ORGAN OF SIGHT IN MAMMALIA. 259 



and the eyeball, the recti perforating this layer or sheath before 

 expanding to their insertions. The upper one, ( rectus superior,' 

 directs the cornea upward, the ( rectus inferior ' downward, the 

 e rectus externus ' outward, the f rectus interims ' inward or 

 toward the nose ; the ( recti ' antagonising, or combining with, 

 each other in all the degrees required to make the cornea assume 

 any intermediate direction : they can thus produce the move- 

 ments analogous to the ( circumduction ' of a limb ; in doing 

 which the centre of the cornea describes a circle. For f rotation' 

 of the eyeball, in which this corneal centre remains fixed as the 

 fore end of an axis, the two muscles called ' oblique ' are added. 



In Mammals the ' superior oblique ' arises from the back part 

 of the orbit with the recti, advances to the upper part of the rim, 

 glides there through a tendinous pulley, returns toward the eye- 

 ball, is reflected backward and outward beneath the rectus 

 superior, and is inserted into the sclerotic between this muscle 

 and the rectus externus. The inferior oblique takes its origin, 

 in advance of the eyeball, from the orbital plate of the maxillary ; 

 passes outward and backward beneath the ' rectus inferior,' and 

 is inserted into the outer and back part of the sclerotic. The 

 two oblique are so disposed as to act, when antagonising each 

 other, in rotating the eyeball 011 its antero-posterior axis : when 

 combining in action they tend to draw forward the eye, and thus 

 antagonise the recti muscles collectively. The trochlear arrange- 

 ment of the superior oblique is peculiar to the present class. 



As habitually antagonistic muscles have nerves from distinct 

 sources, the rectus abductor is supplied by the ' sixth ' nerve, the 

 rectus adductor by the ' third.' The superior oblique, which 

 opposes the inferior one in most movements, is supplied by the 

 6 fourth ' nerve. As the depression of the eyeball can be per- 

 formed by the superior oblique if the downward motion be 

 directed by the lateral muscles, it suffices that it should have the 

 same separate nerve (fourth) for that motion as for antagonising 

 the inferior oblique, which, like the upper, lower, and inner recti, 

 is supplied by the ' third nerve.' l 



In Cetacea the eyelids are represented by a continuous circular 

 fold of the skin, leaving a round opening in front of the eye with 

 a narrow margin unprovided with eyelashes. This ' palpebral ' 

 opening is closed by an orbicular muscle or sphincter, and is 

 expanded by four broad, thin, almost continuous muscles (in the 

 Porpoise). The ( tunica conjunctiva,' fig. 195, y, lines the circular 



1 For Hunter's excellent remarks on ' the use of the Oblique Muscles/ see xciv. 



p. 24. 



VOL. in. *s '2 



