CONTENTS. xxxvii 



SECTION XII 

 THE PROTECTIVE MECHANISMS OF THE EYE. 



PAGE 



608. The eye-lids and their muscles 976 



609. The conjunctiva and its glands. Tears. The secretion of tears . 977 



CHAPTER IV. 

 HEARING. 



SECTION I. 



ON THE GENERAL STRUCTURE OP THE EAR AND ON THE STRUCTURE 

 AND FUNCTIONS OF THE SUBSIDIARY AUDITORY APPARATUS. 



610. The embryonic history of the ear. The otic vesicle . . .980 

 611. The general relations of the parts of the ear ; vestibule and 

 cochlea, membranous and bony labyrinth, tympanum, auditory 

 ossicles, membrana tympani and external meatus . . . 981 

 612. The general use of the several parts 986 



The Conduction of Sound through the Tympanum. 



613. The chain of ossicles as a lever 988 



614. longitudinal and transversal sonorous vibrations. The vibrations 



of the tympanic membrane . . . . . . , . 989 



615. The conduction of vibrations through the chain of ossicles . . 991 



616. The conduction of vibrations through the bones of the skull . . 992 



617. The action of the tensor tympani and stapedius muscles . . 993 



618. The Eustachian tube . 995 



SECTION II. 

 ON AUDITORY SENSATIONS. 



619. Noises and musical sounds 998 



620. The characters of musical sounds ; loudness, pitch and quality ; 



fundamental and partial tones 998 



621. The limits of auditory sensations 1000 



622. Appreciation of differences of pitch 1001 



623. The number of vibrations needed to excite a sensation . . . 1001 



624. The characters of noises 1002 



625. The effects of exhaustion . . . 1002 



626. The fusion of auditory sensations 1003 



627. The interference of vibrations. Beats 1004 



