Taking the human ear as representing that of all the higher vertebrates and as an elab oration of that of lower animals, we find an acoustic nerve emerging from each side of the pons or lower back of the brain, to pass, in close association with the nerve giving motion to the face, into the stony-hard innermost part of the temporal bone which encloses the ear. The facial nerve passes on through and prac tically has but accidental relation to the ear, but may be paralyzed by involvement in tympanic disease. The acoustic nerve, containing nerves for the equilibrium-organ, divides and is dis trihuted to the various parts of the labyrinth. At each point of its distribution cells with hair-like projections form end-organs through which impressions are received to be conveyed to the brain. Three of these points are in the three pear-shaped swellings of the semi-circular canals, each of which is at right angles to the others, so that motion of the head in whatever direction can act upon the fluid in one or more of these canals and thus be analyzed and re ported to the equilibrium center in the brain. Two are in the two vestibular sacs, and the most elaborate is where the real acoustic nerve enters the base of the snail-shell (Fig. 8), and is distributed to the highly elaborated organ of Corti, here located. This consists of a fairly orderly arrangement of hair-cells without otoliths, supported by curiously modified cells of like origin (Fig. 7), all resting upon a deli cate °basilar membrane° narrowest below and broadest above, which winds spirally from the bottom to the top of the shell-like cochlea, and is formed of parallel transversely stretched fibres. Here we have an apparatus comparable to the strings of a harp or piano; and just as each string of a musical instrument can vibrate in accord with those attuned to it in another and can even repeat the tones of an inaudibly distant instrument with which it is connected by a wire; so there is much to support Helm holtz's claim that each fibre of the basilar membrane is a cord tuned to a certain pitch and vibrating responsive to any tone of that exact pitch which reaches it. This serves to excite a special hair-cell resting upon it and send a nerve-impulse along the ultimate nerve fibril of which this forms the terminus. Cer tainly the apparatus in form and elaboration meets most of the requirements of such a theory and its partial destruction by disease has caused loss of hearing for certain tones—low tones if the apex, high if the base, be injured.
Outside of the labyrinth or percipient ap paratus we find a conducting apparatus of ex ternal and middle ear (Fig. 9). The outer ear is formed by the in-growth of a pit of the skin-surface to constitute an external auditory canal, around the edge of which gristly projec tions raise the skin-covering into the prominent but unimportant feature to which the name °ear° is commonly applied. The inward growth of the canal brings its bottom into close relation with the middle ear, so that only a thin parti tion, °the drumhead,° intervenes. The middle ear is an outward development of the mouth cavity, which hollows out the tissues between the external and internal ears— forming the Eustachian tube as its inner portion, the drum cavity or tympanum just within the drumhead at the bottoni of the external canal, and other air-spaces, °mastoid cells," of less constant pres ence and form and of doubtful value, but notable because of serious disease-conditions to which they are subject. Two °windows° make
communication between the middle and internal ear, both closed by membranes shutting in the fluids of the labyrinth — that closing the lower around windows being called the secondary tympanic-membrane. The upper or °oval window° is occupied by the foot-plate of the IPtirrup-bone, which with the little hammer and anvil forms (Fig. 11) a compound lever be tween the drumhead and the labyrinth-fluids. This serves to increase the force while lessen ing the amplitude of the vibrations of the drum head and thus aids the transmission of waves of sound, especially of low tones, from the air to the internal fluids. The higher animals move the external ears as an aid in locating sounds, but the muscles effecting this are rudimentary in man and the external ear can be lost with little recognizable impairment of hearing. More important, although of ill-determined working, are the muscles moving the little bones of the drum-cavity; one, the drumhead tensor, draw ing in the hammer-handle and thus tightening the drum-membrane to which it is attached, and one acting on the stirrup. Even the drumhead and chain of little ear-bones can be actually or virtually destroyed with preservation of a large part of the hearing; but their presence in dam aged condition may be a serious obstacle to use ful function by impeding sound-conduction.
The upper and lower limits of human hear ing are somewhat in dispute, but it is fairly safe to say that tones of 32 double vibrations per second and up through 10 octaves, should be au dible to a healthy ear. If a cog-wheel touching a card or other elastic plate be turned with in creasing speed the individual strokes or vibra tions can be distinguished up to 16 per second; then they blend and a continuous tone is pro duced; hut the tone heard may he an overtone end not that of 16 per second as is often claimed. Through the thin young drumhead or one having an opening in it, still higher tones can be heard, possibly up to 55,000 vibra tions; but age and disease easily effect changes which bar out these high and relatively weak tones: yet it is striking to note how well they may be heard by those deaf to ordinary speech. Those unable to enter general conversation by reason of impairment of the conducting appa ratus may often hear the same voice readily over the telephone, which transposes it to a higher key; and some much-advertised hearing helps have their value as portable telephone lines: Human speech, the hearing of which is in civilized life the most important use of the ear, has a range from near the lowest limit of perception in the sound of R to some 4,000 vibrations per second in the sibilants S and X. We have in speech, therefore, with its easy gradations of intensity from faint whispers to loud shouts, a ready means of testing the hear ing and measuring its defects. Variable as are voices or even the same voice within its range, it constitutes our best practical test of the de gree of deafness. Generally the faintest articu late whisper should he heard at arm's length and a loud "stage whisper" 50 feet or more.