268 GENERAL ORNITHOLOGY part ii 



giving the proper direction to the tendon Z:, and keeping it off the 

 nerve. 



Beneath the eyelids, upon the ball, is a delicate filmy membrane 

 not easily recognised 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 |)a/;.7f/>?'rt^ 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 become 

 congested, when the eye presents the well-known api^earance 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 Harclerian gland, is the smaller, deeply 

 seated behind the winker, upon which it pours a glairy 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 glanil 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, Avhich thus 

 becomes a sort of cesspool to receive the refuse waters of the eye. 

 A third gland about the orbit has been already mentioned (p. 264) 

 as pertaining to the nose, not to the eye. Its site is shown in the 

 crescentic superorbital depression, Fig. 63, w. 



The motions of the eyeball, though more restricted than in 

 mammals, owing to the shape of the ball and its close socketing, 

 are nevertheless subserved by the usual number of six muscles. Of 

 these four are called the recti, or straight muscles, and two the 

 oldiqiii, or oblique muscles ; 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 eyeball at as many nearly equidistant 

 points on its circumference ; the musculus rectus superioi; Fig. 81, a, 

 on top ; m. r. inferior, c, below, antagonising a ; the m. r. externus, 

 h, and interims, cl, respectively to the outer and inner (hindward 

 and forward) sides, also antagonising each other. The two oblique 

 muscles arise farther forward in the bony orbit, near each other, 

 and then diverge obliquely upward, m. o. superioi; e, and downward, 

 m. 0. inferior, f, to be inserted near the margin of the globe of the 

 eye, close by the respective insertions of superior and inferior rectus. 

 All the motions of the ball result from consentaneous or dissen- 

 taneous action along these six lines of traction ; the muscles acting 

 as ropes to pull the Ixxll 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 



