453 



EAR. 



EAR. 



45-1 



of the parts hitherto described, the ear is furnished with two addi- 

 tional provisions, both probably of great consequence to the perfection 

 of the organ. The first is a short meatus auditorius externus, or 

 outer passage, which removes the delicate membrane of the tympanum 

 to some depth from the surface of the head, and thus places it more 

 securely, and at the same time to greater advantage, for observing 

 the direction of sound. The other additional provision in birds is an 

 appendage to the mechanism of the internal ear. This is a small 

 conical cavity in the bone, somewhat curved, with a double spiral 

 ridge winding round the interior, and inclosing a cartilaginous struc- 

 ture so corresponding in form with the ridge as to divide the cavity 

 into two partitions. These communicate with another at the apex, 

 and with the vestibule and tympanum respectively, at their other 

 ends. The cavity is termed the Cochlea, from its resemblance to a 

 spiral shell ; the partition communicating with the internal ear is the 

 Scala (winding stair) of the vestibule ; the other is the Scala Tympani. 

 The opening from the latter into the tympanum is called the Foramen 

 Rotunduin ; it is closed by a membrane to exclude the air of that 

 cavity while it permits the transit of vibration to or from the vesti- 

 bular perilymph within ; for that fluid, passing up the cochlea by the 

 scala vestibuli, descends the scala tympani, and bathes the inner sur- 

 face of the membrane of the fenestra rotunda. The cartilaginous 

 Newel is kept in its place like the semicircular tubes by retiform fila- 

 ments, and is supplied with a separate branch of the acoustic nerve, 

 which ramifies and expands on its surface. The lapilli, which seem 

 to be chiefly a provision for hearing under water, and are therefore 

 large and solid in aquatic and amphibious animals, appear in birds 

 only as fine crystallised grains of chalk in the utricle or sinus of the 

 vestibule, rendering the endolymph somewhat turbid. The columella 

 is straight, and the membrana tympani pressed outwards by it is 

 consequently convex. There is a crescentic fold of skin extending 

 upwards from the superior margin of the meatus externus, sometimes 

 furnished, as in the horned owl, with a fringe of feathers which can 

 be spread at pleasure like a fan to catch the sound. This fold of 

 skin in a rudiment of the concha, or outer ear, of the Mammalia. 



As we have already said, it is only in this last-mentioned class of 

 animals that the ear reaches its complete development. It is nearly 

 the game in all of them ; the difference being only in the comparative 

 size and shape of the component parts of the organ, and not in their 

 essential structure, number, or arrangement. 



We shall therefore describe the organ in one species only. 



There is every reason to suppose that in hearing, as in seeing, man 

 baa no superiority over many of the lower animals except what arises 

 from that intellectual supremacy which enables him to discriminate 

 and compare his sensations more justly than they can do. Indeed it 

 is certain that in the mere perception of sounds he is inferior to most 

 of the Mammalia, and probably to birds ; and if the musical faculty 

 should seem to imply a greater perfection of the organ, the error, for 

 such we believe it to be, may perhaps disappear upon reflection. We 

 therefore select the human ear as the type of the organ in Mammalia, 

 not because it is in any respect more complete than the rest, but 

 as the most interesting. The same description, of the more important 

 parts at least, might be applied nearly word for word to all. 



The parts now to be described fall naturally under a three-fold 

 division into the internal, middle, and external ear. 



1. The Internal Ear, comprising the Acoustic Nerve, Vestibule, and 

 Labyrinth, is deeply placed in the interior of the head, within the 

 most compact and hardest of the bones, denominated from that 

 circumstance the petrous or rocky portion of the temporal bone. 

 This wedge-like or triangular projection passes obliquely inward and 

 forward in the direction of the outer tube of the ear, forming a 

 strongly-marked knobby ridge within the cranium, in the basis or 

 floor of that cavity. Near the inner point, which nearly meets its 

 fellow on the other side, and upon its posterior declivity, there is a 

 large trumpet-like hole (meatus auditorius internus) into which the 

 seventh cerebral nerve enters from the medulla oblongata. [BRAIN ; 

 NKRVK.] The meatus passes in a direction outwards, and therefore 

 obliquely, into the petrous portion for half an inch, and then termi- 

 nates abruptly in two fovea!, or pits : from the upper of these there 

 goeg a winding canal through the substance of the bone, which is the 

 cotyse of the motor nerve of the face (the portio dura of the seventh 

 pair), which, here separating from the auditory nerve, or portio 

 niollis, we need not follow. The latter, splitting into several sets of 

 filaments, finds its way through small sieve-like openings at the 

 bottom of the lower fovea into the internal ear, and is here distri- 

 buted in three separate portion} to the cochlea, the ampullae of the 

 semicircular tubes, and the utricle or vestibular sac. The cochlea is 

 more complicated than in birds ; it consists of a spiral canal iu the 

 bone, gradually diminishing as it ascends to a point, wound round a 

 central hollow pillar of bone called the Modiolus, or Newel. From 

 it inner surface, that namely which may be considered as a groove 

 In the modiolus, a thin and spongy lamella of bone projects rather 

 more than half across the canal, ascending in a similar spiral. From 

 the edge of this lamella (called the Lamina Spiralis) a membrane passes 

 to the outer surface of the canal, where it is attached ; thus com- 

 j>]<'ting the separation of the canal into two scalsc, or winding parti- 

 tions, which unite at the summit, and open (as before), the lower and 

 narrower into the vestibule, the superior and larger into the tympanum ; 



each scala taking two turns and a half round the modiolus in ascending 

 from the base of the cochlea to the Cupola, or inverted cup-shaped 

 cavity at the summit, placed over the funnel (Infvmdibulurn) into 

 which the top of the modiolus expands. The cochlea ia on a level 

 with the vestibule and anterior to it, the base being turned towards 

 the meatus internus ; the summit looking outwards and a little down- 

 wards, is turned towards the sudden bend of the wide canal in the 

 petrous portion of the temporal bone by which the internal carotid 

 artery enters the cavity of the head. It is the close neighbourhood 

 of this artery as it passes through the compact bone that occasions 

 the rushing sound of the pulse to be heard when the ear is placed 

 upon a pillow, or the attention is led to dwell upon what passes 

 within, by deafness arising from some cause not affecting the parts 

 essential to hearing. The modiolus is hollow to some distance from 

 the base. Up this tubular cavity rises the large cochlear branch of 

 the acoustic nerve, giving ofl' lateral filaments through mimite open- 

 ings arranged spirally, which pass through the light spongy bone, 

 and emerge from different points on the spiral floors and sides of the 

 scalse, where they ramify in a delicato pulpy expansion upon the 

 membranous tubes which line the spiral osseous canals : the rest of 

 the cochlear nerve passes through capillary perforations in the cul-de- 

 sac of the tubular cavity ; and ascending in the substance of the 

 central pillar of the modiolua, is distributed through the bone in a 

 similar way to the upper turns of the cochlea and the infundibulum. 

 The two other branches of the acoustic nerve are distributed to the 

 vestibular sac, which lies in a round depression or pit in the barrel- 

 shaped cavity of the vestibule, and to the ampullae of the semicircular 

 tubes. The latter all meet in a membranous sinus, or utricle, which 

 occupies another distinct pit of the vestibule, called from its shape 

 the Elliptic Fovea, much according to the arrangement already 

 described in other animals. The principal opening from the vestibule 

 is the fenestra ovalis, situated on the outer side towards the tympa- 

 num, which is closed by a membrane. At the lower and front part 

 there is another opening into the scala vestibuli of the cochlea. 

 There are five at its posterior aud outer side, which lead into the 

 semicircular canals, of which the superior and posterior enter the 

 vestibule by a common foramen. The sac and utricle each contain 

 a cretaceous deposit called Otocouia, or ear-sand, which in some of the 

 lower Mammalia has the consistence of soft chalk. The cochlea and 

 semicircular canals, from their complexity, are termed the Labyrinth. 

 With respect to the object of their peculiar arrangement, not even a 

 probable conjecture has been hazarded. Yet they appear with sur- 

 prising uniformity in all the Mammalia, and some of them, as we 

 have seen, in the more numerous tribes of birds, reptiles, and fishes. 

 The bony canals of the labyrinth and vestibule are stated to be 

 invested within by a delicate periosteum, the surface of which 

 towards the perilymph is thought to be of the nature of a serous 

 membrane, and to secrete that fluid. 



Fig. 1. 



Fig. 1 . Magnified view of the osseous labyrinth and vestibule as they would 

 appear if the solid bone in which they are imbedded were removed, -with the 

 ossicula auditus in situ : a, ampulla of the posterior semicircular canal ; b, 

 common tube by which this and the superior canal enter the vestibule ; c, pos- 

 terior canal ; d, external canal ; e, superior canal ; /, cochlea ; ,'/, its cupola ; 

 A, fcnestra ovalis covered by the stapes ; *, fenestra rotunda ; (, malleus ; 

 m, incus ; n, vestibule. 



The deafness which arises from causes which affect the feuestra 

 ovalis, or the nerves and canals within the vestibule and labyrinth, is 

 seldom or never cured ; and it is unfortunately very common. There 

 is a very easy way by which the nature of the case may be often 

 sufficiently tested. If the internal ear be affected, especially the nerves 

 of it, the ticking of a watch pressed against the teeth or the outer 

 part of the head on that side, will be very obscurely distinguished. 

 If not, the sound can be easily heard, as the solid bones interposed 

 between the sonorous body and the nerve are excellent conductors of 

 vibration. 



