598 THE NERVOUS SYSTEM. 



beneath the conjunctiva, are the Meibomian glands (Fig. 371), 

 thirty in number on the upper, and fewer on the lower lid. 

 Their ducts open on the edges of the lids, and their secretion 

 prevents the adhesion of the lids and the tears from running 

 over them on to the cheeks. 



The Sense of Hearing. The ear (Fig. 373), the organ of 

 hearing, consists of three subdivisions : (1) External ; (2) middle ; 

 and (3) internal. 



External Ear. The external ear consists of the pinna or auricle, 

 and the external auditory canal or meatus. The function of the 



FIG. 373. Diagram of organ of hearing of left side : 1, the pinna ; 2, bottom of 

 concha; 2, 2', meatus externus; 3, tympanum ; above 3, the chain of ossicles; 3', 

 opening into the mastoid cells : 4, Eustachian tube ; 5, meatus internus. containing 

 the facial (uppermost) and auditory nerves ; 6, placed on the vestibule of the laby- 

 rinth above the fenestra ovalis ; a, apex of the petrous bone ; b, internal carotid 

 artery ; c, styloid process ; d, facial nerve, issuing from the stylomastoid foramen ; 

 , mastoid process; /, squamous part of the bone (Quain, after Arnold). 



pinna is to collect the sound-waves and direct them to the external 

 auditory canal, which they traverse to reach the membrana tym- 

 pani. In some animals, such as the horse, the auricles are very 

 important, enabling the animal to detect the direction from which 

 sounds come, and they are capable of considerable movement ; 

 but in man they are not so important, although when the hearing 

 is defective they are of assistance. That they are not essential to 

 hearing is shown by the fact that when removed, hearing is not 

 affected, and also by the fact that in birds, where they are absent, 

 the sense of hearing is well marked. 



