THE SENSE OF HEARING. 165 



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 outward. 



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 Tears, 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 

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



THE SENSE OF HEARING. 



The Ear or Organ of Hearing is lodged within the petrous portion 

 of the temporal bone. It may be, for convenience of description, divided 

 into three portions, viz : I. The external ear. 2. The middle ear. 3. 

 The internal ear or labyrinth. 



The External Ear consists of the pinna or auricle and the external au- 

 ditory canal. The pinna consists of a thin layer of cartilage, presenting a 

 series of elevations and depressions ; it is attached by fibrous tissue to the 

 outer bony edge of the auditory canal ; it is covered by a layer of integu- 

 ment continuous with that covering the side of the head. The general 



