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4 6 B R A I N "AND o r g a n s S E N S E S IN FISHES. Al
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wards the lower end of the fiiliila, through which the animal refpires ; and, contrary
to what we obferve in man and quadrupeds, it gradually enlarges as it runs
back towards the cavity of the tympanum, in which it terminates (p).
While, therefore, thefe animals float on the furface of the ocean, impreiTion
is made on the fcveral parts of their ear in the fame manner as in man.
The remarkable difl'erence of the fize of the caverns which, in place of the
cells of our mailoid procefs, communicate with the cavity of the tympanum,
leads us to coniider, W^hether .the effect of the found upon the ear be increafed
b y that circumftance ? or, Whether the chief ufe of thefe caverns may be to
render the head fpecifically lighter, and, like fwimming-bladders, to make it
rife mort readily t o the furface of the fea ?
. As the entry into die external meatus auditorius is fo very fmall, there is
rcafon to fuppofe that the animal can fliut it when it dives to a confiderable
depth, and when the weight'of the water might have been in danger of injuring
the membrana tympani. And from obferving the fmallnefs and itructure of the
mouth of the external meatus auditorius in the whale, divers may perceive the
neceifity of plugging the ears accurately, in order to prevent the overilretching
of the membranes of the drums.
§ 2. Of the Ear in Amphib'ioia jinimals, and particularly in the Sea Tortoife or Tiu'llc.
BEFORE I proceed to give an account of the ftrudure of the car in the nantes
pinnati and pifces of Linnaeus, I ihall, in a few words, defcribe the ear in the
intermediate tribe of the amphibious animals, taking tlie fea tortoife or turtle as
my chief example of the clafs.
In this animal, as well as in the frog and moil others of the clafs, there is no
external ear nor meatus aii<iitojdus.j^tcrnus; but we find a large Euftachian tube
on the back part of the roof of the mouth," near to the articulation of the under
jaw with the upper. This tube has a winding courfe behind the condyle of the
upper jaw, and leads to a large cavity, rcfembling (y) our cavity of the tympanum
(r), covered by the flcin of the temple and a tough fubftance with a thin
cartilage on its inner fide; the whole of which taken together are nearly one
fixth part of an inch in thicknefs (j).
. T o thefe teguments a cartilaginous body, nearly of the fize of a fmall probe,
and upwards of tlu'ce quarters of an inch in length, is connccled; and paiTes,
nrlT: to the bottom of the tympanum, then through a bone, and through another
fmaller cavity, in which a watery humour is lodged; and at the bottom of
wliich the cartilaginous body is connected t a a membrane which fills a hole (i).
W i t h i n that membrane or hole a third cavity is found, which lodges tiiree
femi-
(/-) Stc Tab. XXXV. r.s, 4. O. ii£. 5- L M N.
(r) See Tab. XXXVL C-g. z. E. lig. j. B.
1/1 S« Tab. XXXVI. fig. 2. E. fij. 3. E F. (If. 4.
(7) i'ab. XXXVI. lig. I. Ii I K- Ig- i- C.
CO Tab. XXXVI. fiB- 2. A. fig. f A'
fcmicircular canals, to wit, arl anterior, a pofterior, and a middle horizontal
canal (?/), and alfo a fmall fac, which contains a foft cretaceous fubftance (ly).
The femicircular canals contain, and are furrounded by, a vifcid watery humour.
O n the membranes of thefe canals, and on the fac, nerves are difperfed (x).
When we comparc the parts in this animal with the human ear, the cartilaginous
body fcems to fupply the place of the fmall bones of our ear; and the
membrane to which the inner end of it is connedted feems analagous to the
membrane of our foramen ovale.
T h e fac containing the cretaceous matter, with the three femicircular canals
and nerves difperfed upon them, refemble the labyrinth of our ear.
§ 3« Of the Ear in the Cartilaginous and i r Eißes.
THE itrudure of the ear in tlie cartilaginous and oflcous fiflies has been fo
little examined, till of late, that, for upwards of two thoufand years, it has
been a queftion whether they poifeiTed organs appropriated for hearing.
Swammerdam {y) mentions a wonderful labyrinth of the ear in fiihes: but
moil anatomifts fince his time contented themfelves with pointing out, as the
organ of hearing, facs at the fides of the brain of the moil common fiflies containing
ilony fubilances, without pretending to fliow external paifages leading
to thefe facs, or the nerves or medium by which thefe facs were conneiled with
the brain of the animal.
About the beginning of the year 1779, the learned and accurate Dr Soemmering,
now profeiTor at CaiTcl, and who at that time did me the honour of attending
my courfe of leftures, told me, that the ingenious Dr Camper, in a
letter he had received, mentioned his having difcovcrcJ rcuudrcular canals in
the ears of fiflies. I therefore determined to look for thefe when I fliould come
to that part of my courie in which I treat of Comparative Anatomy: And accordingly
I found, and iliowed to the iludents, in tlie mOnth of April following,
femicircular canals both in the fkate and in the cod.
In the following month of May 1779, I traced the communication of the
fcveral canals of the ear with each other, and the diftribution of the nerves upon
t h e m ; and was alfo fo fortunat e as to difcover the entry into the external ear of
the ilcate, its concha, meatus auditorius externus, and its communication with
the interior canals: All which T have fince annually demonflrated to the iludents
in my courfe of leilures.
In the month of Augufl: following, I fliowed to the Philofophical Society of
this place the tables of thofe parts in the fkate and cod which I now publifh,
along with the tables which reprefent their circulating and abforbent veiTels.
Since
See Tab. XXXVI. fig. 3. G HI.
(r) Swammerdam, Bib!. Natur, p. 111.
(au) Tab. XXXVI. fig. 4- C. W Tab. XXXVI. lig. 4.D.