MUSCLES OF THE EYEBALL. 811 



oviparous Vertebrata, and in like manner are supported in their move- 

 ments by a quantity of semifluid fat, with which the orbital cavities are 

 filled up. In Man, as in Birds, Reptiles, and Pishes, six muscles are 

 appropriated to the movements of each eyeball, viz. four recti and two 

 obliqui. The four recti muscles have the same disposition in Mammalia 

 as in Birds ; that is, they arise from the margin of the optic foramen, 

 and run forward to be inserted opposite to each other upon the superior, 

 inferior, and lateral surfaces of the sclerotic coat. The inferior oblique 

 likewise offers a similar arrangement in all the Yertebrata, arising from 

 the margin of the internal wall of the orbit, and running outwards to be 

 attached to the external surface of the globe of the eye. But the supe- 

 rior oblique, in the class before us, takes a very peculiar course. Arising 

 like the rest, it passes forward to the upper and inner margin of the 

 orbit, where its tendon is reflected over a little cartilaginous pulley 

 (fig. 410, c), and turns back again to be inserted into the external and 

 posterior aspect of the eyeball. 



Fig. 410. 



Muscles of the eyeball. 



(2379.) In addition to the six muscles appointed for the movements 

 of the eye in Man and the Quadrumana, other Mammalia have a 

 seventh, called the choanoid or funnel-shaped muscle. This likewise 

 arises from the borders of the optic foramen, and, gradually expanding, 

 forms a hollow cone interposed between the recti muscles and the optic 

 nerve, the base of the cone being attached to the sclerotic behind the 

 insertion of the recti. Frequently, indeed, this choanoid or suspensory 

 muscle is divided into four portions, in which case the animals so pro- 

 vided would seem to have eight recti muscles. 



(2380.) The eyelids of Mammalia resemble the human in every 



