CONTENTS. 



XXX VII 



SECTION XIL 

 The Protective Mechanisms of the Eye. 



§ 608. The eye-lids and their muscles 



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



PAGE 



976 

 977 



CHAPTER IV. 

 Hearing. 



SECTION I. 



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 

 §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 



