Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/613

Rh both of which are found in Central Asia, as well as in South-eastern Europe—may also be employed. Of the other more or less nearly allied species or races want of room forbids the consideration, but there is a smaller form on which a few words may be said. This has usually gone under the name of A. ncevia, but is now thought by the best authorities to include three local races, or, in the eyes of some, species. They inhabit Europe, North Africa, and Western Asia to India, and two examples of one of them—A. danga, the form which is somewhat plentiful in North-eastern Germany—have occurred in Cornwall. The smallest true Eagle is A. pennata, which inhabits Southern Europe, Africa, and India. Differing from other Eagles of their genus by its wedge- shaped tail, though otherwise greatly resembling them, is the A. audax of Australia. Lastly may be noticed here a small group of Eagles, characterized by their long legs, forming the genus Nisaetus, of which one species, N. fasciatus, is found in Europe. The Osprey (Pandion), though placed by many among the Aquilince, certainly does not belong to that subfamily.  EAR. The simplest form of the organ of hearing is a small sac containing fluid, with the auditory ngrve expanded upon it. Sonorous vibrations are communicated to this sac either directly through the hard parts of the head, or at the same time by a membrane exposed to the surround ing medium. Such is the form of ear found in many of the Crustacea and in the Cephalopoda. In the Vertebrata, there is a progressive development and increasing com plexity from the fishes up to Mammalia. For details as to the structure of the ear in the different subdivisions of the Vertebrata, reference is made to the articles treating of these, such as,, &c.; and the structure of the human ear will be found fully described in the article ANATOMY, vol. i. p. 891. It is the object of this article to describe the phenomena of auditory sensation from the physiological point of view. The sense of hearing is a special sensation the cause of which is an excitation of the auditory nerves by the vibrations of sonorous bodies. A description of sonorous vibrations and of their transmission is given in the article ACOUSTICS ; here we shall consider, first, the transmission of such vibrations from the external ear to the auditory nerve, and secondly, the physiological characters of auditory sensation.

I.—1.—Transmission in External Ear.—The external ear consists of the pinna, or auricle, and the external auditory meatus, or canal, at the bottom of which w r e find the membrana tympani, or drum head. In many animals the auricle is trumpet-shaped, and, being freely movable by muscles, serves to collect sonorous waves coming from various directions. The auricle of the human ear presents many irregularities of surface. If these irregularities are abolished by filling them up with a soft material such as wax or oil, leaving the entrance to the canal free, experi ment shows that the intensity of sounds is weakened, and that there is more difficulty in judging of their direction. When waves of sound strike the auricle, they are partly re flected outwards, while the remainder, impinging at various angles, undergo a number of reflections so as to be directed into the auditory canal. Vibrations are transmitted along the auditory canal, partly by the air it contains and partly by its walls, to the membrana tympani. The absence of the auricle, as the result of accident or injury, has not caused diminution of hearing. In the auditory canal, waves of sound are reflected from side to side until they reach the membrana tympani. From the obliquity in position and peculiar curvature of this membrane, most of the waves must strike it nearly perpendicularly, and in the most advantageous direction.

2.—Transmission in Middle Ear.—The middle ear is a small cavity, the walls of which are rigid with the exception of the portions consisting of the membrana tympani, and the membrane of the round window and of the apparatus filling the oval window. This cavity communicates with the pharynx by the Eustachian tube, which forms a kind of air-tube between the pharynx and the tympanum for the purpose of regulating pressure on the membrana tympani. It is generally supposed that during rest the tube is open, and that it is closed during the act of deglutition. As this action is frequently taking place, not only when food or drink is introduced, but when saliva is swallowed, it is evident that the pressure of the air in the tympanum will be kept in a state of equilibrium with that of the external air on the outer surface of the membrana tympani, and that thus the membrana tympani will be rendered independent of variations of atmospheric pressure such as may occur within certain limits, as when we descend in a diving bell or ascend in a balloon. By a forcible expiration, the oral and nasal cavities being closed, air may be driven into the tympanum, while a forcible inspiration (Valsalva s experi ment) will draw air from that cavity. In the first case, the membrana tympani will bulge outwards, in the second case inwards, and in both, from excessive stretching of the membrane, there will be partial deafness, especially for sounds of high pitch. Permanent occlusion of the tube is one of the most common causes of deafness. The membrana tympani is capable of being set into vibration by a sound of any pitch included in the range of perceptible sounds. It responds exactly as to number of vibrations (pitch), intensity of vibrations (intensity), and complexity of vibration (quality or timbre). Consequently we can hear a sound of any given pitch, of a certain intensity, and in its own specific timbre or quality. Generally speaking, very high tones are heard more easily than low tones of the same intensity. As the membrana tympani is not only fixed by its margin to a ring or tube of bone, but is also adherent to the handle of the malleus, which follows its movements, its vibrations meet with con siderable resistance. This diminishes the intensity of its vibrations, and prevents also the continued vibration of the membrane after an external vibration has ceased, so that a sound is not heard much longer than it lasts. The tension of the membrane may be affected (1) by differences of pres sure on the two surfaces of the membrana tympani, as may occur during forcible expiration or inspiration, or in a patho logical condition, and (2) by muscular action, due to con traction of the tensor tympani muscle. This small muscle arises from the apex of the petrous temporal and the car tilage of the Eustachian tube, enters the tympanum at its anterior wall, and is inserted into the malleus near its root. The handle of the malleus is inserted between the layers of the membrana tympani, and, as the malleus and incus move round an axis passing through the neck of the malleus from before backwards, the action of the muscle is to pull the membrana tympani inwards towards the tympanic cavity in the form of a cone, the meridians of which, according to Helmholtz, are not straight but curved, with convexity out wards. When the muscle contracts, the handle of the malleus is drawn still farther inwards, and thus a greater tension of the tympanic membrane is produced. On relaxation of the muscle, the membrane returns to its position of equilibrium by its own elasticity and by the elasticity of the chain of bones, This power of varying the tension of the membrane is a kind of accommodating mechanism for receiving and transmitting sounds of different pitch. With different degrees of tension, it will respond more readily to sounds of different pitch. Thus, when the membrane is tense, it will readily respond to high sounds, while relaxation will be the condition most 