[d] I have before, in [Book IV. Chap. 3. Note (u)], taken notice of what others have observed concerning the inner Ear of Birds, reserving my own Observations for this Place: Which I hope may be acceptable, not only for being some of them new, but also shewing the Mechanism of Hearing in general.
In this Organ of Birds, I shall take notice only of three Parts, the Membranes and Cartilages; the Columella; and the Conclave: The Drum, as some call it, or Membrana Tympani, as others, consists of two Membranes, the Outer, which covers the whole Meatus, Bason or Drum, (as some call it) and the inner Membrane. To support, distend and relax the outermost, there is one single Cartilage, reaching from the Side of the Meatus, to near the middle of the Membrane. On the top of the Columella is another Cartilage, consisting of three Branches, a.b.c. in [Fig. 23.] The longest middle Branch a. is joined to the top of the single upper Cartilage before spoken of, and assists it to bear up the upper outer Membrane: The two Branches, b.c. are joined to the Os Petrosum, at some distance from the outer Membrane: Upon this inner Cartilage, is the inner Membrane fixed, the two outer Sides of which, a.b. and a.c. are joined to the outer Membrane, and make a kind of three-square Bag. The Design of the two Branches or Legs of the Cartilage, b.c. are I conceive to keep the Cartilage and Columella from wavering side-ways, and to hinder them from flying too much back: There is a very fine slender Ligament extended from the opposite side, quite cross the Meatus or Bason, to the Bottom of the Cartilage, near its joining to the Columella. Thus much for the Membranæ Tympani, and their Cartilages.
The next Part is the Columella (as Schelhammer calls it.) This is a very fine, thin, light, bony Tube; the Bottom of which spreads about, and gives it the Resemblance of a wooden Pot-lid, such as I have seen in Country-Houses. It exactly shuts into, and covers a Foramen of the Conclave, to which it is braced all round, with a fine subtile Membrane, composed of the tender Auditory Nerve. This Bottom or Base of the Columella, I call the Operculum.
The last Part, which some call the Labyrinth and Cochlea, consisting of Branches more like the Canalos Semicirculares in Man, than the Cochlea, I call the Conclave Auditûs. It is (at in most other Animals) made of hard context Bone. In most of the Birds I have opened, there are circular Canals, some larger, some lesser, crossing one another at right Angles, which open into the Conclave. But in the Goose it is otherwise, there being cochleous Canals, but not like those of other Birds. In the Conclave, at the Side opposite to the Operculum, the tender Part of the Auditory Nerve enters, and lineth all those inner retired Parts, viz. the Conclave and Canals.
As to the Passages, Columnæ, and other Parts observable in the Ear of Birds, I shall pass them by, it being sufficient to my Purpose, to have described the Parts principally concerned in the Act of Hearing. And as the Ear is in Birds the most simple and incomplex of any Animals Ear; so we may from it make an easy and rational Judgment, how Hearing is performed, viz. Sound being a Tremor, or Undulation in the Air, caused by the Collision of Bodies, doth as it moves along, strike upon the Drum, or Membrana Tympani of the Ear: Which Motion, whether strong or languid, shrill or soft, tuneful or not, is at the same Instant impressed upon the Cartilages, Columella, and Operculum, and so communicated to the Auditory Nerve in the Conclave.
And now if we compare the Organ and Act of Hearing, with those of Sight, we shall find, that the Conclave is to Hearing, as the Retina is to Sight; that sonorous Bodies make their Impressions thereby on the Brain, as visible Objects do by the Retina. Also, that as there is an Apparatus in the Eye, by the opening and shutting of the Pupil, to make it correspond to all the Degrees of Light, so there is in the Ear to make it conformable to all the Degrees of Sound, a noble Train of little Bones and Muscles in Man, &c. to strain and relax the Membrane, and at the same Time to open and shut the Basis of the Stapes (the same as what I call the Operculum in Birds:) But in Birds, there is a more simple, but sufficient Apparatus for this Purpose, tender Cartilages, instead of Bones and Joints, to correspond to the various Impressions of Sounds, and to open and shut the Operculum. Besides which, I suspect the Ligament I mentioned, is only the Tendon of a Muscle, reaching to the inner Membrana Tympani, and joined thereto (as I find by a stricter Scrutiny) and not to the Cartilage, as I imagined. By this Muscle, the inner Membrane, and by Means of that the Outer also can be distended or relaxed, as it is in Man, by the Malleus and its Muscle, &c.
[e] Flat-billed Birds, that grope for their Meat, have three Pair of Nerves, that come into their Bills, whereby they have that Accuracy to distinguish what is proper for Food, and what to be rejected by their Taste, when they do not see it. This was most evident in a Duck’s Bill and Head; a Duck having larger Nerves that come into their Bills than Geese, or any other Bird that I have seen; and therefore quaffer and grope out their Meat the most. But then I discovered none of these Nerves in round-bill’d Birds. But since, in my Anatomies in the Country, in a Rook, I first observed two Nerves that came down betwixt the Eyes into the upper Bill, but considerably smaller than any of the three Pair of Nerves, in the Bills of Ducks, but larger than the Nerves in any other round-bill’d Birds. And ’tis remarkable that these Birds, more than any other round-bill’d Birds, seem to grope for their Meat in Cow-dung, &c. Mr. J. Clayton, in Philos. Transact. Nᵒ. 206.
I observ’d three Pair of Nerves in all the broad-bill’d Birds that I could meet with, and in all such at feel for their Food out of Sight, as Snipes, Woodcocks, Curlews, Geese, Ducks, Teals, Widgeons, &c. These Nerves are very large, equalling almost the Optic Nerve in Thickness.——Two are distributed nigh the End of the upper Bill, and are there very much expanded, passing through the Bone into the Membrane, lining the Roof of the Mouth. Dr. A. Moulen. Ibid. Nᵒ. 199. Or both in Mr. Lowthorp’s Abridg. V. 2. p. 861, 862.
[f] The Gizzard is not only made very strong, especially in the Granivorous; but hath also a Faculty of Grinding what is therein. For which Purpose, the Bird swalloweth rough Stones down, which, when grown smooth, are rejected and cast out of the Stomach, as useless. This Grinding may be heard in Falcons, Eagles, &c. by laying the Ear close to them, when their Stomachs are empty, as the famous Dr. Harvey saith. De Generat. Exer. 7.
As to the Strength of the Gizzard, and the Use of Stones to the Digestion of Fowls, divers curious Experiments may be met with, try’d by Seigneur Redi, with glass Bubbles, solid Glass, Diamonds, and other hard Bodies. See his Exp. Nat.