upward over the eyeball through a puUy of 

 ligamentous tissue and from here backward and downward 

 to insert on the eyeball. The action of the superior oblique 

 is the same as the action in other vertebrates but the form is 

 quite distinctive. The occurrence of a pully suggests that the 



origin had shifted backward to this new position. The supe- 

 rior rectus may also be divided into a levator palpebrae and a 

 .superior rectus. The retractor bulbi of the mammal is highly 

 variable. There may be several parts paralleling the optic 

 nerve. The retractor bulbi is innervated along with the ex- 

 ternal rectus by the abducens nerve. 



The six basic extrinsic eye muscles develop from the three 

 pairs of preotic somites of the head or from three pairs of 

 mesenchymatous condensations. In the shark there are three 

 distinct mesodermal vesicles from whose parts these muscles 

 condense (Figure 14-3). There is some disagreement about 

 the exact details for the shark. Head cavities occur in many 

 groups: shark, Amia. Leptsosleus, alligator, turtle, and some 

 birds. In some fish (salmon), the chick, and cat, head cavi- 

 ties as such do not appear, only mesenchymal condensations. 



The evolution of eye muscles of vertebrates suggests a 

 basic pattern, at least among gnathostomes, which heis 

 undergone some modifications in accordance with the use of 

 the eyes and changes of the orbit area. The retractor bulbi 

 appearing in the Amphibia was probably developed to pro- 

 tect the eye by retracting it into the orbit at the same time 

 the eyelids are closed. 



SKELETAL MUSCULATURE 



423 



