margin is thin. Two strong tendinous retractor muscles, two inches in length, are
prolonged forwards and upwards from the anterior margin of the posterior nares along
the floor of the sac and appear to he attached to the posterior palatine process of
the pterygoids. They are separated by an interval anteriorly, but unite posteriorly
and surround the margin of the orifice like a sphincter. The whole surface of this
sac, from the orifice forwards to the narial septum, is richly covered with the orifices
of small mucous glands (PL XXVIII, fig. 3). The surface also of the sac-like
portion is thrown into strong tendinous bands or folds, resembling chorda tendinece.
In all the specimens I have examined there are indications, more or less distinct, on
the lateral walls of the sac of nearly longitudinal tendinous bands separated from
each other by intervals varying from 0-25 inch to nearly an inch in extent, these
interspaces being divided into long arched crypts varying in size and number and
disposed more or less vertically. In one of these crypts the Eustachian tube (Eig. 3,
eus) seems to open.
Eustachian Tube.—The opening to this tube (eu2) appears, as I have just
said, to be situated in the posterior nares,—at least the only orifice communicating
with the canal that I have been able to perceive is situated in that locality. The
opening is placed on the lateral wall of the nares, 0'75 of an inch above the narial
rima in an adult in which I had no difficulty in detecting the orifices and to the
position of which I had been led by what I had observed in a semi-adult individual.
I must state, however, that it was a matter of considerable difficulty to
find the orifice in the last-mentioned specimen, but that its presence in the adult
and in the position indicated was easily demonstrated. In looking into the post-
xiarial pharyngeal sac or dilatation of the Eustachian tube in the semi-adult, a patulous
diverticulum from the external wall of the sac, near its anterior end, was observed
to pass outwards in the direction of the wall of the posterior nares: but on attempting
to pass a probe along this, its progress was impeded by valvular folds of the walls
of the passage. On directing my attention, however, to the external wall of the posterior
nares I found that the probe passed along one of the crypts (eu2) and ran
upwards, and on introducing the probe again from the sac I found it appearing on
the posterior nares through the crypt in question. In the adult I passed no probes,
but observing an oval orifice in about the same position as the crypt, I slit open its
upper margin and found it to constitute a canal (d) which ran upwards and was
about half an inch in length and a quarter in breadth, and, at the former distance
from the opening, the outer wall of the passage abruptly stopped as a crescentic
fold, the inner or narial wall being prolonged upwards for 0-40 inch beyond it, so that
an elongated slit occurred in this position opening into a. considerable cavity.
This was found to result from the union of the Eustachian tube with its long
posterior prolongation or guttural pouch, thus forming a cavity common to
both, and which sends up valvular passages around the sac of the Eustachian
orifice which is placed opposite to the end of its tube (PL XXVIII, fig. 3). The
postnarial end (pn) of the Eustachian tube and the commencement of the guttural
pouch are each protected by thin folds of membrane which must act more or less
as valves. The Eustachian tube, in the adult, at its termination has a diameter of
040 inch, and the guttural pouch is half an inch broad at its beginning. Erom the
crescentic fold the direction of the guttural pouch is downwards, backwards, and
inwards, and the postnarial passage lies on the commencement of the outer wall of
the pouch, the two lying back to back; hence the difficulty of passing a probe
from the guttural pouch into the posterior nares, for, when it reaches the orifice
above the crescentic fold, it meets with the outer wall of the external passage, and
a similar difficulty is encountered in tracing the Eustachian canal from its auditory
end.
As these prolongations of the Eustachian tube are undoubtedly air receptacles,
and in all probability subservient to audition, as seems to be the function of their
homologues in the horse, we have an explanation of the position, direction, and
valvular character of the external orifice in an animal which has to breathe by
rising to the surface of water. .
When the Eustachian tube reaches the outer wall of the posterior nares, it
dilates into a tube of considerable capacity, being half an inch in diameter (in
an individual 4J feet long) at the point where it opens into the posterior nares,
and immediately behind this, it suddenly expands into a large sac, lying internal
to the stylohyoid, to which its outer wall is attached. Turning round that bone
f r o m behind forwards, this sac lies between it and the thyrohyal, and its internal
wall lies against the outer wall of the back of the pharynx, while the roof of
the sac lies below the exoccipital and basi-occipital bones. In the same individual,
it has a length from before backwards of 1*75 inch, and a vertical height of
1-25 inch. The inner surface is white smooth glistening and tendinous in appearance,
and its wall has numerous deep recesses of various dimensions formed by
arching folds of the membrane constituting the walls of the sac; while other parts
of it are covered by a tendinous meshwork, defining shallow crypts, of the same
structure as those found on the walls of the posterior nares. ,
Some of the recesses lead into small secondary pouches, and from these into a
labyrinth of smaller passages. A rather wide orifice, lying at the posterior margin
of the stylohyoid, leads into a pouch divided into crypts and little pockets by valvular
folds which turn round the outer margin of that bone, and lie along the cranial
end of the thyrohyal. About the middle of the great sac from which this little pouch
is given off, a strong valvular fold arches across it from the external to the internal
walls, and serves to close a wide orifice leading into a second great sac hereafter to
be considered. Behind this valve, and at the bottom of the sac, there are seven
¡Bma.ll orifices arranged in linear series, and each separated from its fellow by a pillar-
like fold; and immediately above these occurs the opening leading into the secondary
sac just described. These orifices, which, however, vary in number in
different individuals, when traced out are found to lead into a third secondary sac,
which passes downwards behind the thyrohyal. This orifice leading from the
strong valvular fold just mentioned opens into the secondary sac, which is divided into
compartments by large valves, and communicating with the sac behind the thyro