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Mammals generally.

Platysma-sphincter-group.

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and in the Dog (PL 12) it is even rather well developed; in both,
the cervical part of the muscle is the strongest, the cephalic part
not extending far forward and upward. In Myopotamus, where
it is rather thin, it is far extended on the face, as far as the
angle of the mouth. In Sus (PL 9) the muscle, which is rather
bulky, is present only on the neck, extending on to the angle of
the lower jaw; an anterior, narrow portion extends below the
platysma and on to the inner side of this muscle, and a few
fascicles thereof are directly continued into the sphincter profundus
(PL 10, fig. 3); the greater, posterior, part of the muscle, as
usual, lies on the outer side of the platysma. In other Ungulata
it is also restricted to the neck, not extending on to the head.
In the Marsupials, the Prosimiaz and Primates, it is not present
(at most in the Prosimise there may be traced vestiges of it).

The sphincter profundus when most completely developed
extends forward to the anterior extremity of the head; upward
it extends to the eye, and the ear (PI. 17, fig. 2). The hindmost
part is inserted into the tragus of the auricle, acting as a depressor
of the ear, portio auricularis. A middle portion of the
muscle extends into the lower eyelid, acting as a depressor thereof:
portio palpebralis. The part of the muscle which connects these
two portions we term portio intermedia. Finally there is a front
part of the muscle, the portio oris, generally termed muse, orbicularis
oris1). The fibres are here curved in such a manner, that
they more or less perfectly surround the cleft of the mouth. The
direction of the fibres in the rest of the muscle is mainly transverse
, but in the hind part of the muscle the direction is ventrally
more antero-posterior; and sometimes these fibres are inserted
into the anterior end of the sternum. Ventrally the transverse
bundles of the right and left muscle are sometimes directly continued
into one another, or the bundles may cross one another,
or they may be separated in the middle line by a linea tendinea,
or they fail to reach one another, or parts of the muscles may
behave in one, parts in another manner in the same animal.

A complete development of the muscle as just described is
not found in many Mammals. The portio oris we know only in
the Dog and the Rabbit. The portio intermedia has often been
suppressed, the portio palpebralis and auricularis thus representing
two distinct muscles.

As a continuous muscular plate, extending from one end of
the head to the other, the muscle is developed in the Dog (PL 15,
fig. 2). The portio oris, which extends round the angulus oris into
the upper and lower lip (but not to the anterior end thereof), proceeds
posteriorly in the following part of the sphincter, the fibres
lying in the same plane and becoming gradually less curved, and
more transverse; thus passing over into the portio palpebralis,
which is large and intimately connected with the orbicularis
oculi. The portio auricularis is very distinct, the portio intermedia
being thinner and composed of more loosely connected bundles.

In the Prosimio2 the sphincter profundus is a continuous
rather well developed thin plate, but an insertion into the auricle
does not always take place, and the portio oris is not present,
or at most only a few fibres are continued into the upper lip2).

In the Rabbit the sphincter profundus extends as a continuous
plate from the anterior end of the sternum, from which
a part of its fibres takes origin, unto the angle of the mouth.
A powerful portio auricularis is present, but the portio palpebralis
does not reach the orbicularis oculi. The portio oris extends
only into the upper lip.

In the following forms there is no portio oris; the sphincter
does not extend so far forward on the face.

In Centetes (fig. G) and Halmaiurus (PL 15, fig. 2) the sphincter
profundus is well developed, and continuous; the portio auricu-

*) Not every muscle, which is termed »musculus orbicularis oris«, is the portio
oris of the sphincter prof. In most Mammals the »m. orbicularis oris« is wholly,
or in part, a portion of the buccinator (v. below).

2) What Ruge (Gesichtsmusk. d. Halbaffen, in: Morph. Jahrb. 11. Bd.) terms
the orbicularis oris (and the caninus) in the Prosimise is the superficial transverse
layer of m. buccinatorius, behaving in a quite similar manner as the corresponding
part of the Dog and Hedgehog; in the figures of Ruge it appears as if his
orbicularis oris and caninus were a direct continuation of the sphincter profundus
— but we have not found it so. Comp. the description of the m. buccinatorius
, hereafter.

laris is connected with the ear; the portio palpebralis extends, in
Halmaturus, generally unto the orbicularis oculi and is inserted
into it; but in Centetes, and in one of the specimens of Halmaturus
examined, it does not reach the orbicularis. In Centetes
the bundles of the hinder part of the muscle ventrallv meet
the corresponding muscle of the other side in a tendinous line,
whereas the bundles of the greater anterior part of the muscles
are directly continued from one side to the other. In Myopotamus
the muscle is very strong, the caudal end thereof being connected
with the front end of the sternum; the hindmost- part of the
muscle, as usual, forms a fleshy portio auricularis, but further
forward the dorsal part of the muscle is represented by an aponeurosis
, the flesh}'- part of the muscle being situated for the
most part ventrally.

In the Ungulata generally the portio intermedia has totally disappeared
, the portio auricularis and the portio palpebralis being
quite separated; at most a small part of the portio intermedia has
been preserved in connection with the portio auricularis. In all
the Artiodactyla examined by us (viz. Sus, Camelus, Alces, Cervus
canadensis and Aristotelis, Bos) both the said portions are well
developed. In the Elk (PL 15, fig. 6) the portio auricularis is a true
sphincter, whose anterior bundles run as an arch from one auricle
to the other below the neck; the hind bundles radiate like
a fan on the underside of the neck, in the middle line crossing
those of the other side. In Cervus Aristotelis the very well developed
portio auricularis is not a true sphincter, the bundles in
the middle line being separated from those of the other side by
a tendinous line. In this species there is a distinct remnant of
the portio intermedia, attached to the front margin of the portio
auricularis, extending a little on the surface of the masseter. In
the Perissodactyla, viz. Tapir and Horse, the portio palpebralis is
only slightly developed, in the Tapir (PL 7) being connected with
the buccinator (PI. 8, fig. 2), of which it is in this animal appa-
rentLy a detached portion; in the Horse (PL 8, fig. 1) it is a very
thin weak muscle, the ventral ends of whose bundles are directed
somewhat more forward than usual; in both animals the portion
is connected with the orbicularis oculi. The portio auricularis
in the Tapir (PL 7) is partially a true sphincter, the hind bundles
forming a sling from one ear to the other; in the Horse there is
no such connection.

In Echidna (PL 10) the muscle presents rather striking peculiarities
. Only the hind part of it, behind the ear, is developed;
it is in the usual manner attached to the ear-conch, but is much
larger than the portio auricularis of other Mammals, being a powerful
muscle extending beyond the median side of the fore-limb
and on the ventral side of the trunk, where it is connected with
the cutaneous muscles of the latter. A point of great interest is
that the sphincter profundus and the sphincter superficialis in Echidna
ventrally run into one another (PL 10, fig. 2), so that both, to
some extent, form one muscle, with a deep cleft, in which the
platysma has its place1). This arrangement in a form such as
Echidna makes it rather probable that the two sphincteres were
originally one muscle: there seems to be no reason to suppose
that the connection of the sphincter superficialis and profundus
in Echidna is a secondary one2).

In Ornithorhynchus there is no sphincter profundus.

The intermediary sheet of our complex is the platysma
(PL 17, fig. 1), whose bundles have chiefly, and markedly, a longitudinal
direction. It is always present. Primarily it is a great
continuous muscular plate extending from the neck to the angle
of the mouth or still further forward; occupying the side of
the head below the eye and the ear (seldom extending above
them). But it may be transformed in various ways: sometimes
either the hind or the front part of the muscle has been reduced
to an aponeurosis, the front or the hind part of the
muscle respectively being muscular; and instead of being continuous
the muscle may have been split in two or more portions,

x) In the individual examined an aberrant bundle from the sphincter prof, extends
on the outer surface of the plat3^sma (a PI. 10, fig. 1—2).

2) Comp. also the singular connection of some fibres of sphincter superficialis
with the auricular portion of sphincter profundus in Sus. (p. 7).