EAR 



EAR 



45* 



oooipieuotu, appearing externally in the form of round white point* 

 on the head, s little nearer the middle line, and somewhat higher than 

 the ba*e of the long outer antenna!. In the lobetor they are eon 

 tained in a small nipple-like prominence or papilla upon the undei 

 part of the moreabfe baw of the nkn^ looking downwards am 

 forward*. This papilla consists of a substance harder and more 

 brittle and probably more vibratile than the rest of the shell 



The Jfotfwco, though placed higher in the scale of animals, by 

 Cuvier, do not afford so many examples of animals possessing a 

 distinct organ of hearing as the Articulata. Such as have been 

 discovered all belong to the order of the Cephalopoda with two 

 branchiic, or gills, which approach more nearly to the true fishes in 

 their structure than the other mollusks. 



In the Sfpia, or Cuttle-Fish, which belongs to this order, anc 

 which may be taken as a type of the rest, there is a protuberance 

 under the elastic gristly integument at the back part of the heac 

 which contains the ear. It consists of a pair of symmetrical vestibules 

 each containing an oval sac filled and surrounded with fluid. On the 

 interior surface of this sac the acoustic nerve is expanded in the 

 form of a white mucous pulp. The sac is supported in the perilymph 

 not only by an adhesion to the inner side of the parietes of the 

 vestibule at the entrance of the nerve, but also by a fine net-work 

 of fibrils which pass from its outer surface to numerous prominent 

 points on the inner surface of the vestibule. There is no fenestra 

 ovalia, or membrane, as in the lobster and the air-breathing insect* 

 but the sac contains a small loose bony or chalky concretion, calloc 

 an otolithe (off Arot, the ear, and AiOot, a stone), which answers the 

 name purpose, namely, to indicate the degree and direction of sound 

 for juit as we estimate a weight by poising it in the hand, or, if it 

 be suspended, by gently pushing it from us thus measuring in our 

 minds the muscular tension necessary to support it, or the force 

 required to overcome its inertia, and conscious of the direction in 

 which we exert our muscles so, conversely (the weight and inertia 

 of the lapillus always remaining the same), the degree and direction 

 of a vibratory force affecting it from without through the medium <>l 

 the integuments, the parietes of the vestibule, and the fluids within, 

 may be estimated by a consciousness on the part of the animal o! 

 the nature of the stress on the sensitive membranes and fibrils which 

 support it, which by their elasticity restrain and redress the slight 

 movements impressed upon it. This should be borne in mind ; for, 

 as we shall see further on, it is in some degree by the exertion of the 

 muscular sense, as Sir Charles Bell has called that by which we judge 

 of weight and tension, that the human ear is enabled to estimate 

 the intensity of sound. Other curious particulars as to the function 

 of otolithes might be enlarged upon ; but we have said enough to 

 explain, as we think, the most important of them ; and to correct 

 the misstatementa of authors who tell us that they are intended to 

 increase the intensity of the vibrations of sound : they appear to us 

 rather calculated to diminish it, as the board floating in the bucket 

 of the water-carrier tends to pevent the fluid from dashing over the 

 side. They undoubtedly play an important part in the organ of 

 hearing, especially in the larger fishes, where they are more numerous, 

 and attain a considerable size ; but it is difficult to conceive that they 

 are possessed of any intensative power. 



The vertebrated classes of the animal kingdom, comprising the true 

 fuhes, reptiles, birds, and the mammals, are all provided with acoustic 

 organ, which are very various in their degrees of complexity, but 

 much exceed in that respect the comparatively simple organs of the 

 inferior divisions. 



In the Cartilaginous Fishes, mich as the ray and the shark, the vesti- 

 bule is deeply imbedded in the elastic walls of the back part of the 

 cranium, near iU junction with the spine. The fenestra ovalis, closed 

 by a tense transparent membrane, faces upwards, backwards, and 

 towards the middle line. The membrane is placed obliquely at the 

 bottom of a more superficial flattened tubular cavity, which termi- 

 nates beneath the integument in a kind of forked extremity, and may 

 be oonsidered as a rudiment of the tympanum, or middle ear, .(' tlT.- 

 higher Vtrltbrala. with its Eustachian tube. The inner surface of the 

 membrane is turned towards three sacculi, one of which is much 

 tager than the rest, arranged at the opposite side of the cavity of the 

 vertibule, and containing eash an otolithe. The sacs are filled with a 

 thick gelatinous endolymph, which adheres to the lapilli, and serves, 

 with minute filaments, such u those in the Sepia, to stead v 

 The vestibule.is filled with a limpid aqueous ],. rilyniph, traversed in 

 all directions by a fine cellular network, by means of which its con- 

 tent* are supported in their relative situations. Besides the fenestra 

 ovalia, other perforations lead out of the vestibule into three arched 

 cylindrical canals of considerable diameter and dimensions, the diverg- 

 ing curves of which take a wide circuit within the cranial cartilage, 

 and terminate at both ends in this central cavity. These passages, 

 from thrir situation and form, are called the anterior, posterior, and 

 noriiontal semicircular canals. Within the canals, In which the 

 vwtibiiUr perilymph freely circulates, there are three similarly curved 

 more slender membranous elastic tubes: they are nowhere in 

 Wrtact with the sides of the canals, but are suspended in the mi. 1st 

 them by means of the cellular net-work above u,. nii..i,. ,1. They 

 1 swell out at one end like a flask (ampulla) as they enter the vesti- 

 ule, after which the anterior and horizontal tubes separately enter 



a common pouch or sinus ; into this their other ends likewise open 

 by a conduit common to both. The posterior tube, which is the 

 largest and longest, after forming its ampulla resumes its t 

 calibre, and passing along the floor of the vestibule under the largest 

 sac, to which it is connected by the network, returns into itaelf, thus 

 completing a separate circuit 



The fluid contents of the several membranous cavities do not com- 

 municate with each other or with the vestibular perilymph ; though, 

 as they lie in close apposition, their vibrations ore mutually inter- 

 changeable. 



The acoustic nerve U distributed in two principal branches only to 

 the sacs and the ampulla' ; chiefly to the latter, to which it gives a 

 white colour. The filaments form a fine network on the outside of 

 the ampulln?, and then, piercing their parietes, are raised up within 

 into a kind of crescentic screen, in order probably that they may be 

 more exposed to the impulse of the vibrations descending along the 

 aqueous cudolymph of the semicircular tubes. All the parts we have 

 described are transparent except the opaque ampulla} and the solid 

 cretaceous otolithes. We have been particular in our account of these 

 membranous parts, which are found with little essential variation in 

 all the superior animals, man included, because in the cartilaginous 

 fishes they admit of more easy examination from their great size and 

 firmer texture, and from the softness of the cartilage that incloses 

 them. In man and the mammals they are not only much smaller 

 and more delicate, but incased iu the hardest bone in the body, from 

 which it is almost impossible to separate them with sufficient accuracy 

 to be certain that the description is correct. 



In some cartilaginous fishes, as the sturgeon, the fenestra ovalis is 

 not closed by a membrane, but by a round button-like piece of semi- 

 transparent cartilage called an operculum, or lid. 



The parts are similar in the osseous fishes, except that they have 

 generally no fenestra ovalis. 



In Serpents there is but one sacculus containing chalky matter, 

 and all the semicircular tubes communicate with a central mem- 

 branous sinus, which the anterior and posterior tubes enter by a 

 common trunk. The fenestra ovalis is closed, not as in fishes by a 

 membrane, but by the expanded trumpet-shaped extremity of a 

 slender bone (ossiculum or columella) attached at the other extremity 

 by a ligament to the outer end of the intermaxillary bone. 



Nearly the same arrangement of the internal ear prevails in the 

 Four-Footed Reptiles (turtle, crocodile, frog, lizard) ; but a new and 

 important step is here made towards the ultimate perfection of the 

 organ by the development of an air-cavity, called the tympanum or 

 ear-drum, between the vestibule and the surface of the head. This 

 mlilitioii, which as we said first becomes more than a mere rudiment 

 in the four-footed reptiles, permits the vestibule to be placed with 

 equal advantage at a comparatively greater depth, and therefore in 

 greater security ; but it has more important uses in rendering the 

 sound more clear, and facilitating in several ways (to be presently 

 explained) its communication to the auditory nerve. Like the musical 

 instrument from which it takes its name, the tympanum is provided 

 with a membrane tightly stretched upon the margin of a round open- 

 ing in the outer part of its bony or cartilaginous wall ; and has an 

 open vent or passage called after the anatomist who discovered it the 

 Eustachian Tube, leading forwards from the cavity to the throat or 

 back part of the nostrils, by means of which the air within it is 

 adjusted to the variable state of the atmospheric pressure without. 

 If the animal be amphibious, as many of the four-footed reptiles are, 

 the membrana tympani is still covered entirely by integument ; some- 

 times, as in the crocodile, by a moveable flnp of the scaly hard skin, 

 which can be raised up when the animal is out of the water. More 

 frequently however the membrane lies entirely beneath the skin, here 

 thinner than elsewhere on the head, as in the tortoise. The Lacerta 

 agilit, or Basking Lizard, alone, which lives entirely on the land, has 

 the membrane naked to the air. In this class of animals the columella 

 is not directed forwards to the angle of the jaw as in serpents, but is 

 attached by a cartilaginous extremity to the centre of the membrana 

 tympani, and thus conveys the collected effect of its \ -ibnit i 

 to the fenestra ovalis : the effect of this arrangement in n-iulering the 

 Impression of sound more definite must be obvious. In some K| 

 the cartilaginous portion of the columella is joined to the bony port ,i< m 

 at an acute angle, like the letter V, which adds an elasticity to the 

 mechanism very serviceable as a protection to the delicate parts within 

 Lhe fenestra ovalis from the injury they might otherwise sustain by a 

 alow or undue pressure upon the membrana tympani. This is the 

 case with the lizard mentioned above, in which there is also a rudi- 

 ment of the muscle which serves in the higher animals to tighten the 

 membrane ; a circumstance which makes this elbow in the columella 

 a still more essential provision against sudden changes in the distance 

 Between the centre of the membrane and the fenestra ovalis. It is 

 worthy of remark that in one class of serpent*, the Cactiia (Illincl- 

 Worms), the ear is as complete as in any of the four footed terrestrial 

 ; possessing a tympanum with its membranes, a Kustachian 

 tube, and a columella bent to an angle. Thi* departure from the 

 unual rule in nerpenta appears to be one of those compensations so 

 -. in. -I witli in the animal kingdom, the organ of sight in the 



loped. 

 In Birds, besides a greater nicety and tenuity in the conformation 



