CONTENTS. 



SECTION XIL 

 THE PROTECTIVE MECHANISMS OF mi. ETB. 



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 L 



ON THE GENERAL STRUCTURE OF THE EAR AND ON THE STRUCTURE 

 AND FUNCTIONS OF THE SUBSIDIARY AUDITORY APPARATUS. 



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

 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 996 



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 



