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Mammals generally.

Ear-muscles of Monotremes. Concluding remarks.

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the platysma lying behind the ear. Further we note, that some
of the platysma-bundles going forward below the eye take their
origin from the mediad side of the ear near the hind margin.

Already the facts found in the Marsupials suggest that the
muscles of the outer ear may be derived from the platysma.
What we have seen in Echidna strongly supports this suggestion.
As far as the greater muscular complexes of the ear, the scutu-
laris-group and the auriculo-occipitalis-group, are concerned, it
may, from the facts adduced, be taken as granted that they are
originally parts of the platysma. Unfortunately the conditions
found in Echidna are so far remote from those of other Mammals
that the course of development which has taken place cannot
be demonstrated in detail at present.

8. CONCLUDING REMARKS.

As is well-known Gegenbaur and Ruge have very positively
advanced the view, that the totality of the facial muscles of the
Mammals should be ultimately derived from one primary muscle,
viz. the sphincter colli of the Amphibians and Reptilians, which
should have extended over the head and split up into numerous
muscles. This idea was pronounced for the first time some twenty
years ago by Gegenbaur, and later on further worked out by
Ruge in his excellent treatise of the facial muscles of the Prosi-
mise, and in other papers.

This theory decidedly is not a little engaging; and it is also
beyond dispute that to a large extent differentiations have taken
place, so that in many cases one muscle has been split up into

a number of muscles. But the facts at hand do not prove the
inference that this differentiation was of such a nature, that finally
the whole of the facial muscles were derived from the single muscle
of the Amphibians and Reptilians. That these muscles all are supplied
with branches from the nervus facialis, cannot settle the
question; the n. facialis supplies, besides the facial muscles, the
mm. stapedius, stylo-hyoideus and digastricus, which have nothing
to do with the facial muscles. It appears then quite possible,
that among the muscles termed facial muscles, there may be
some, which have also an origin independent of the rest. On the
whole, one nerve may, in many cases, supply organs which have
nothing in common.

Neither does the comparison furnish us with conclusive facts.
It is true, that in the lowest Mammals, the Monotremata, the facial
muscles are continuous to an extent that we do not find in
any other Mammals; but in no way can what is found in the
Monotremes be regarded as demonstrating the above-mentioned
theory: the system of facial muscles of the Monotremes is already
richly differentiated, and widely different from the supposed
simple starting point. Still less do the facial muscles in other
Mammals present conditions which could be regarded as supporting
the idea of such an origin. And it does not suffice to
say, that the development of the system of facial muscles is only
conceivable in this manner; at all events it were not beyond the
possibilities that various facial muscles had taken origin independently
of one another from indifferent mesoblastic elements in
connection with the development of the eye-lids, the lips, the
external ear etc.

Thus we must agree, that the question of the origin of the
facial muscles of Mammals is still an open one.