THE ANATOMY OF BIIWS. —NEUROLOGY. 181 



the eyes from the ghire of light; aud doubtless the eagle throws tin- same screeu over its siglu 

 when soaring towards the sun. When not in action, the winker lies curled up in the corner of 

 the eye, like those patent window shades which stay up of themselves till pulled down. The 

 ingenious mechanism of the movement of the winker across the lid uuiy be understood with the 

 help of fig. 81, which represents the hack of the eye-ball. The winker lies in front, on the left 

 hand of the picture, and is to be puUcd across the front by the slender tendon, k, of the x>!/rami- 

 (lalis muscle, h. As h contracts it pulls on k, and k, winding round to the front, pulls the 

 winker to the right hand. But i is the optic nerve, entering the ball; k would press upon 

 it, were it not fended off by passing, as seen by the dotted hue, through a puUey in the end 

 of the qiiadratus muscle, g. The harder h pulls, the harder does g also pull, their consentane- 

 ous action at once giving the proper direction to the tendon k, aud keeping it off the nerve. 



Beneath the eye-lids, upon the baU, is a delicate filmy membrane not easily recognized on 

 ordinary inspection : this is the conjunctiva, so called because it joins the eye to the lids. The 

 ocular layer is transparent where it passes over the cornea : it is then reflected away from the 

 ball, to form the palpebral layer, — a folding between being the nictitating membrane. The 

 conjunctiva is highly vascular, but the blood-vessels are too small to be seen unless they be- 

 come congested, when the eye presents the well-known appeai'ance called blood-shot. Though 

 birds can hardly be said to cry, they have a well-developed apparatus for the manufacture of 

 tears. The lacrymal are two small glands lying one in each corner of the eye, inner and 

 outer. The former, called the harderian gland, is the smaller, deeply seated behind the 

 winker, upon which it pours a glary fluid : it is an oil-can which not only supplies but 

 applies the fluid to the winker, which needs constant lubricating to work well. The lac- 

 rymal gland proper is the outer one, which prepares the tears to moisten and cleanse the 

 conjunctiva; after which they are drained off by the lacrymal duct into the cavity of the 

 nose, which thus becomes a sort of cesspool to receive the refuse waters of the eye. A third 

 gland about the orbit has been ali'eady mentioned (p. 178) as pertaining to the nose, not to the 

 eye. Its site is shown in the crescentic super-orbital depression, fig. G3, w. 



The motions of the eye-ball, though more restricted than in mammals, owing to the shapi' 

 of the ball and its close socketing, are nevertheless subserved by the usual number of six mus- 

 cles. Of these four are called the recti, or straight muscles, and two the obliqui, or oblique 

 nuiscles; though they are all "straight" enough, the terms applying to their lines of traction. 

 The four recti arise from the bony orbit, near together, about the optic foramen, and pass to 

 be inserted in the eye-ball at as many nearly equidistant points on its circumference ; the 

 musculus rectus superior, fig. 81, a, on top; m. r. inferior, c, below, antagonizing a ; the m. r. 

 externus, b, and internus, d, respectively to the outer and inner fhindward and forward) sides, 

 also antagonizing each other. The two oblique muscles arise further forward in the bony or- 

 bit, near each other, and then diverge obliquely upward, m. o. superior, e, and do\^aiward, m. 

 0. inferior, f, to be inserted near the margin of the globe of the eye, close by the respective in- 

 sertions of superior and inferior rectus. All the motions of the ball result from consentaneous 

 or dissentaneous action along these six lines of traction ; the muscles acting as ropes to puU 

 the ball about, and to steady it in any direction of its axis. The peculiarity of mechanism in a 

 bird is, that the superior oblique goes straight to its insertion, instead of passing through a 

 pulley which changes its line of actitm in mammals. The special nerves presiding over 

 these muscles (3, 4, 6) have been pointed out already (p. 177). In the figure, the cut orbital 

 ends of them all are reflected away from the ball to disclose the underlying muscles of the 

 winker : the reader must mentally bring the six loose ends together and fasten them to the 

 bony orbit at points near about opposite i, as above said of their origins. 



The above are the principal circumstances and accessories of the optic api)aratus ; we may 

 now examine the eye itself, of which fig. 82 gives an enlarged view, in longitudinal vertical 

 section, — the nerve, marsupium, and ciliary processes not inleed lying as shown in this section. 



