SENSE OF HEARING. 223 



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

 is to rotate the eyeball and to direct the pupil upward and outward. 



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

 forming 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; this covers the edge of the 

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



The lacrymal glands are ovoid in shape, and are 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 lacrymal 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 lacrymal 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.: 



1. 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 auditory 

 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 integument continuous 

 with that covering the side of the head. The general shape of the pinna is 

 concave, and presents, a little below the center, a deep depression the concha. 

 The external auditory canal extends from the concha inward for a distance of 

 about i| inches. It is directed somewhat forward and upward, passing 

 over a convexity of bone, and then dips downward to its termination; it is 

 composed of both bone and cartilage, and is lined by a reflection of the skin 

 covering the pinna. At the external portion of the canal the skin contains 

 a number of tubular glands the ceruminous glands which in their con- 

 formation resemble the perspiratory glands. They secrete the cerumen, or 

 ear-wax. 



The middle ear, or tympanum, is an irregularly shaped cavity hollowed 

 out of the temporal bone and situated between the external ear and the 



