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A.

THE FACIAL MUSCLES

OF

MAMMALS GENERALLY.

In the history concerning our knowledge of the facial muscles
of Mammalia, the works of Georg Ruge mark a new era,
their starting-point being an actually and designedly comparative
one: the comparative aim being pursued with all consistency:
and coupled with the greatest care in investigation. Unfortunately
the papers of Ruge only deal with Prosimise and Primates,
which groups he worked out first; and the Monotremata, which
he dealt with later on. Of the other orders of Mammalia no
such investigations of the facial muscles are on record. Even in
the domesticated mammals, whose anatomy — next to that of
man — has been dealt with in the most thorough manner, the
facial muscles have not been treated in this way. The papers of
Ruge as we have said only deal with a few groups af Mammals;
and these groups being very distant from the Elephant, we have
found it necessary to work out, on similar lines, the facial
muscles of a series of Mammals of various groups in order to
form a somewhat broader basis, from which we might estimate
the facts which we found in the Elephant. The more comprehensive
material thus at our command — viz. the investigations of
Ruge and those of ourselves — have naturally led us to results,
in some points differing from Ruge1), which differences will be
evident from what is communicated in the sequel.

The Mammals, whose facial muscles in a more or less complete
manner are examined by us, are the following: Echidna,
Ornithorhynchus, Halmaturus, Didelphys, Centetes, Erinaceus,

x) F'or the sake of comparison with the results arrived at through our investigations
and exposed in the sequel, we summarize here the principal points of the
view of the facial muscles set forth by Ruge in his paper: Uber die Gesichts-
muskulatur der Halbaffen, in: Morphol. Jahrb. 11. Bd.:

The whole of the facial muscles is differentiated from two muscles, both origi
nating from the neck, viz.

I) a superficial layer, platysma myoides.

II) a deep layer, sphincter colli.

From the platysma Ruge derives the following muscles:

1) M. auriculo-occipitalis — m. auriculo-occipitalis nob., from which again a deep
layer is split off: M. auricularis post. — m. postauricularis nob.

2) M. auriculo-labialis inf. — m. adductor infer, nob. From this is derived the
m. trago-antitragicus = m. posted septimi nob.

3) M. mentalis = m. mentalis nob.

4) M. auriculo-labialis sup. — m. postorbicularis nob., from which the following
muscles are derived:

a) M. orbicularis oculi == m. orbicularis oculi nob., from which the m. levator
labii (= m. nasolabials nob.) originates.

Talpa, Myogale, Lepus, Myopotamus, Canis, Nasua, Alces, Cervus
canadensis, C. Aristotelis, Bos, Camelus, Sus, Tapirus, Equus, Lemur
, Nycticebus. The types, whose names are italicized, are those
which we have examined in the most complete manner.

A general survey of the facial muscles of Mammals may be
made in grouping them in the following manner.

1. PLATYSMA-SPHIIMCTER-CROUP.

On the ventral and lateral hinder part of the head (the region
below the outer ear and further forward) and on the neighbouring
parts of the neck is developed a three-layered complex
of muscles, consisting of a superficial, transverse layer, which
we designate the sphincter superficialis, an intermediary, longitudinal
layer, the platysma, and a deep, transverse layer, the
sphincter profundus. These three muscle-plates are generally all
present, but often the sphincter superficialis is restricted to the
neck alone, not reaching the head.

The sphincter superficialis (PI. 17, fig. 1) is in many forms
a true sphincter, the right and left part of the muscle running
ventrally into each other. It is most powerfully developed in
Echidna (PI. 10, fig. 1, 2) and Ornithorhynchus, where it forms a
continuous muscular plate underneath the neck and the head,
proceeding unto a line from the outer ear to the angle of the
mouth. Similarly, in Centetes (fig. B) it is a very strong muscle,

b) M. depressor helicis = m. scutularis, ventral portion, nob.

c) M. helicis = m. anteri sexti nob.

d) M. orbito-auricularis = m. scutularis nob., from which again are derived
the m. auricularis sup. (= pars transiens nob.).

From the sphincter colli (= m. sphincter profundus nob.) Ruge derives the following
muscles:

1) M. depressor tarsi = portio palpebralis m. sphinct. profundi nob.

2) M. orbicularis oris = pars rimana m. buccinatorii nob.
From the last are derived:

a) M. caninus = part of pars rimana nob., inserting into the maxillary bone.

b) M. nasalis = m. nasalis nob.

c) M. maxillolabialis = m. maxillolabialis nob.

d) M. buccinatorius — m. buccinatorius nob.

Finally Ruge enters the in. mandibulo-auricularis (~ m. temporo-auricularis nob.)
as a muscle independent of the system of facial muscles; his motive of this opinion
is that he has not been able to find a branch of the n. facialis to the muscle,
which he is of opinion is innervated by a branch of n. trigeminus and therefore
should be regarded as standing in nearer relation to the masticatory muscles.