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9 Mammals generally.

one of which appears as the principal platysma. The platysma
may be a pure cutaneous muscle, but sometimes parts of it are
connected with certain of the underlying bones.

In Echidna (PI. 10) the muscle is more powerfully developed
than in any other Mammal; extending not only under the ear
and the eye, but also above the ear, where it meets the corresponding
muscle of the other side, and posteriorly being continued into
the cutaneous muscle of the back of the trunk. The part of the
muscle lying above and behind the external ear is very thick
and studded with the deep follicles of the spines of that part of
the body. Most of the bundles have the usual, longitudinal direction
, but in the part of the muscle which is placed before
the auricle the bundles have a more oblique direction. The post-
auricular part of the muscle is continued below the ear into a
rather narrow band, which extends forward and is inserted on
the superior maxilla below the eye. The preauricular portion
of the muscle extends below the eye and is here, ventrally, intimately
connected with the postauricular portion and also inserted
on the superior maxilla; dorsally and in front, it is intimately
connected with, or rather continued into the orbicularis oculi
(comp. below).

In Ornithorhynchus also the platysma forms a large muscular
plate extending from the fore-limb to the angle of the mouth,
but the greater part of the muscle here lies below the ear-opening
. The lower parts of the muscle end in the skin ventrally,
the upper parts are inserted on the maxillaiw bone. The part of
the muscle lying between the eye and the ear (which two are
only separated by a very short distance) is very small.

In Halmaturus (PI. 13, fig. 1) the platysma is a continuous,
strong, muscular plate; extending from the neck to the angle of
the mouth, reaching dorsally very nearly to the external ear.
In Didelphys (fig. A), Centetes (fig. B) and the Dog (PL 12) the
muscle is similarly developed as a strong muscular plate, extending
from the neck, below the ear, to the angle of the mouth. —
In Myopotamus, in which the platysma is still a continuous muscle
, the muscular fibres only extend a short way upon the head
(on the hind part of masseter), the front part of the muscle being
represented by an aponeurosis extending further upon the face;
the hind part of the muscle, lying on the neck, forms a rather
strong muscular plate.

In the Ungulata (PI. 5, 7, 9) the dorsal part of the platysma
has generally been separated from the major part of the muscle
as a fleshy band, which takes its origin from the zygoma, and
runs to the angle of the mouth. We term it the pars zygomatica
platysmatis (muse, zygomaticus autt.). In Alces the pars zygomatica
and the rest of the platysma, pars principalis, at the fore-end,
near the angle of the mouth, to the extent of a couple of cm,
are closely united, so that it is impossible to point out any limit
between them; the fibres of the pars principalis, which is a
rather narrow band on the lower part of the side of the head,
have a longitudinal direction. In all other Ungulata examined by
us the pars zygomatica and the pars principalis are separated
throughout their whole extent. In the Camel (PL 5) the fibres of
the pars zygomatica have the same direction as in the pars principalis
(which is a broad band of longitudinal fibres) and the
front ends of both are inserted into the buccinatorius. In both
the hind end of the muscle is represented by an aponeurosis, that
of the pars zygomatica connected with the zygoma, and that of
the pars principalis extending from the angulus mandibular round
the neck. But in most Ungulata the direction of the fibres in the
two parts is not quite the same and they may cross each other
under obtuse angles (Tapir, PL 7, etc.) and behave as muscles
which have nothing to do with each other. In Sus (PL 9) the
upper part of the pars principalis spreads like a fan on the face,
below and behind the eye: and this feature is still more marked
in some of the Ruminants; for instance Bos, where this part of
the muscle is dissolved into a number of disconnected thin and
narrow muscular bands spreading over the face.

In the Primates there is, as is well known, a muscle very
similar to that which we have termed the pars zygomatica platysmatis
in Ungulata. This muscle — musculus zygomaticus —

Platysma-sphincter- and Orbicularis-oculi-group. 10

has, according to Ruge1), been derived from the m. auriculo-
labialis superior of the Prosimise, and we think he is quite right
in this. The m. auriculo-labialis sup. of the Prosimise is our m.
postorbicularis. As we must derive our pars zygomatica in the
Ungulata from the platysma, it is probable that it has been
developed from other elements than the similar muscle in Primates
. As we shall see later on, some muscles which in various
other Mammals have been termed »musculus zygomaticus« have
also an independent origin.

2. ORBICULARIS-OCULI-GROUP.

The group of muscles, which we term the orbicularis-oculi-
group (P. 17, fig. 1; PL 16, fig. 20—27) consists firstly of the well-
known muscle surrounding the opening between the eye-lids:
secondly of several other muscles, which evidently are derived
from the orbicularis. The orbicularis itself has, as is also insisted
on by Ruge, probably been derived from the platysma, to the
dorsal margin of which its ventral part fits in in such a manner
in Echidna, that it has quite the appearance of their being of
one set.

As to the orbicularis proper; the fibres composing it in the
forms which we have examined, generally do not wholly encircle
the palpebral fissure. In the Ungulata, which we have principally
studied in this connection, they are always interrupted
at the front angle of the eye by the front palpebral ligament.
Sometimes, as in the Horse (PL 16, fig. 26), the fibres with this
exception go all round the eye; but generally a number of them
have a more limited extension: 1) some only lying in the upper
lid cross others only lying in the lower lid at the posterior
angle of the eye (Camel, PL 16, fig. 21); 2) fibres lying principally
in the upper lid go from this round the posterior angle
into the lower lid but not wholly round to the anterior canthus
(Zebu, Wapiti, fig. 23—24); 3) fibres lying principally in the lower
lid go round the posterior canthus into the hind part of the superior
eyelid (Alces, fig. 22); 4) some fibres go from the anterior
canthus only a short way into the lower (fig. 23—24), resp. the
upper eyelid; 5) some fibres only encircle the posterior angle of
the eye (fig. 27).

From such incompletely developed parts of the orbicularis-
fibres the annexed muscles of the group are derived. From fibres
encircling only the posterior angle of the eye (5) is derived the
musculus postorbicularis, a muscular plate whose fibres sometimes
have a similar direction to those of the orbicularis proper
(Tapir, PL 16, fig. 27), but generally have taken a more transverse
(dorso-ventrad) direction. The muscle is in some Mammals of
considerable size, sometimes being very broad as in the Didelphys
figured (fig. A), in others being far extended dorsad and ventrad
as in Centetes (fig. B). Sometimes the fibres have been divided
transversely (Horse, PL 16, fig. 26), the muscle being thus separated
into a dorsal and a ventral portion.

In Bos, Alces (PL 16, fig. 22), Wapiti (fig. 24), Dog (PL 12) at
the hind angle of the eye there lies a little narrow flat muscle,
m. horizontalis nob., consisting of longitudinal fibres, taking its
origin in the fascia of the m. temporalis. As to the homology of
this muscle we waver. The muscle has much the appearance of
an intruder into the orbicularis-group, belonging to quite another
system than the rest of the orbicularis, and we have for a time
been inclined to think, that it was a detached piece of muscle belonging
originally to the scutularis-group. But the superficial position
of it goes against this supposition, the scutularis lying always
inside the postorbicularis, if extending into the part of the face
occupied by this muscle; while the m. horizontalis lies outside the
postorbicularis. Perhaps the horizontalis consists of orbicularis-
fibres which have taken a deviating direction; sometimes (comp.
the upper fibres in the diagram of the Wapiti, PI. 16, fig. 24) some
of the fibres have a direction, reminding one somewhat of that
of the orbicularis-fibres, which might support this supposition.

At the anterior canthus, parts of the orbicularis, in a similar

but more complicated manner than at the posterior canthus, have

formed annexed muscles. In Echidna (PL 10, fig. 1), fibres lying

before and intimately connected with the orbicularis proper form

a flat muscle in front of the eye: m. pra?orbicularis; the fibres are

*) Gesichtsmuskulatur d. Primaten. Leipzig 1887. p. 61.

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