THE SENSE OF SIGHT. 143 



Presbyopia is a loss of the power of accommodation of the eye to near 

 objects, and usually occurs between the ages of 40 and 60 ; it is remedied 

 by the use of convex glasses. 



Accessory Structures. The muscles which move the eyeball are six 

 in number ; the superior and inferior recti, the external and internal recti, 

 the superior and inferior oblique muscles. The four recti muscles, arising 

 from the apex of the orbit, pass forward and are inserted into the sides of 

 the sclerotic coat ; the superior and inferior muscles rotate the eye around 

 a horizontal axis ; the external and internal rotate it around a vertical 

 axis. 



The Superior oblique muscle, having the same origin, passes forward to 

 the inner and upper angle of the orbital cavity, where its tendon passes 

 through a cartilaginous pulley ; it is then reflected backward and inserted 

 into the sclerotic just behind the transverse diameter. Its function is to 

 rotate the eyeball in such a manner as to direct the pupil downward and 

 outward. 



The inferior oblique muscle arises at the inner angle of the orbit and 

 then passes outward and backward, to be inserted into the sclerotic. 

 Its function is to rotate the eyeball and direct the pupil upward and 

 out%vard. 



By the associated action of all these muscles, the eyeball is capable of 

 performing all the varied and complex movements necessary for distinct 

 vision. 



The Eyelids, bordered with short, stiff hairs, shade the eye and protect 

 it from injury. On the posterior surface, just beneath the conjunctiva, are 

 the Meibomian glands, which secrete an oily fluid ; it covers the edge of 

 the lids, and prevents the tears from flowing over the cheek. 



The Lachrymal Glands are ovoid in shape, and situated at the upper 

 and outer part of the orbital cavity ; they open by from six to eight ducts 

 at the outer portion of the upper lids. 



The TearSy secreted by the lachrymal glands, are distributed over the 

 cornea by the lids during the act of winking, and keep it moist and free 

 from dust. The excess of tears passes into the lachrymal ducts, which begin 

 by two minute orifices, one on each lid, at the inner canthus. They con- 

 duct the tears into the nasal duct, and so into the nose. 



