THE SENSE OF HEARING. 249 



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 ex- 

 ternal 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*4 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 ex- 

 ternal portion of the canal the skin contains a number of tubular 

 glands, the' ceruminous glands, which in their conformation re- 

 semble 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 



