Hearing depends upon the sonorous vibrations of the atmosphere. The waves of sound strike the sensitive portions of the ear, and their impressions upon the auditory nerves are termed the sensations of hearing. The ear is divided into three parts, called respectively the External, Middle, and Internal ear.
The external organs of hearing are two in number, and placed on opposite sides of the head. In most of the higher order of vertebrates, they are so situated as to give expression and proportion to the facial organs, and, at the same time, to suit the requirements of actual life.
The External ear is connected with the interior part by a prolongation of its orifice, termed the external auditory meatus. In man, this gristly portion of the auditory apparatus is about one inch in length, lined by a continuation of the integument of the ear, and has numerous hairs on its surface, to prevent the intrusion of foreign substances. Between the external meatus and the cavity of the middle ear is the membrana tympani, which is stretched across the opening like the head of a drum. The tympanum, or ear-drum, communicates with the pharynx by the eustachian tube, which is a narrow passage lined with delicate, ciliated epithelium. On the posterior portion it is connected with the mastoid cells. Three small bones are stretched across the cavity of the tympanum, and called, from their form, the malleus, incus and stapes, or the hammer, anvil, and stirrup. Agassiz mentions a fourth, which he terms the os orbiculare. Each wave of sound falling upon the membrana tympani, throws its molecules into vibrations which are communicated to the chain of bones, which, in turn, transmits them to the membrane of the foramen ovale. The three muscles which regulate the tension of these membranes are termed the tensor tympani, laxator tympani, and stapedium tympani.
The Labyrinth, or Internal ear, is a complicated cavity, consisting of three portions termed the vestibule, cochlea, and semi-circular canals. The vestibule is the central portion and communicates with the other divisions. The labyrinth is filled with a transparent fluid, termed perilymph, in which are suspended, in the vestibules and canals, small membranous sacs, containing a fluid substance, termed endolymph (sometimes called vitrine auditive from its resemblance to the vitreous humor of the eye). The filaments of the auditory nerve penetrate the membranous tissues of these sacs, and also of those suspended at the commencement of the semi-circular canals. These little sacs are supposed to be the seat of hearing, and to determine, in some mysterious way, the quality, intensity and pitch of sounds.
The determination of the direction of sound is a problem of acoustics. Some have contended that the arrangement of the semi-circular canals is in some way connected with this sensation. But this supposition, together with the theory of the transmission of sound through the various portions of the cranial bones, has been exploded.
From the foregoing description, it will be seen that the labyrinth and tympanum are the most essential parts of the organs of hearing. In delicacy and refinement this sense ranks next to sight. The emotions of beauty and sublimity, excited by the warbling of birds and the roll of thunder, are scarcely distinguishable from the intense emotions arising from sight. It is a remarkable fact, that the refinement or cultivation of these senses is always found associated. Those nations which furnish the best artists, or have the highest appreciation of painting and sculpture, produce the most skillful musicians, those who reduce music to a science.
SMELL.
Next in order of delicacy, and more closely allied with the physical functions, is the sense of smell. Delicate perfumes, or the fragrance of a flower, impart an exhilarating sensation of delight, while numerous odors excite a feeling of disgust. The organ of smell is far less complicated in its structure than the eye or the ear. It consists of two cavities having cartilaginous walls, and lined with a thick mucous coat, termed the pituitary membrane, over which are reflected the olfactory nerves. Particles of matter, too minute to be visible even through the microscope, are detached from the odorous body and come in contact with the nerves of smell, which transmit the impressions or impulses thus received to the brain. Fig. 65 shows the distribution of the olfactory nerves in the nasal passages. The nose is supplied with two kinds of filaments which are termed respectively nerves of special and nerves of general sensation. Compared with the lower animals, especially with those belonging to the carnivorous species, the sense of smell in man is feeble. The sensation of smell is especially connected with the pleasures and necessities of animal life.