146 ■ ON THE SENSES. 



conducting vehicle between distant sounds and the nerve needs no special proof; 

 wherefore we conclude that, of all conceivable passages by which sound-pro- 

 ducing vibrations penetrate to the sensorium, that alone deserves physiological 

 consideration through which undulations of slight intensity, or proceeding from 

 distant objects, attain with facility the nerve of hearing. 



It is this one passage, therefore — the passage through the outer ear or auricle, 

 the auditory canal, the tympanum, the ossicles or small movable bones, the laby- 

 rinth, both osseous and membranous — to which we shall turn our special attention, 

 while M'e accompany the vibratory movement communicated by the au--waves 

 through all these several stations in the order in which we have named them. 

 We trust that it may be in our power to convey a clear comprehension of the 

 apparatus, in part so greatly complicated, which the vibrations have thus to 

 traverse. In beginning, however, we are somewhat embarrassed by the doubt 

 as to how far we may venture to presume our readers already acquainted with 

 the nature and laws of those periodic movements outside of our organs of hear- 

 ing, designated as vibrations and waves — an embarrassment which recurs in a 

 different form with the popular exposition of each of our physiological divisions, 

 since physiology in all its parts presupposes a knowledge of physics and chem- 

 istry as its basis. We have before expressed our belief that, of all departments 

 of physics, acoustics is that respecting which some degree of information is most 

 widel}' diffused ; in declining, therefore, the long digression which any adequate 

 acoustical introduction would render necessary, and confining ourselves to the 

 physiological view of our subject, we shall hope to be excused if Ave refer any 

 reader who may happen to be little familiarized with it to some popular treatise 

 for the doctrine of sounds. For the sake of simplicity and clearness in our state- 

 ment, we shall substitute, in place of an analysis of all possible sources of sound, 

 .a single concrete example of a string vibrating under conditions which give rise 

 to the ixndulations whose destination in the organ of hearing it is our purpose to 

 follow. We take for this purpose a stretched string, supposed to be at a distance 

 ■of some feet from the ear and of such a degree of tension that, when made to 

 vibrate by a smart blow, it executes in a second four hundred entire vibrations — 

 that is to say, four hundred forward and as many backward movements, with a 

 •constantly diminishing magnitude of the path described by its several constituent 

 particles, until it returns to a state of rest. In such a string, the movement of 

 the individual particles lying behind one another in the direction of its length is 

 of very different magnitude ; those at either end, a l>, by which the string is 

 pj J fixed, are entirely motionless ; those next to them towards the middle 

 have a certain, but very small movement, being restrained by their 

 fixed neighbors, and the amount of movement increases as the middle 

 of the string is approached, until the maximum of movement is reached 

 in the particle c situated midway between a and h. If, therefore, we 

 set the string in vibration by pulling it by the middle as far as c' and 

 then releasing it, it will oscillate between the two limitary positions 

 indicated by the dotted lines a c' b and a c" h and its position of 

 equilibrium a c h, which it passes at each successive oscillation. The 

 curvature which the figure presents on both sides of the central line 

 is the necessary result of the differences in the movement of the par- 

 ticles as just described. With the continuance of the oscillations, the 

 number occurring in a second of time does not change, depending a,s 

 this number does only on the length and tension of the string ; but, 

 it is otherwise with the magnitude of the oscillations; as these pro- 

 ceed, the particle c deviates less and less from the position which it 

 occupied when at rest, the curvature becomes constantly weaker, and 

 the string finally forms a right line, as before the impulse. Did there 

 exist now no material medium between the string and our ear, were 

 this space wholly devoid of air, the vibrations of the string would 



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