PARTS FOR THE PROTECTION" OF THE EYEBALL. 811 



the obliquity of the assumed square of the retina must be exactly the same for the two 

 eyes, or the coincidence of the corresponding points would be disturbed and we should 

 have double vision. When we clearly understand that deviation of one eye in the hori- 

 zontal or the vertical plane disturbs the relation of the corresponding points, which is 

 sufficiently easy of comprehension, and that a deviation from exact coincidence of action 

 in torsion of the globes twists, as it were, the corresponding points, so that their rela- 

 tion is also disturbed, we can see that the varied movements of the globes, by the com- 

 bined action of the recti and oblique muscles, must correspond for each eye, in the move- 

 ments of torsion upon an antero-posterior axis, as well as in movements of rotation upon 

 the horizontal or the vertical axis. 



Parts for the Protection of the Eyeball. 



The orbit, formed by the union of certain of the bones of the face, receives the eyeball, 

 the ocular muscles, the muscle of the upper lid, blood-vessels, nerves, part of the lachry- 

 mal apparatus, and contains, also, a certain amount of adipose tissue, which latter never 

 disappears, even in extreme marasmus. The bony walls of this cavity protect the globe 

 and lodge the parts above enumerated. The internal, or nasal wall of the orbit projects 

 considerably beyond the external wall, so that the extent of vision is far greater in the 

 outward than in the inward direction. As the globe is more exposed to accidental injury 

 from an outward direction, the external wall of the orbit is stroog, while the bones which 

 form its internal wall are comparatively fragile. The upper border of the orbit (the 

 superciliary ridge) is provided with short, stiff hairs (the eyebrows) which serve to shade 

 the eye from excessive light and to protect the eyelids from perspiration from the fore- 

 head. 



The eyelids are covered by a very thin integument and are lined by the conjunctival 

 mucous membrane. The subcutaneous connective tissue is thin and loose and is entirely 

 free from fat. The skin presents numerous short papilla and small sudoriparous glands. 

 At the borders of the lids, are short, stiff, curved hairs, arranged in two or more rows, 

 called the eyelashes or cilia. Those of the upper lid are longer and more numerous than 

 the lower cilia. The curve of the lashes is from the eyeball. They serve to protect the 

 globe from dust, and, to a certain extent, to shade the eye. 



The tarsal cartilages are small, elongated, semilunar plates, extending from the edges 

 of the lids toward the margin of the orbit, between the skin and the mucous membrane. 

 Their length is about an inch. The central portion of the upper cartilage is about one- 

 third of an inch broad, and the corresponding part of the lower cartilage measures about 

 one-sixth of an. inch. At the inner canthus, or angle of the eye, is a small, delicate liga- 

 ment, or tendon, the tendo palpebrarum, which is attached to the lachrymal groove 

 internally, passes outward, and divides into two lamella, which are attached to the two 

 tarsal cartilages. At the outer canthus, the cartilages are attached to the malar bone by 

 the external tarsal ligament. The tarsal cartilages receive additional support from the 

 palpebral ligament, a fibrous membrane attached to the margin of the orbit and the con- 

 vex border of the cartilages and lying beneath the orbicularis muscle. This membrane is 

 strongest near the outer angle of the eye. 



On the posterior surface of the tarsal cartilages, partly embedded in them and lying 

 just beneath the conjunctiva, are the Meibomian glands. The structure and functions 

 of these glands have already been considered in connection with secretion. They pro- 

 duce an oily fluid, which smears the edges of the eyelids and prevents the overflow of 

 tears. 



Muscles which open and close the Eyelids. Leaving out the corrugator supercilii, 

 which draws the skin of the forehead downward and inward, we have the orbicularis 

 palpebrarum, which closes the lids, and the levator palpebraa superioris, which raises the 

 upper lid. The tensor tarsi, called the muscle of Horner, is a very thin, delicate muscle, 



