MYOLOGY
The first point to decide in commencing this study is the order in which
we shall consider the different muscles which we have to examine. It
must not be forgotten that in the present work we compare the
organization of animals with that of man, which we already know, and
that it is on the construction of this latter that, in these studies,
the thought must at each instant be carried back in order to establish
this comparison. Now, the general tendency which we notice in our
teaching of anatomy, when one regards the region of the trunk in the
human figure (a living model or a figure in the round), is first to
consider the anterior aspect. It is the latter that, for this reason, we
study at the very beginning; we next deal with the posterior surface of
the trunk, because it is opposite; lastly, the lateral surfaces, because
they unite with the preceding surfaces, the one to the other.
In studying an animal, it is usually by one of its lateral aspects that
one first observes it; it is, in fact, by these aspects that it presents
its greatest dimensions, and that the morphological characters as a
whole can be more readily appreciated. Hence, possibly, the order of
description adopted in most texts, or in the figures which accompany
them. The first representation of the human figure as a whole, in a
treatise on anatomy, represents the anterior aspect; the first view of
the horse as a whole, in a treatise on veterinary anatomy, for example,
is, on the other hand, a lateral view.
We break with this latter custom, and, without taking into account the
tendency above indicated, we will commence our analysis with the study
of the aspect of the trunk, which corresponds to the anterior aspect of
the same region in man.
The first muscles usually presented for study to artists being the
pectorals, it is their homologues that we will first describe here. We
will afterwards describe the abdominal region, then the muscles which
occupy the dorsal aspect of the trunk. With regard to the lateral
surfaces, they will be found, by this fact alone, almost completely
studied, since the muscles of the two preceding (back and abdomen),
spreading out, so to speak, over them, contribute to their formation.
Nothing further will remain but to incorporate with them the muscles of
the shoulder; but these will be studied in connection with the anterior
limbs, from which they cannot be separated.
The neck, in man, may be considered in an isolated fashion, because, on
account of its narrowness in proportion to the width of the shoulders,
it is clearly differentiated from the trunk; for this reason we combine
the study of it with that of the head. In animals, because of the
absence or slight development of the clavicles, the neck is generally
too much confounded with the region of the shoulders to make it
legitimate to separate it from that region in too marked a fashion. It
will, accordingly, be considered next.
We will then undertake the study of the muscles of the limbs, and end
with the myology of the head.
THE MUSCLES OF THE TRUNK
We shall divide them into muscles of the thorax, of the abdomen, and of
the back.
Muscles of the Thorax
=The Pectoralis Major= (Fig. 66, 1, 2; Fig. 67, 3, 4; Fig. 68, 7; Fig.
69, 10; Fig. 70, 11).--Further designated by the name of _superficial
pectoral_, this muscle is described in treatises on veterinary anatomy
as formed of two portions: an anterior one, called the _sterno-humeral_
muscle; the other, situated below and behind the preceding, bearing the
name of _sterno-aponeurotic_.
It occupies the region of the breast, and, as a whole, it takes origin
from the median portion of the sternum, from which it is directed
towards the arm and forearm.
The anterior portion (sterno-humeral muscle)--thick, forming an
elevation under the skin, and really constituting the pectoral
region--is directed downwards and outwards to be inserted into the
anterior margin of the humerus--that is to say, to the ridge which
limits in front the spiral groove of this bone.
The other part (sterno-aponeurotic muscle) is situated more posteriorly,
and corresponds to the region known in veterinary anatomy as the
_inter-fore-limb space_, which is limited laterally on each side by the
superior portion of the forearm, of which the point of junction with the
trunk bears the name _ars_. Arising from the sternum, as we have above
indicated, this portion is directed outwards, to be joined with the
terminal aponeurosis of the sterno-humeral, and with that which covers
the internal surface of the forearm.
All things considered, the sterno-humeral muscle may be regarded as the
representative of the upper fibres of the great pectoral of man, of
which the attachments, owing to the more or less complete absence of the
clavicle in the domestic mammals, the fibres must be concentrated on
the sternum; the sterno-aponeurotic portion then representing the
inferior fasciculae of the same muscle.
[Illustration: FIG. 66.--MYOLOGY OF THE HORSE: ANTERIOR ASPECT OF THE
TRUNK.
1, Pectoralis major (sterno-humeral); 2, pectoralis major
(sterno-aponeurotic); 3, mastoido-humeralis; 4, point of the shoulder;
5, sterno-mastoid or sterno-maxillary: 6, inferior portion of the
platysma myoides of the neck, divided; 7, triceps cubiti; 8, brachialis
anticus; 9, radialis (anterior extensor of the metacarpus); 10, scapular
region.]
The great pectoral muscle of one side is separated from that of the
opposite side along the median line, and especially above and in front,
by a groove which is more or less deep, according as the muscles are
more or less developed. At the bottom of this groove, suggestive of that
which exists in the corresponding region in man, is found, as in this
latter, the median portion of the sternum.
The preceding description particularly applies to the arrangement which
the great pectoral presents in the horse; in other animals it is marked
by some distinctive characters. In the pig, it is inserted into the
sternum as far only as the level of the third costal cartilage; in the
ox and sheep, it extends as far as the sixth; in the dog, it is
attached to the two first sternal pieces only--that is to say, as far as
the third costal cartilage. Moreover, in the latter, as in the cat, the
two portions which we have indicated are less readily distinguished.
The great pectoral, by its contraction, draws the fore-limb towards the
middle line--that is to say, adducts it.
=The Pectoralis Minor= (Fig. 67, 6; Fig. 68, 8; Fig. 69, 11; Fig. 70,
12, 26).--This muscle, also called the _deep pectoral_, is, in animals,
larger than the superficial pectoral, therefore certain authors prefer
to give to this muscle and the preceding one the names of deep and
superficial pectoral respectively. This nomenclature is evidently
legitimate, and conforms more to reality, since it does not bring in the
notion of dimensions which here is found in contradiction to
nomenclature; but, in order to establish more clearly the parallelism
with the corresponding muscles in man, we think it better, nevertheless,
to give them the names by which it has been customary to designate them
in connection with the latter.
We will recall at the outset that the lesser pectoral muscle in man is
completely covered by the great. In animals this is not the case; the
lesser pectoral being very highly developed, projects beyond the great
pectoral posteriorly, and occupies to a greater or less extent the
inferior surface of the abdomen.
It also consists of two parts: one anterior, which we designate by the
name of _sterno-prescapular_; the other, posterior, bearing that of
_sterno-humeral_.[20]
[20] This division of the pectorals certainly complicates the
nomenclature of these muscles; nevertheless, it introduces no
insuperable difficulty from the mnemonic point of view. But where
the study becomes less profitable, and comparison with the
corresponding muscles in man more complicated, is in adopting the
nomenclature of Bourgelat. Indeed, the great pectoral is
designated by this author the 'common muscle of the arm and
forearm,' while the lesser pectoral (or deep pectoral) is called
the 'great pectoral' in its sterno-trochinian and 'lesser
pectoral' in its sterno-prescapular portion. We do not consider
it necessary to give the other theories relative to the
homologies of these, notwithstanding the very real interest which
they present from the purely anatomical point of view, as they
have but few applications in the study of forms.
The sterno-prescapular muscle, being covered by the sterno-humeral, has
little interest for us. It arises from the sternum, and is directed
towards the angle formed by the junction of the scapula and humerus;
then it is reflected upwards and backwards, to terminate on the anterior
margin of the shoulder by insertion into the aponeurosis, which covers
the supraspinatus muscle.
We can, especially in the horse after removal of the skin, recognise it,
at the level of this region, in the interspace limited by the
superficial muscles (Fig. 70, 26).
In the dog and cat this portion of the muscle does not exist. The other
division of the muscle, the sterno-trochinian, is more interesting. It
arises from the abdominal aponeurosis and the posterior part of the
sternum; hence it passes forward, turns under the superficial pectoral,
and is inserted into the lesser tuberosity (trochin) of the humerus.
In the pig, dog, and cat, it is inserted into the greater tuberosity
(trochiter) of the bone of the arm.
The superior border of this muscle is in relation with a superficial
vein, which is distinctly visible in the horse--the subcutaneous
thoracic vein, which in this animal is called the vein of the spur.
The sterno-humeral muscle, in contracting, draws the shoulder and the
whole anterior limb backwards.
=Serratus Magnus= (Fig. 67, 2; Fig. 69, 8; Fig. 70, 9).--This muscle,
which is situated on the lateral aspect of the thorax, is covered to a
considerable extent by the shoulder, the posterior muscular mass of the
arm, and by the great dorsal muscle.
It arises by digitations from the external surface of the dorsal
vertebrae; from the first eight in the horse, ox, and dog.
[Illustration: FIG. 67.--MYOLOGY OF THE HORSE: INFERIOR ASPECT OF THE
TRUNK.
1, Anterior extremity of the sternum; 2, point of the shoulder and
inferior portion of the mastoido-humeral muscle; 3, pectoralis major
(sterno-humeral); 4, pectoralis major (sterno-aponeurotic); 5, point of
the elbow; 6, pectoralis minor (sterno-trochinian); 7, serratus magnus;
8, external oblique; 9, sheath of the rectus abdominis; 10, linea alba;
11, the umbilicus; 12, external oblique divided in order to expose the
rectus abdominis; 13, rectus abdominis.]
The muscular bundles, converging as they proceed, towards the scapula,
pass under this bone, to be inserted into the superior portion of the
subscapular fossa, near the spinal border. The inferior portion of its
posterior digitations is visible in the ox and in the horse; these
digitations are less visible in the pig. They are not seen at all in the
dog (Fig. 68) or cat, for in these animals the great dorsal muscle
covers them completely.
The great serratus muscle, by the position which it occupies and the
arrangement that it presents, forms with the corresponding muscle of the
opposite side a sort of girth, which supports the thorax, and at the
same time helps to fix the scapula against the latter.
When it contracts, in taking its fixed point at the ribs, it draws the
superior portion of the scapula downwards and backwards in such a way
that this bone has its inferior angle directed forwards and upwards. If
it takes its fixed point at the shoulder, it then acts on the ribs,
raises them, and so becomes a muscle of inspiration.
Because of the connections of the serratus magnus with the levator
anguli scapulae, some authors consider it as united with the latter. But
as the latter muscle is visible only in the region of the neck (see p.
157), and as it is separately described in man, we prefer to distinguish
them from one another. We shall recall the connections to which we have
just made allusion when describing the cervical region.
Muscles of the Abdomen
The abdominal wall is, as in man, formed by four large muscles: the
external oblique, the internal oblique, and the transversalis, which
form the lateral walls, and the rectus abdominis, situated on each side
of the middle line of the abdomen. This latter, because of the general
direction of the trunk in quadrupeds, has its superficial surface
directed downwards.
The arrangement of these muscles closely corresponds to that which we
find in the human species.
=The External Oblique Muscle= (Fig. 67, 8, 12; Fig. 68, 5; Fig. 69, 9;
Fig. 70, 10).--This muscle arises, by digitations, from a number of
ribs, which varies according to the species, the number of the ribs
being itself variable for each of them, as we pointed out in connection
with the osteology of the thorax. Indeed, the great oblique arises from
the eight or nine posterior ribs in the dog and the ox, and from the
thirteen or fourteen posterior in the horse. It is attached, besides, to
the dorso-lumbar aponeurosis.
These attachments are arranged in a line directed obliquely upwards and
backwards, and the first digitations--that is to say, the most anterior
ones--dovetail with the posterior digitations of origin of the great
serratus muscle.
The fleshy fibres are directed downwards and backwards, and terminate in
an aponeurosis which covers the inferior aspect of the abdomen, and
proceeds to form the linea alba by joining with that of the muscle of
the opposite side, and also to be inserted into the crural arch.
This aponeurosis of the external oblique is covered by an expansion of
elastic fibrous tissue, which doubles it externally, and which is known
as the _abdominal tunic_. This latter is further developed as the organs
of the digestive apparatus are more voluminous, and their weight,
consequently, more considerable. For this reason, in the large
herbivora, as the ox and the horse, this tunic is extremely thick,
whereas in the pig, cat, and dog it is, on the contrary, reduced to a
simple membrane. Indeed, in these latter, the abdominal viscera being
less developed, the inferior wall of the abdomen does not require so
strong a fibrous apparatus for supporting them. The great oblique, when
it contracts, compresses the abdominal viscera in all circumstances
under which this compression is necessary; it also acts as a flexor of
the vertebral column.
=The Internal Oblique Muscle.=--This muscle, which is covered by the
preceding, arises from the anterior superior iliac spine (external angle
in ruminants and solipeds) and the neighbouring parts. From this origin
its muscular fibres, the general direction of which is opposite to that
of the fibres of the external oblique, diverging, proceed to terminate
in an aponeurosis, which contributes to the formation of the _linea
alba_, and to be attached superiorly to the internal surface of the last
costal cartilages. It has the same action as the great oblique. What it
presents of special interest is the detail of form which it determines
in the region of the flank; this detail is _the cord of the flank_. It
is characterized by an elongated prominence which, starting from the
iliac spine, is directed obliquely downwards and forwards, to terminate
near the cartilaginous border of the false ribs.
Often very apparent in the ox, and still more so in the cow, the cord in
question contrasts with the depression which surmounts it; this
depression is situated below the costiform processes of the lumbar
vertebrae, and is called the _hollow of the flank_. It is so much the
more marked as the mass of the intestinal viscera is of greater weight.
We sometimes meet with a case of the presence of this hollow in the
horse. But when in the latter, the flank is well formed, the hollow is
scarcely visible, and the cord but slightly prominent. It is only in
emaciated subjects that these details are found clearly marked.
=Transversalis Abdominis.=--This muscle being deeply situated does not
present any interest for us. We will, however, point out, in order to
complete the series of muscles which form the abdominal wall, that the
direction of its fibres is transverse, and that they extend from the
internal surface of the cartilages of the false ribs, and the costiform
processes of the lumbar vertebrae to the _linea alba_.
=The Rectus Abdominis= (Fig. 67, 13; Fig. 68, 6).--This muscle,
enclosed, as it is in man, in a fibrous sheath (Fig. 67, 9) formed by
the aponeuroses of the lateral muscles of the abdomen, is a long and
wide fleshy band, which, as in the human species, reaches from the
thorax to the pubis.
What distinguishes it in quadrupeds is that there are costal attachments
which extend further on the sternal surface of the thorax, and the
number of its aponeurotic insertions, which, in general, is more
considerable. These are, indeed, six or seven in number in the pig and
in ruminants, and about ten in the horse.
It is true that we may find but three in the cat and dog; still, we
often find as many as six. These intersections are not marked on their
exterior by transverse grooves, such as we find in the human species in
individuals with delicate skin and whose adipose tissue is not very much
developed.
The rectus abdominis is covered, in its anterior portion, by the
sterno-trochinian muscle (posterior segment of the small pectoral). In
contracting, this muscle brings the chest nearer the pelvis, and as a
result flexes the vertebral column. It also contributes to the
compression of the abdominal viscera.
=Pyramidalis Abdominis.=--This unimportant little muscle, which in man
is situated at the lower part of the abdomen, extends from the pubis to
the _linea alba_. It is not present in the domestic animals.
We consider it interesting, however, to point out, although the fact is
not a very useful one as regards external form, that this muscle is
distinctly developed in marsupials.
We know that in the opossum, the kangaroo, and the phalanger fox, the
young are brought forth in an entirely incomplete state of development,
and that, during a certain period, they are obliged to lodge in a pouch
which is placed at the lower part of the abdomen of the mother. Now,
this pouch contains the mammary glands; but the young, being too feeble
to exercise the requisite suction, the pyramidal muscles come to their
assistance. These muscles, in contracting, approximate to one another
two bones which are placed above the pubis, the (so-called) marsupial
bones (see Fig. 80); by their approximation the bones in question, which
are placed behind and on the outer side of the mammary glands, compress
the latter, and thus is brought about the result which the little ones,
on account of their feebleness, would, without that intervention, be
incapable of obtaining for themselves.
Muscles of the Back
=Trapezius= (Fig. 68, 1, 2; Fig. 69, 1, 2; Fig. 70, 1, 2).--This muscle,
more or less well developed, according to the species, is divided into
two portions, of which the names indicate the respective situations--a
cervical and a dorsal.
These two parts, considered in the order in which we find them, take
their origin from the superior cervical ligament and from the spinous
processes of the first dorsal vertebrae. From these different points the
fibres are directed towards the shoulder; the anterior are,
consequently, oblique downwards and backwards, and the posterior are
directed downwards and forwards. They are inserted into the scapula in
the following manner: the fibres of the dorsal portion are attached to
the tuberosity of the spine; those of the cervical region are also fixed
into the same spine, but into a considerably larger surface.
The cervical portion occupies, in the region of the neck, an area
relatively smaller than the corresponding portion of the trapezius in
man. This diminished degree of development results from the absence,
complete, or nearly so, of the clavicle in the animals which we are now
considering. We remember, that the trapezius of man is partly inserted
into the clavicle, and the disappearance of this latter cannot fail to
bring modifications in the general disposition of the corresponding
portion of the muscle. There results a disconnection of this latter, and
it becomes united to other muscular fibres to form a muscle with which
we shall soon have to deal--the mastoido-humeral (see p. 150).
As specific differences we should add that the trapezius occupies a more
or less extensive portion of the median and superior regions of the
neck; terminating at a considerable distance from the head in the dog
and horse, it, on the contrary, approaches it in the pig and in
ruminants. The cervical portion, when it contracts, draws the scapula
upwards and forwards, the dorsal portion draws it upwards and backwards.
When the trapezius acts as a whole the scapula is raised.
[Illustration: FIG. 68.--MYOLOGY OF THE DOG: SUPERFICIAL LAYER OF
MUSCLES.
1, Trapezius, cervical portion; 2, trapezius, dorsal portion; 3,
superior outline of the scapula; 4, latissimus dorsi; 5, external
oblique muscle; 6, rectus abdominis; 7, pectoralis major of the right
side; 8, pectoralis minor (sterno-trochinian); 9, 9, mastoido humeral
muscle; 10, tendinous intersection, at the level of which is found a
rudimentary clavicle; 11, sterno-mastoid muscle; 12, infrahyoid muscles;
13, omo-tracheal or acromio-tracheal muscle; 14, splenius; 15, levator
anguli scapulae; 16, deltoid muscle, spinal portion; 17, deltoid,
acromial portion; 18, superior extremity of the humerus; 19,
supraspinatus; 20, infraspinatus; 21, biceps cubiti; 22, brachialis
anticus; 23, triceps cubiti, long head; 24, triceps cubiti, external
head; 25, olecranon process; 26, radialis (anterior extensor of the
metacarpus); 27, iliac crest; 28, gluteus maximus; 29, gluteus medius;
30, biceps cruris; 31, semitendinosus; 32, semi-membranosus; 33,
gastrocnemius; 34, tensor of the fascia lata; 35, sartorius; 36, fascia
lata drawn up by the triceps; 37, the patella or knee-cap; 38,
ischio-coccygeal muscle; 39, superior sacro-coccygeal; 40, lateral
sacro-coccygeal; 41, inferior sacro-coccygeal.]
=The Latissimus Dorsi= (Fig. 68, 4; Fig. 69, 5; Fig. 70, 5).--This
muscle arises by an aponeurosis, the so-called dorso-lumbar aponeurosis,
from the spinous processes of the last dorsal vertebrae (the seven last
in the dog, fourteen or fifteen last in the horse), from the spinous
processes of the lumbar vertebrae, and from the last ribs. Its fleshy
fibres are directed downwards and forwards, being more oblique in
direction posteriorly, and pass on the inner side of the posterior
muscular mass of the arm, to be inserted into the internal lip of the
bicipital groove of the humerus, or, a little lower down, on the median
portion of the internal surface of the same bone. This latter mode of
insertion is met with in the horse and the ox.
The anterior fibres cover the posterior angle of the scapula (as in man,
where the corresponding angle, but in this case inferior, is covered by
the same muscle), and, a little higher up, are in their turn concealed
by a portion of the dorsal fibres of the trapezius. It covers, to a
greater or less extent, the great serratus muscle. These relations are
similar to those found in the human species.
We find that the fleshy fibres of the great dorsal are prolonged more or
less backwards if we examine this muscle in the dog, the ox, the pig,
and the horse. Indeed, the fibres reach to the thirteenth rib in the dog
and the cat (that is to say, the last rib), the eleventh in the ox,
tenth in the pig, and twelfth only in the horse. We say 'only' in
connection with this last because it is necessary to remember that the
ribs are eighteen in number on each side of the thorax of this animal,
and that, accordingly, the fleshy fibres of the great dorsal muscle are,
relatively, of small extent.
When this muscle contracts it flexes the humerus upon the scapula, and
helps to draw the whole of the anterior limb backwards and upwards.
There is a muscular fasciculus which, because of its relations with the
muscle we have just been studying, is known as the _supplementary muscle
of the latissimus dorsi_. But as, on the other hand, this fasciculus is
in relation with the triceps, we shall in preference consider it in
relation with this latter (see p. 173).
[Illustration: FIG. 69.--MYOLOGY OF THE OX: SUPERFICIAL LAYER OF
MUSCLES.
1, Trapezius, cervical portion; 2, trapezius, dorsal portion; 3, outline
of the scapula; 4, spine of the scapula; 5, latissimus dorsi; 6, small
posterior serratus; 7, prominence caused by the costiform processes of
the lumbar vertebrae; 8, serratus magnus; 9, external oblique; 10,
pectoralis major (sterno-humeral); 11, mastoido-humeralis; 12, atlas;
13, atlas; 14, parotid gland; 15, sterno-mastoid muscle; 16, infrahyoid
muscles; 17, omo-trachelian or acromio-trachelian muscle; 18, deltoid;
19, brachialis anticus; 20, triceps, long head; 21, triceps, external
head; 22, olecranon; 23, radialis (anterior extensor of the metacarpus);
24, anterior iliac spine; 25, gluteus maximus; 26, gluteus medius; 27,
biceps cruris; 28, semitendinosus; 29, gastrocnemius; 30, tensor of the
fascia lata; 31, fascia lata covering the triceps of the thigh; 32,
patella; 33, ischio-coccygeal muscle; 34, superior ischio-coccygeal; 35,
lateral ischio-coccygeal; 36, inferior ischio-coccygeal.]
The aponeurosis by which the great dorsal arises from the vertebral
column covers, as in man, the muscles which occupy the grooves situated
on each side of the spinous processes--the spinal muscles or common
muscular mass, if we regard them as a whole (Fig. 70, 7); the
sacro-lumbar and the long dorsal muscles covering the transverse spinal,
if we consider them as distinct.
It would be superfluous to enter here into a detailed examination of
these muscles.
If they are but little developed the spinous processes become prominent
under the skin; if they are more so they may by their thickness project
beyond the level of these processes, and these latter thus come to lie
in a groove more or less marked, which, on account of the division which
is determined by its presence, has caused the regions which it occupies
to be designated by the names _double back_ and _double loins_.
The muscles are extensors of the vertebral column.
Under the aponeurosis of the great dorsal muscle there is found in man
another muscle, the serratus posticus inferior, which, on account of
being deeply placed and its slight thickness, offers nothing of interest
in connection with the study of external form. It arises from the
spinous processes of the three last dorsal vertebrae and those of the
three first lumbar; it then passes upwards and outwards, and divides
into four digitations, to be inserted into the inferior borders of the
four last ribs. We repeat that it is covered by the great dorsal muscle.
In the pig, ox, and horse, which have this latter muscle less developed
in its posterior portion, the same small serratus muscle, known as the
_posterior serratus_, is visible in the superficial layer of muscles
(Fig. 69, 6; Fig. 70, 6). The number of its digitations is more or less
considerable according to the species examined.
=The Rhomboid Muscle= (Fig. 70, 21).--In order to make intelligible the
position of the rhomboid in the superficial layer in quadrupeds, it
appears to us necessary to recall the anatomical characters of the
muscle as found in man. The rhomboid arises from the inferior portion of
the posterior cervical ligament, from the spinous process of the seventh
cervical vertebrae and the four or five upper dorsal; thence passing
obliquely downwards and outwards, it is inserted into the spinal border
of the scapula, into the portion of this border which is situated below
the spine; it sometimes extends to the middle of the interval which
separates this latter from the superior internal angle of the same bone.
The portion of the muscle which arises from the cervical ligament and
the seventh cervical vertebra is often separated from the lower portion
by a cellular interspace. For this cause some anatomists have described
the rhomboid as consisting of two parts--the superior or small rhomboid
and the inferior or large rhomboid, on account of the position occupied
by each, and of their difference in volume.
This muscle can only be seen in the region of the back, in the space
limited externally by the spinal border of the scapula, below by the
latissimus dorsi, and internally by the trapezius, which covers it in
the rest of its extent. It is not in this space that it is seen in
certain quadrupeds. As we pointed out in the section on osteology, the
spinal border of the scapula is short, and it seems to be due to this
limitation in length that the trapezius and the latissimus dorsi muscle
are, at this level, in contact the one with the other in such a way that
they fill up the interval in which the rhomboid is seen in man.
In the horse we can partly see it in the superficial muscular layer, but
in the region of the neck only, at the superior border of the shoulder.
Indeed, as we have already pointed out, the trapezius does not reach the
occipital protuberance; for this reason a part of the anterior portion
of the rhomboid may be seen--that is, the portion which corresponds to
the superior part of the human muscle.
But whether it be covered by the trapezius, or, as we find in the cat
and dog, by the _mastoido-humeral muscle_ (see p. 150), which is very
broad in this region, we do not the less recognise its presence; and in
the horse and ox, in particular, it forms an elongated prominence
beginning at the level of the scapula, and tapering as it ascends,
towards the posterior part of the head.
[Illustration: FIG. 70.--MYOLOGY OF THE HORSE: SUPERFICIAL LAYER OF
MUSCLES.
1, Trapezius, cervical portion; 2, trapezius, dorsal portion; 3,
superior outline of the scapula; 4, spine of the scapula; 5,
latissimus dorsi muscle; 6, small posterior serratus; 7, spinal muscles,
or common muscular mass; 8, ribs; 9, serratus magnus; 10, external
oblique; 11, pectoralis major (sterno-humeral); 12, pectoralis
minor (sterno-trochinian); 13, atlas; 14, parotid gland; 15,
mastoido-humeralis; 16, point of the arm; 17, sterno-mastoid, or
sterno-maxillary; 18, jugular groove; 19, infrahyoid muscles; 20,
omo-trachelian muscle; 21, rhomboid; 22, splenius; 23, levator anguli
scapulae; 24, deltoid; 25, supraspinatus; 26, terminal part of the
sterno-prescapular, a portion of the small pectoral muscle; 27,
brachialis anticus; 28, triceps cubiti, middle or long head; 29, triceps
cubiti, external head; 30, olecranon; 31, radial extensor (anterior
extensor of the metacarpus); 32, anterior iliac spine; 33, anterior
portion of the gluteus maximus--the aponeurosis of the muscle has been
divided in order to expose the gluteus medius; 34, posterior portion of
the gluteus maximus; 35, gluteus medius; 36, biceps cruris; 37,
semitendinosus; 38, point of the buttock; 39, gastrocnemius; 40, tensor
of the fascia lata; 41, triceps cruris; 42, ischio-coccygeal muscle; 43,
superior sacro-coccygeal; 44, lateral sacro-coccygeal; 45, inferior
sacro-coccygeal.]
Its origins are similar to those which we have already described in the
human rhomboid. It arises from the cervical ligament and the spinous
processes of the foremost dorsal vertebrae; its fibres converge and pass
to the scapula, to be inserted into its superior or spinal border, or
into the internal surface of the cartilage of prolongation.
It assists in keeping the scapula applied to the thoracic cage, and when
it contracts, draws the scapula upwards and forwards.
Taking its fixed point at the scapula, it acts on the neck by its
anterior fibres, and extends it.
We shall soon have occasion to mention this muscle again, in connection
with the study of the muscles of the neck.
=The Cutaneous Muscle of the Trunk= (Fig. 71).--Immediately beneath the
skin which covers the neck, shoulders, and trunk is found a vast
cutaneous muscle, analogous to that which, in the human species, exists
only in the cervical region.
This thin muscle, whose function is to move the skin which strongly
adheres to it, and in this way to remove from it material causes of
irritation (insects, for example), is of considerable thickness in the
region of the trunk; where it constitutes what certain authors have
designated by the name of _panniculus carnosus_. In this region it
extends from the posterior border of the shoulder to the thigh, and, in
the vertical direction, from the apices of the spinous process of the
dorso-lumbar vertebrae to the median line of the abdomen.
Arising above from the supraspinous ligament of the dorso-lumbar and
sacral regions (except in the carnivora; see below) by an aponeurosis
which, posteriorly, covers the muscles of the hind-limbs, its fibres are
directed to the elbow, on which they are arranged in two layers: a
superficial, which becomes continuous with the panniculus muscle of the
shoulder; and a deep, which passes on the inner side of the shoulder to
be inserted into the internal surface of the humerus; this latter exists
only in the dog and cat.
The most inferior fibres, behind, at the level of the knee-cap form a
triangular process which in the horse receives the name of the _stifle
fold_, from the name veterinarians give to the region of the
articulation of the knee. This fold of skin, which commences on the
antero-internal surface of this region, is directed upwards, and then
forwards, to end by gradually disappearing over the corresponding part
of the abdomen.
[Illustration: FIG. 71.--MYOLOGY OF THE HORSE: PANNICULUS MUSCLE OF THE
TRUNK.]
In the same animal the muscular fibres of the panniculus of the trunk
arise along a line which connects the stifle-joint to the withers, a
line which is, consequently, oblique upwards and forwards. Now, as the
fleshy layer is thicker than the aponeurosis, the result is that the
mode of constitution of this muscle can be recognised under the skin.
Indeed, we can see in some animals, occasionally very distinctly, a
slight elevation starting from the region of the abdomen in the
neighbourhood of the knee, and thence directed obliquely upwards and
forwards. This elevation is produced by the fleshy portion of the
panniculus.
In the carnivora, the panniculus of the trunk is not attached to the
supraspinous ligament; it is blended with the same muscle of the
opposite side, passing over the spinous region of the vertebral column.
From this arrangement results a great mobility of the skin which covers
the back. Further, it explains why it is possible to lift up this skin
along with the panniculus which it covers, and to which it adheres,
throughout the whole extent of the dorso-lumbar column. As we pointed
out above, there is also a panniculus muscle of the shoulder and one of
the neck. We will deal with them when treating of the regions to which
those muscles belong.
The Coccygeal Region
As a sequel to the study of the muscles of the region of the trunk very
naturally comes the description of those which, belonging to the region
of the coccyx, are destined for the movements of the caudal appendix, of
which this latter constitutes the skeleton. The muscles may not seem to
be of much importance with regard to external form, but, as they form
part of the superficial muscular layer, and as the mass of each is
seen in the form of the tail in some animals (the lion, for example),
they merit our attention for a moment. A few lines will suffice to
give an idea of them. They are: the _ischio-coccygeal_, _superior
sacro-coccygeal_, _lateral sacro-coccygeal_, and _inferior
sacro-coccygeal_.
=The Ischio-coccygeal= (Fig. 18, 38; Fig. 69, 33; Fig. 70, 42).--This
muscle, triangular in shape, better developed in the carnivora than in
the horse, arises from the spine of the ischium, or from the
supracotyloid crest, which replaces this latter in the solipeds and the
ruminants. Thence its fleshy mass is directed upwards, expanding as it
proceeds to be inserted into the transverse processes of the first two
coccygeal vertebrae after insinuating itself between two of the following
muscles, the lateral and inferior sacro-coccygeal.
In the dog and cat, the muscle is in great part covered by the great
gluteal. In the ox, by a peculiar arrangement of the corresponding
region of the muscles of the thigh--an arrangement which we will examine
in connection with the study of the latter--it is more exposed than in
the horse, and gives origin to an outline which corresponds to its
general form in the region situated immediately below the root of the
tail.
It is a depressor of the whole caudal appendix.
=The Superior Sacro-coccygeal= (Fig. 68, 39; Fig. 69, 34; Fig. 70,
43).--The fasciculi which form this muscle arise from the crest of the
sacrum, and proceed thence to end successively on the coccygeal
vertebrae. It is in contact in the middle line with the corresponding
muscle of the opposite side.
It raises the tail and inclines it laterally; if the muscle of one side
contracts at the same time as that of the other the tail is elevated
directly.
=The Lateral Sacro-coccygeal= (Fig. 68, 40; Fig. 69, 35; Fig. 70,
44).--Situated on the lateral part of the caudal region, this muscle
arises, in the dog, from the internal border of the iliac bone and the
external border of the sacrum; in the horse, it arises from the crest of
the sacrum. It is inserted into the coccygeal vertebrae.
It produces lateral movement of the tail.
=The Inferior Sacro-coccygeal= (Fig. 68, 41; Fig. 69, 36; Fig. 70,
43).--This muscle, which is fairly thick, arises from the inferior
surface of the sacrum and the corresponding surface of the sacro-sciatic
ligament; it is inserted into the coccygeal vertebrae.
It depresses the caudal appendix.
Muscles of the Neck
=Mastoido-humeralis= (Fig. 66, 3; Fig. 68, 9, 9, 10; Fig. 69, 12; Fig.
70, 15).--One of the most important muscles of the region of the neck in
man is the sterno-cleido mastoid. We recollect that, in its inferior
part, it is divided into two bundles, one of which arises from the
manubrium of the sternum, and the other from the inner third of the
clavicle, whence the denominations of the _sternal_ portion and
_clavicular_ portion. The muscle formed by the union of these two
portions is then directed obliquely outwards, backwards, and upwards, to
be inserted into the mastoid process of the temporal bone and the two
external thirds of the superior curved line of the occipital bone.
Now, the animals which we are here considering have but a rudimentary
clavicle or are entirely without it. From the absence of this item of
the skeleton there necessarily result modifications in the arrangement
of the muscles of this region, which we must at the very outset explain,
before undertaking the special study of the muscle which is the subject
of the present paragraph.
Let us suppose, for the more definite arrangement of our ideas, that the
clavicle is altogether absent, although we do find it in a rudimentary
state in some animals and completely developed in others (marmot, bat),
and we will proceed to indicate what this absence determines.
The great pectoral muscle in man arises in part from the clavicle; this
origin not being possible in animals which have no clavicle, its
attachments, as we have already seen, are concentrated on the sternum.
The trapezius in man similarly arises in part from the clavicle; for the
reasons above indicated its clavicular fasciculi cannot exist in
distinct form in the animals which have no clavicle.
The sterno-cleido mastoid, whose inferior attachments we mentioned
above, cannot have a clavicular portion.
It is the same in the case of the deltoid, which, we know, arises in
part from the anterior bone of the shoulder.
Of the four muscles which have partial clavicular origins in man, two
are known to us in connection with animals--the great pectoral and the
trapezius. What has become of the other two, the sterno-cleido mastoid
and the deltoid?
It is this which we now proceed to investigate. After a fashion
simple enough, but which it is necessary to describe, the clavicular
fasciculi of the trapezius and the corresponding fasciculi of the
sterno-cleido mastoid are united the one to the other; the portion
of the deltoid which in man arises from the clavicle, by reason of the
absence of this latter, is also combined with the fleshy mass formed by
the preceding muscles. From this fusion results the muscle known as the
mastoido-humeral. This muscle, which consists of a long fleshy band
situated on the lateral aspect of the neck, takes its origin, as a
general rule, from the posterior surface of the skull and the upper part
of the neck, from which it passes obliquely downwards and backwards,
covering the scapulo-humeral angle--that is, the region known as the
point of the shoulder or arm--and is inserted into the anterior border
of the humerus, the border which, limiting anteriorly the musculo-spiral
groove, forms a continuation of the deltoid impression. On account of
the regions with which it is related, Bourgelat named this muscle _the
muscle common to the head, neck, and arm_.
It is at the level of the scapulo-humeral angle that the vestiges of the
clavicle are found.
This bone is represented in some animals--the pig, ox, and horse--by a
single tendinous intersection, more or less apparent, which extends
transversely from the scapula to the anterior extremity of the sternum.
In the dog and the cat, we find, besides, on the deep surface of the
muscle and at the level of this tendinous intersection, the rudiment of
the clavicle of which we made mention in the section on Osteology (see
p. 25).
It is beneath the intersection, the existence of which we have just
pointed out, that is found that portion of the mastoido-humeral muscle
which corresponds to the clavicular fasciculi of the deltoid; that
portion which is situated above the intersection corresponds to the
clavicular fibres of the sterno-cleido-mastoid and of the trapezius.
The mastoido-humeral presents certain varieties in different animals.
In the dog and the cat, this muscle, which is blended above with the
sterno-mastoid (see p. 153), to be inserted with it into the mastoid
process and the mastoid crest, covers the neck for a considerable extent
from the superior curved line of the occipital bone to which it is
attached, to the trapezius with which it unites posteriorly, but from
which it separates below. Between these two extreme points of its
superior portion it is attached to the cervical ligament.
In the pig and in ruminants, in which the trapezius approaches more
closely to the head, the mastoido-humeral occupies, in consequence, a
less extent of the cervical region.
In the horse, the mastoido-humeral neither covers the neck nor joins the
trapezius; indeed, we have already shown that it is separated by a
considerable distance from the head. In the limited interval between
these two muscles a part of the rhomboid and parts of other muscles are
seen with which we shall soon be occupied.
This muscle, as regards the horse, is described by some anatomists as
consisting of two parts: one anterior, or superficial; the other
posterior, or deep. In reality, the first only corresponds to the
mastoido-humeral, which we are considering; the posterior may be more
exactly regarded as representing a special muscle of quadrupeds, but
which is here a little deformed, the _omo-trachelian_ (see p. 155).
When the mastoido-humeral contracts, taking its fixed point above, it
acts as an extensor of the humerus, and carries the entire fore-limb
forwards. If it takes its fixed point below--that is to say, at the
humerus--it inclines the head and neck to its own side. If it contracts
at the same time as the mastoido-humeral of the opposite side, then the
head and the neck are carried into the position of extension.
=The Sterno-mastoid= (Fig. 66, 5; Fig. 68, 11; Fig. 69, 15;
Fig. 70, 17).--Having described the clavicular portion of the
sterno-cleido-mastoid in connection with the mastoido-humeral, because
it forms a part of the latter, we have, in order to complete the
homologies of this muscle, to study now that which corresponds to its
sternal portion. This is the _sterno-mastoid_ muscle. In all the
quadrupeds with which we are here concerned this muscle arises from the
anterior extremity of the sternum; narrow and elongated in form, it
passes towards the head in a direction parallel to the anterior border
of the mastoido-humeral, from which it is separated by an interspace
which, along its whole length, lodges superficially the jugular vein;
hence the name of _jugular groove_, which is given to this part of the
neck (Fig. 10, 18).
It is inserted, in the case of the dog and cat, into the mastoid
process, where it is united with the mastoido-humeral; in the ox it is
divided into two portions--one which goes to the base of the occipital
bone, the other passing in front of the masseter is by the medium of
the aponeurosis of this latter attached to the zygomatic crest. This
latter part is considered by some writers as forming a portion of the
panniculus muscle of the neck.
In the horse it is attached to the angle of the lower jaw by a tendon,
which an aponeurosis that passes under the parotid gland binds to the
mastoido-humeral muscle and the mastoid process.
By reason of this insertion into the jaw, in the case of the solipeds,
this muscle is further named the _sterno-maxillary_.
When it contracts, it flexes the head, and inclines it laterally. This
movement is changed to direct flexion when the two sterno-mastoid
muscles contract simultaneously.
In man, the sterno-cleido-mastoid and the trapezius leave a triangular
space between them, which, being limited inferiorly by the middle third
of the clavicle, is known as the supraclavicular region; this region,
being depressed, especially in its inferior part, has also been given
the name of supraclavicular fossa--popularly called the '_salt-cellar_.'
The muscles which form the floor of this region, passing from above
downwards, are: a very small portion of the complexus, splenius, levator
anguli scapulae, posterior scalenus, and anterior scalenus; then,
crossing these latter, and most superficial, is the omo-hyoid muscle.
An analogous region, but of only slight depth, exists in quadrupeds; its
borders are formed by the mastoido-humeral and trapezius muscles.
It is not limited below by the clavicle--we know, indeed, that this, or
the intersection which represents it, belongs to the mastoido-humeral
muscle--but by the inferior portion of the spine of the scapula.
It is of greater or less extent according to the species considered.
In the dog, cat, pig, and ox, it is narrow, for the muscles which bound
it approach one another pretty closely. It has, as in man, the form of a
triangle, with the apex above. In the horse it is much broader, and,
contrary to the arrangement which it presents in the human species, the
widest part is directed upwards.
The muscles which we find there are, consequently, more or less
numerous. In the dog and cat they are: a portion of a muscle which we do
not normally meet with in man--the _omo-trachelian_--then in a
decreasing extent: supraspinatus, levator anguli scapulae and splenius.
In the pig: the omo-trachelian, supraspinatus, and the terminal portion
of the sterno-prescapular--the anterior part of the lesser or deep
pectoral muscle.
In the ox: the omo-trachelian only.
But in the horse we find the omo-trachelian, the supraspinatus, and the
terminal extremity of the sterno-prescapular; then in a larger extent of
area the levator anguli scapulae and the splenius; and, finally, the
anterior portion of the rhomboid.
Among the muscles which we have just enumerated are some that we have
already studied; these are the sterno-prescapular and the rhomboid. We
will examine the supraspinatus muscle in connection with the region of
the shoulder.
As to the scaleni muscles and the complexus, they are deeply situated,
whereas the omo-hyoid is visible in the anterior region of the neck
only.
There remain for us, accordingly, to examine, at the present juncture,
but the omo-trachelian, levator anguli scapulae, and splenius muscles.
=The Omo-trachelian Muscle= (Fig. 68, 13; Fig. 69, 17; Fig. 70,
20).--Also called the _acromio-trachelian_, _levator ventri
scapulae_,[21] the _angulo-ventral muscle_, and the
_transverso-scapular_,[22] etc., this muscle is described by some
hippotomists as belonging to the mastoido-humeral, of which it then
forms its posterior or deep portion (see p. 153).
[21] Ventri, because inserted into the inferior part of the spine of
the scapula, towards the acromion--that is, on the ventral
side--by contrast with the trapezius, which is attached higher up
(dorsal side) on the same process.
[22] Among the many names given to this muscle, Arloing and Lesbre
recommend the adoption of the name 'transverse scapular' given by
Straus-Durckheim, or 'transverse of the shoulder' (Arloing and
Lesbre, 'Suggestions for the Reform of Veterinarian Muscular
Nomenclature,' Lyons, 1898).
The omo-trachelian muscle is found in all mammalia, man alone excepted.
It is, however, sometimes found in the human being; but it then
constitutes an anomaly.
In the dog, pig, and ox, it arises from the inferior part of the spine
of the scapula, in the region of the acromion, and terminates on the
lateral portion of the atlas.
In the cat it is attached besides to the base of the occipital bone. It
is visible in the space limited by the trapezius and the
mastoido-humeral, the direction of which it crosses obliquely.
In the horse it appears to be blended in clearly defined fashion with
the mastoido-humeral. Attached below, like this latter, to the anterior
border of the humerus, it covers the scapulo-humeral angle; and is
attached by its upper portion to the transverse processes of the first
four cervical vertebrae.
We remember that the transverse processes are often, from their relation
with the trachea, known as the tracheal processes. Hence the word
'trachelian,' which forms part of the name of the muscle with which we
are now dealing.
By its contraction it helps to draw the anterior limb forwards.
When this muscle, as an abnormality, exists in man, it arises from
the clavicle or the acromion process, traverses the supraclavicular
fossa, and is inserted into the transverse processes of the atlas or
axis, or of both these vertebrae, or of the cervical vertebrae below
these latter. It is then known by the names of the _elevator of the
clavicle_ or _elevator of the scapula_, and, finally, as the
_cleido-omo-transversalis_ (Testut).[23]
[23] L. Testut, 'Les anomalies musculaires chez l'homme expliquees par
l'anatomie comparee,' Paris, 1884, p. 97. A. F. Le Double,
'Traite des variations du systeme musculaire de l'homme et de
leur signification au point de vue de l'anthropologie
zoologique,' Paris, 1897, t. i., p. 235.
=The Levator Anguli Scapulae= (Fig. 68, 15; Fig. 70, 23).--As we have
pointed out (p. 136), the levator anguli scapulae, because of its
connections with the great serratus, is sometimes described with it. But
inasmuch as in human anatomy these two muscles are considered
separately, and that, in the superficial layer of muscles, they are seen
in different regions--the great serratus in the thoracic, and the
levator anguli scapulae in the cervical--we prefer to study them
separately.
We remember that in man this muscle arises from the transverse processes
of the upper cervical vertebrae and is inserted into the superior portion
of the spinal border of the scapula, into the portion of this border
which is situated above the spine; it also contributes to the formation
of the floor of the supraclavicular region.
When it contracts, it draws the superior portion of the scapula forwards
and upwards, and causes a see-saw movement, for at the same time the
inferior angle of the scapula is directed backwards. Taking its fixed
point at the shoulder, it directly extends the neck if the muscle of one
side acts at the same time as that of the opposite; but if only one
muscle contracts it inclines the neck to the corresponding side.
It is to be noticed that during movements a little more active than the
ordinary the levator anguli scapulae, as moreover the other muscles of
the neck do, becomes very distinct. We have, indeed, often remarked
that, apart from these movements, each time the support of one of the
fore-limbs is brought into requisition a brusque contraction of the
muscles of this region accompanies it.
This contraction gives the impression that, as on the one hand, each
support determines a momentary arrest of progression, a jolt, and on the
other hand, the head continues to be projected in the forward direction,
the latter should be retained. But it cannot be so except by an effort
in the opposite direction--that is to say, by the brusque contraction
which we have just pointed out.
Analogous contractions also take place in a man while running at the
beginning of each contact of the lower limbs with the ground.
We may add, apropos of this latter, that displacements of the head,
sometimes in very pronounced fashion, take place during simple walking,
and that every time one of the lower limbs is carried forwards the head
is projected in the same direction. These displacements, which we also
find take place in the horse in pacing, especially in the region of the
neck and head, seem then to have the effect of aiding the progression of
the body forwards.
They occur especially in animals when drawing a heavy load, and in
individuals whose walking movements are executed with difficulty.
It is necessary to repeat that, in these cases, the individual appears
to assist the movement of his body by the impetus which the projection
of his head forward determines, in order to add--and it is for this that
we have referred to the subject--that during the intervals between each
projection the head is carried backwards by a muscular contraction
similar to that above discussed.
=The Splenius= (Fig. 68, 14; Fig. 70, 22).--In man, this muscle is
attached in the median line to the inferior half or two-thirds of the
posterior cervical ligament, to the spinous processes of the seventh
cervical, and four or five upper dorsal vertebrae; it passes obliquely
upwards and outwards, becomes visible in the supraclavicular region,
passes under the sterno-cleido-mastoid, and proceeds to duplicate the
cranial insertions of this latter; and, further, the most external
fasciculi of this muscle are inserted into the transverse processes of
the atlas and the axis.
These separate superior attachments, and the division of the muscle
which results, have caused the splenius to be regarded as formed of two
portions: splenius of the head, and splenius of the neck.
In the horse, this muscle, which is of voluminous dimensions, arises
from the superior cervical ligament, and the spinous processes of the
first four or five dorsal vertebrae; thence it proceeds to be inserted
into the mastoid crest, and the transverse processes of the atlas and
three or four vertebrae following.
The region occupied superficially by the splenius is remarkable for the
prominence which this muscle, with the deeply-seated complexus, which is
equally bulky, determines at this level; it is situated above that
region of the neck, in which are seen in part the fasciculi of the
levator anguli scapulae. It terminates above and in front in the ridge,
which is sometimes very pronounced, which the transverse processes of
the atlas make on each side of this part of the neck.
In the dog and the cat, the superior and anterior region of the neck is
thick and of rounded form, on account of the development which the
splenius presents in those animals; but it is covered by the
mastoido-humeral.
This latter relation is also found in the ox, but the splenius in this
case is but slightly developed.
When the splenius contracts it extends the head and neck, while
inclining them to its own side.
If the splenius of one side contracts at the same time as that of the
opposite, the extension takes place in a direct manner--that is to say,
without any modifying lateral movement.
Infrahyoid Muscles
Having studied the lateral surfaces of the neck, we must now examine the
anterior part of this region. Here, between the two sterno-mastoid
muscles, we find a space broader above than below, in which are situated
the larynx and the trachea, to the general arrangement of which is due
the cylindrical form which this region presents. This space corresponds
to that which in the neck of man is limited laterally by the
sterno-cleido-mastoid muscles, below by the fourchette of the sternum,
and above by the hyoid bone. In animals, as in man, it is called the
infrahyoid region.
The hyoid bone in quadrupeds is situated between the two rami or
branches of the lower jaw. Owing to this disposition, the region above
this bone, instead of having its surface projecting a little beyond the
inferior border of the maxillary bone, is depressed. This is especially
so in the horse. It is there that we find in this animal the region
known as the _trough_ (_auge_); the larynx corresponds to that part
known as the _gullet_.
The muscles which occupy the infrahyoid region are: the sterno-thyroid,
the sterno-hyoid, and the omo-hyoid. There is also a thyro-hyoid, but
because of its deep situation and its slight importance it offers no
interest from our point of view.
=Sterno-thyroid and the Sterno-hyoid Muscles.=--These two muscles, long,
narrow, and flat, arise from the anterior extremity of the sternum;
then, covering the anterior surface of the trachea, they proceed to
terminate, the one on the thyroid cartilage, and the other on the hyoid
bone. The sterno-hyoid is superficial; it covers the sterno-thyroid,
which, however, projects a little on its outer side.
=Omo-hyoid.=--This muscle does not exist in the dog or cat. It arises,
in the horse, from the cervical border of the scapula, where it blends
with the aponeurosis that envelops the subscapularis muscle, but in the
pig and the ox it arises from the deep surface of the mastoido-humeral
muscle. It is directed obliquely upwards and inwards, becoming
superficial at the internal border of the sterno-mastoid, and is
inserted into the hyoid bone.
The region in which are united the portion of the neck which we have
just studied and the neighbouring part of the thorax--that is, the
breast--has certainly, in our opinion, a form less expressive than the
corresponding region in man.
In the latter, indeed, the fourchette of the sternum, with the
hollow which it determines, the heads of the clavicles, and the
sterno-cleido-mastoid muscles, by the elevations which they produce, and
the trachea, by the situation which it occupies in the inferior part,
constitute a whole in which are admirably indicated, not only the forms
of the organs which constitute this region, but also the relations which
these organs have one with another; and, to a certain extent, their
respective functions.
In making an exception in the case of the ox, in which a fold of skin,
the _dewlap_, which passes from the neck to the breast, constitutes an
element of form which possesses some expressive value; in the horse and
in the dog, which possess no sternal fourchette and no heads of
clavicles, the bones and the muscles are found nearly on the same plane.
This produces a uniformity which is evidently inferior, from an
aesthetic point of view, to the modelling of the corresponding region of
the human body. Such, at least, is our impression.
Suprahyoid Muscles
As their name indicates, these muscles are found above the hyoid bone;
amongst those which should arrest our attention for a moment are the
mylo-hyoid and the digastric.
=Mylo-hyoid.=--This muscle, forming a sort of fleshy sling which
contributes in great measure to form the floor of the mouth, is situated
between the lateral halves of the inferior maxillary bone. Arising on
each side from the internal oblique line of the mandible, its fibres are
directed towards the median line, to be inserted posteriorly into the
hyoid bone, and, between this bone and the anterior part of the
mandible, into a median raphe which unites these latter.
=Digastric.=--This muscle arises from the styloid process of the
occipital bone and from the jugular process; it thence passes downwards
and forwards, and terminates variously, in different species. In the ox
and the horse it terminates in its anterior portion on the internal
surface of the inferior maxillary bone, close to the chin. But in the
horse a bundle of fibres is detached from the upper portion of the
muscle, to be inserted into the recurved portion of the jaw. It is to
this fasciculus that Bourgelat has given the name of '_stylo-maxillary
muscle_.'
In the pig, dog, and cat, the digastric differs more from the
corresponding muscle in man; it is not, as in the latter, formed of two
parts. The anterior portion only exists. This consists of a thick
muscular mass, which is inserted into the middle of the internal surface
of the lower jaw.
In the dog and cat it is clearly recognisable in the superficial layer
of muscles by the long and thick prominence which it produces below the
masseter, against the inferior border of the mandible (see pp. 235 and
237, the two figures showing the myology of the head of the dog).
By its contraction, it draws the lower jaw downwards and backwards.
=Panniculus of the Neck.=--This very thin muscle, which cannot be
recognised on the exterior, calls for little notice.
We shall merely point out that it duplicates the skin of the cervical
region; but as the latter is only slightly adherent to it, the
panniculus of this region seems rather destined to maintain in position
the muscles which it covers than to displace the cutaneous covering.
We recall the fact that in man, on the contrary, the muscle is very
evident at the instant of its contraction, and, for this reason, it
presents a very great interest with regard to external modelling, and it
plays an important part in the expression of the physiognomy.
MUSCLES OF THE ANTERIOR LIMBS
Muscles of the Shoulder
=Deltoid= (Fig. 68, 16, 17; Fig. 69, 18; Fig. 70, 24).--This is the
first muscle we study in connection with the shoulder in human anatomy.
Indeed, its wholly superficial position, and especially the manner in
which it is separated from the surrounding muscles, its volume, and its
characteristic modelling, give it such an importance that, from the
didactic point of view, there is every indication for commencing with
this muscle in studying the region to which it belongs. If, in regard to
quadrupeds, we also commence with it, it is merely in deference to the
spirit of method, and for the sake of symmetry; for it is far from
presenting, in the latter, characters so distinctive and so clearly
defined.
It is necessary to remark, at the outset, that in quadrupeds, on account
of the absence or slight development of the clavicle, the clavicular
portion of this muscle is, as we have shown, united to bundles of the
same kind belonging to the sterno-cleido-mastoid and trapezius to form
the mastoido-humeral (see p. 151). There exists, therefore, in an
independent form, the scapular portion only.
It is this latter which, by itself alone, forms the deltoid of
quadrupeds, a muscle known, in veterinary anatomy, as _the long abductor
of the arm_.
In the dog and the cat it consists of two parts, one of which arises
from the spine of the scapula; the other from the acromion process.
Thence it passes to the crest of the humerus, which limits the
musculo-spiral groove anteriorly, to be attached at a point which is
found, as in other quadrupeds, to be the homologue of the human deltoid
impression, or deltoid [V], of the human humerus.
In the ox, in which the acromion process, which is very rudimentary,
does not attain the level of the glenoid cavity, the acromion portion is
but slightly marked off from that which takes its origin from the spine
of the scapula.
Still, in the horse, which is completely deprived of an acromion
process, the deltoid muscle is correspondingly divided into two parts,
separated from one another by superficial interstices, but of which the
arrangement differs from that of the portions above indicated; one part,
the posterior, arises above from the superior part of the posterior
border, and the postero-superior angle of the scapula (exactly as if, in
man, certain fasciculi of the deltoid took their origin from the
axillary border and inferior angle of the scapula); the other, anterior,
arises from the tuberosity of the spine of the same bone. The two parts,
united inferiorly, proceed to be inserted into the deltoid impression or
infratrochiterian crest of the humerus.
It is necessary to add that the deltoid is inserted into the humerus,
above the insertion of the mastoido-humeral.
This muscle flexes and abducts the humerus, and also rotates it
outwards.
With regard to the other muscles of the human shoulder, subscapularis,
supraspinatus, infraspinatus, teres minor and teres major, they are also
present in quadrupeds, but in a form more elongated, as the scapula has
its dimensions more extended from below upwards--that is, from the
glenoid cavity towards the superior or spinal border.
=Subscapularis.=--This muscle occupies the subscapular fossa, from which
it takes its origin, leaving free the superior part where the surface is
found, to which are attached the serratus magnus and the levator anguli
scapulae. It passes towards the arm, to be inserted into the small
tuberosity of the humerus. It is an adductor of the arm.
The subscapularis does not offer any interest from the point of view of
external form, for it is completely covered by the scapula.
We speak of it, however, because we mention it in human anatomy, and
that it affords us here a new opportunity of bringing into prominence
the differences which exist in connection with the mobility of the
shoulder.
We remember that in man, when the arm is abducted, and then raised a
little above the horizontal, the scapula see-saws, is separated, to a
certain extent, from the thoracic cage inferiorly and externally, and
that, on the superficial layer of muscles, we are then able to see in
the bottom of the armpit, at the level of the deep portion of the
posterior wall of the latter, a small part of the subscapularis muscle.
In the animals with which we are here occupied it is not the same; for
they are incapable of performing with their fore-limbs a movement
analogous to that to which we have just referred, the humerus in their
case being retained in contact with the trunk by the muscular masses
which surround it.
=Supraspinatus= (Fig. 68, 19; Fig. 70, 25; Fig. 72, 7).--This muscle, as
its name indicates, occupies the supraspinous fossa--that is to say,
that which, by reason of the direction of the scapula in quadrupeds, is
situated in front of rather than above the spine. It arises from this
fossa; and, further, from the external surface of the cartilage which
prolongs the scapula upwards in solipeds and ruminants. It projects more
or less beyond the supraspinous fossa in front.
After passing downwards towards the humerus, it is inserted into the
summit of the great tuberosity or trochiter--that is to say, to a part
of this osseous prominence which represents the anterior facet of the
great tuberosity of the human humerus, into which, as we know, the
corresponding muscle is inserted.
In solipeds and ruminants it is inserted, by a second fasciculus, into
the small tuberosity.
In the pig and the horse its anterior border is in relation with the
terminal portion of the sterno-prescapular anterior portion of the small
or deep pectoral.
The supraspinatus, which in man is completely covered by the trapezius,
is partly visible in the superficial layer of the cat, dog, pig, and
horse, in the lower part of the space limited by the mastoido-humeral
and the trapezius. It is crossed by the scapulo-trachelian.
It is, in the ox, completely covered by these muscles, but its form,
notwithstanding this, is easily discerned by the prominence which it
produces. When it contracts, the supraspinatus muscle carries the
humerus into the position of extension.
=Infraspinatus= (Fig. 68, 20; Fig. 72, 8).--This muscle, which occupies
the infraspinous fossa, which, in quadrupeds, is situated behind the
spine of the scapula, arises from the whole extent of this fossa, and in
solipeds and ruminants encroaches on the cartilage of prolongation. Its
fibres are directed downwards and forwards, to be inserted into the
great tuberosity of the humerus--the trochiter--below the insertion of
the supraspinatus.
It is completely covered (ox and horse), or in part only (cat and dog),
by the portion of the deltoid which arises from the spine of the
scapula; nevertheless, its presence is revealed by the prominence which
it produces.
It is an abductor and external rotator of the humerus.
In connection with this muscle, which, as we have just pointed out, is
less seen in the superficial muscular layer than the supraspinatus, we
will draw attention to the fact that this arrangement is exactly the
reverse of that which is found in the human shoulder. In this latter it
is the supraspinatus which is not visible; while, on the contrary, the
infraspinatus is uncovered in a considerable part of its extent. We
further notice that it is accompanied by the teres minor, and that the
teres major, situated inferiorly, forms with these two muscles a fleshy
mass which, below, ends on the superior border of the great dorsal
muscle.
In quadrupeds, in which the infraspinatus is so slightly visible, the
teres major and minor are not found at all in the superficial muscular
layer.
Accordingly, we will say but few words about them.
[Illustration: FIG. 72.--MYOLOGY OF THE HORSE--SHOULDER AND ARM: LEFT
SIDE, EXTERNAL SURFACE.
1, Cartilage of prolongation of the scapula; 2, tuberosity of the spine
of the scapula; 3, superior extremity of the humerus; 4, inferior
extremity of the humerus; 5, radius; 6, ulna; 7, supraspinatus muscle;
8, infraspinatus; 9, teres minor; 10, biceps; 11, tendon of the biceps
passing over the anterior surface of the superior extremity of the
humerus; 12, brachialis anticus; 13, triceps, long head; 14, external
head of the triceps divided; 15, external head of the triceps reflected,
in order to expose the anconeus; 16, region normally occupied by the
external head of the triceps; 17, anconeus.]
=Teres Minor= (Fig. 72, 9).--This muscle, also called in veterinary
anatomy _the short abductor of the arm_, arises from the posterior
border of the scapula (the external border in man), and is inserted
below the great tuberosity of the humerus, between the attachments of
the infraspinatus and deltoid.
It is covered by the deltoid and the infraspinatus.
=Teres Major.=--This muscle is known to veterinarians as _the abductor
of the arm_; it arises from the postero-superior angle of the scapula
(the inferior angle of the human scapula), from which it passes to be
inserted into the internal surface of the humerus.
It is covered by the latissimus dorsi and the posterior muscular mass of
the arm.
In brief, for the better understanding of the relations of the teres
major and minor muscles in quadrupeds, we may fancy the corresponding
muscles in man modified in the following manner: The infraspinatus,
thicker, covering the teres minor; latissimus dorsi, more extended in
its superior part, covering a large proportion of the teres major. As to
the relations of the teres minor with the deltoid, they exist in man,
seeing, in this case, the same muscle is, in its external portion,
covered by this latter. With regard to the relations of the teres major
with the posterior muscular mass of the arm, they also exist in man,
since the external surface of this muscle is covered by the triceps.
These modifications are sufficient to render the small and large teres
muscles completely invisible in the superficial layer.
The muscles of the shoulder which we have just been studying fulfil,
with regard to the articulation which they surround, the function of
active ligaments. This role is made necessary by the laxity of the
scapulo-humeral capsule--a laxity which renders it incapable by itself
of maintaining the bones in contact at this joint.
The same condition exists in man.
=Panniculus Muscle of the Shoulder.=--This thin muscle covers, as its
name implies, the region of the shoulder, and is the continuation
forward of the panniculus muscle of the trunk.
It arises, by its superior part, from the region of the withers and from
the superior cervical ligament; thence its fibres descend directly
towards the elbow, to terminate at the level of the region of the
forearm.
The muscle is not found in the pig or in the carnivora.
Muscles of the Arm
We should remember, at the outset, that in man the muscles of the arm
are divided into two groups: one anterior, which contains the biceps,
brachialis anticus, and the coraco-brachialis; the other, posterior,
which is constituted by a single muscle, the triceps.
In animals, we find them in the same number and arranged in analogous
fashion--that is to say, in two groups--with respect to the bone of the
arm. But then we find that they have undergone a transformation with
regard to their length, and it is the change of general aspect which
results from this modification that we proceed to examine.
We know that in quadrupeds, and especially in the domestic animals, the
humerus is relatively short in proportion to the forearm. We have
already seen, in dealing with the bones, that whilst in the human
species the humerus is longer than the forearm, in the dog and cat these
two segments of the fore-limb are of equal length, and that the humerus
of the horse is, on the contrary, much shorter. Now, let us suppose the
human humerus to be shorter than it is in reality; the anterior muscles
undergoing, very naturally, the same reduction, will be uncovered only
slightly by those above--the deltoid and the great pectoral--or will
remain completely hidden by them. Thus would be found realized the
disposition which we meet with in quadrupeds of the muscles of this
region.
With regard to the posterior muscular mass of the arm, it does not
undergo the same change. The muscle which constitutes it--the triceps
cubiti--occupies, on the contrary, a greater area. Let us suppose,
further--for it is the best method of comprehending the homologies which
now occupy our attention--the humerus of man to be shortened as before,
and directed downwards and backwards (as in quadrupeds), this bone would
form an acute angle with the axillary border of the scapula. Let us
suppose also that the long portion of the triceps, instead of arising
solely from the superior part of this axillary border, is attached to
the whole length of the latter, and that the triceps fills the whole
interior of the angle formed by the arm and the shoulder. We then shall
have an idea of what the triceps is in quadrupeds. It is necessary to
add that the general resemblance would be still more complete if the arm
were firmly supported by the side of the thorax, because in quadrupeds
it occupies an analogous position, determined by the arrangement of the
muscles which, proceeding from the trunk and neck, are attached to it.
Anterior Region
=Biceps Cubiti= (Fig. 68, 21; Fig. 72, 10, 11).--This muscle, also
called _the long flexor of the forearm_, does not merit the name except
by its analogy with the corresponding muscle in man. Indeed, in the
domestic animals it is not divided into two parts; it is represented by
a single fasciculus, long and fusiform, situated on the front of the
humerus, and directed obliquely downwards and backwards, as the latter,
on its part, is also inclined.
It arises above from a tubercle at the base of the coracoid process,
which surmounts the glenoid cavity of the scapula. Its tendon, which is
highly developed in the solipeds, occupies the bicipital groove. We
remember that in these latter the groove in question is divided into two
channels by a median prominence.
The tendon in which the muscle ends is inserted into a tuberosity,
situated on the internal surface of the superior extremity of the
radius--the bicipital tuberosity. In the pig, the cat, and the dog,
there is detached from the tendon to which we have just referred a
fasciculus of the same nature, which, after having wound round the
radius, is inserted into the internal surface of the ulna, towards the
base of the olecranon process. From the inferior part of the muscle
arises a fibrous band, comparable to the aponeurotic expansion of the
human biceps; but, instead of passing downwards and inwards, as does the
latter, it terminates on the muscular mass which constitutes the
antero-external part of the forearm.
The biceps is not seen in the superficial layer, except in the dog and
cat (in which the humerus is, in fact, proportionately long); and even
in them only to the slightest extent. It is covered partly in these
latter, and completely in other animals, by the great pectoral and the
inferior portion of the mastoido-humeral--that is to say, that part of
the latter which represents the whole of the clavicular fibres of the
human deltoid.
The biceps is a flexor of the forearm on the arm. It also contributes to
the movement of extension of the humerus.
=Brachialis Anticus= (Fig. 68, 22; Fig. 69, 19; Fig. 70, 27; Fig. 72,
12).--In veterinary anatomy further designated as _the short flexor of
the forearm_, this muscle, which is thick, occupies the musculo-spiral
groove, and arises from it, reaching upwards to just below the head of
the humerus. But it does not, as in man, extend to the internal surface
of the bone.
Situated on the outside of the biceps, it is directed towards the
forearm, and terminates by a flattened tendon, which, dividing into two
slips, passes below the bicipital tuberosity, on the internal surface of
the radius, into which one of these slips is inserted, while the other
proceeds to terminate on the ulna.
The inferior half of this muscle is visible on the superficial layer, in
the space limited posteriorly by the triceps brachialis, and below by
the muscles of the forearm, which correspond to the external muscles of
the human forearm, and in front by the great pectoral and the
mastoido-humeral. It is in the upper part of the interspace which
separates these latter from the brachialis anticus that the deltoid
insinuates itself to proceed to its insertion into the humerus.
These relations precisely recall those which we meet with when we
examine the external surface of the human arm, with this difference,
however--that in the latter the anterior brachialis anticus is
extensively related, in front, to the biceps. However, in animals it is
not absolutely the same, since, as we have shown above, the biceps is
covered, more or less completely, by the mastoido-humeral and the great
pectoral.
The brachialis anticus flexes the forearm on the arm.
=Coraco-brachialis.=--In man this muscle, which occupies the superior
half, or third, of the internal surface of the humerus, is visible only
when the arm is abducted, and then especially when it approaches the
vertical position; indeed, it is only in this attitude that the region
which it occupies is accessible to view.
But an analogous attitude not being possible in domestic animals, in
which the arm is fixed along the corresponding parts of the trunk, the
result is that the coraco-brachialis is always covered, and that,
consequently, it presents nothing of interest from our point of view. We
speak of it, then, merely in order to complete the series of the muscles
of the anterior surface of the arm, among which we rank it, in spite of
the fact that in veterinary anatomy it is described as a muscle of the
shoulder.
It arises above from the coracoid process, and thence passes downwards
towards the internal surface of the humerus into which it is inserted,
more or less high up, according to the species. The coraco-brachialis is
an adductor of the arm.
Posterior Region
=Triceps Cubiti= (Fig. 68, 23, 24; Fig. 69, 20, 21; Fig. 70, 28, 29;
Fig. 72, 13, 14, 15, 16).--This muscle, which is voluminous in the
quadrupeds with which we are here concerned, fits more or less
completely the angular space between the scapula and the humerus. Its
bulk forms a thick prominence, which surmounts the elbow and the
forearm.
We should say, with regard to this mass, that if the deltoid does not
constitute in quadrupeds a prominence sufficient to remind one of that
which this muscle produces in man, the triceps, in producing an
analogous elevation, seems to replace in the general form of the body
the relief which the deltoid is incapable of producing.
The triceps is divided into three portions, which, as in man, have the
names middle, or long head; external and internal heads. But that which
renders the nomenclature a little complicated is that veterinary
anatomists have given other names to these three parts: that of _great
extensor of the forearm_ (caput magnum) to the long head; _the short
extensor of the forearm_ (caput parvum) to the external head; and of
_medium extensor of the forearm_ (caput medium) to the internal.[24]
[24] Other names given by certain authors to the parts of this muscle
which we have just enumerated still further complicate this
nomenclature.
The long head is further designated by them under the names of the
_long_ or _great anconeus_; the _external head_ under those of _external
anconeus_, or _lateral_ or _short anconeus_; whilst the internal head
becomes the _internal anconeus_, or _median_.
It is more especially the long portion and the external head which,
being visible on the external surface of the arm, contribute to the
external form.
The long portion, which is triangular in shape and of considerable
development, arises in the cat and the dog from the inferior half or
two-thirds of the posterior border of the scapula (axillary border);
from the whole extent of that border as far as the superior posterior
angle in the pig, the ox, and the horse; it then passes downwards
towards the articulation of the elbow, to terminate in a tendon which is
inserted into the olecranon process. The portion of this muscle which is
next the scapula is covered by the deltoid.
The external head, situated below the long portion, is directed
obliquely downwards and backwards. It arises from the curved crest
which, from the deltoid impression of the humerus, is directed upward to
meet the articular head of the same bone. This crest limiting the
musculo-spiral groove superiorly, and the brachialis anticus arising
from the whole extent of this groove, the result is that at this level
the external head is in relation with the brachialis anticus. From this
origin it is directed towards the elbow, to be inserted into the
olecranon, either directly or by the medium of the tendon of the long
portion. The part of this muscle which arises from the humerus is
covered by the deltoid.
As for the internal head (Fig. 76, 4), which, in the superficial layer,
is only visible in its inferior part, on the internal aspect of the arm
in those animals in which the elbow is free of the lateral wall of the
thorax (the dog and the cat, for example), it arises from the internal
surface of the humerus, and thence proceeds to be inserted into the
olecranon.
The triceps extends the forearm on the arm.
A fourth muscle exists, which veterinary anatomists include in the study
of the three portions of the triceps which we have just been discussing,
in giving it the name of _small extensor of the forearm_. But, as this
muscle is no other than the anconeus, and as, in human anatomy, we
describe the latter, according to custom, in connection with the
forearm, it is when on the subject of the latter that we will concern
ourselves with it. This grouping of muscles cannot fail to give greater
clearness to the description of the muscles of these regions.
=The Supplemental or Accessory Muscle of the Latissimus Dorsi= (Fig. 76,
2; Fig. 77, 1).--Because of the relations, to which we have already
referred (see p. 142), of this muscle with the triceps cubiti, its
description very naturally follows that of the latter.
Indeed, this supplementary muscle of the great dorsal is further
designated in zoological anatomy under the name of _long extensor of the
forearm_; and this name indicates that its study may be united to that
of the triceps.
Situated on the internal surface of the arm, it arises from the external
aspect of the tendon of the latissimus dorsi; it is very highly
developed in the horse, in which it also arises from the posterior
border (axillary) of the scapula; then, covering in part the internal
head of the triceps and also the long portion, on the superior border of
which it is folded, it proceeds to be inserted into the olecranon
process and the anti-brachial aponeurosis.
It extends the forearm on the arm. Further, it makes tense the
aponeurosis into which it is inserted; this explains the name of _tensor
of the fascia of the forearm_, which is sometimes given to it.
It seems to us interesting to add that, abnormally, we sometimes find in
man an analogue of this muscle. It is given off from the latissimus
dorsi, near the insertion of the latter into the humerus; it accompanies
the long head of the triceps and becomes fused with it. Sometimes it is
inserted into the olecranon process, at other times into the
antibrachial aponeurosis or the epitrochlea. It is on account of its
insertion into the last-mentioned, in some cases, that it is also
designated by the name of _dorso-epitrochlear_ muscle.[25]
[25] L. Testut, 'Anomalies musculaires chez l'homme expliquees par
l'anatomie comparee,' Paris, 1884, p. 118. A. F. Le Double,
'Traite des variations du systeme musculaire de l'homme et de
leur signification au point de vue de l'anthropologie
zoologique,' Paris, 1897, t. i., p. 203. Edouard Cuyer,
'Anomalies musculaires' (_Bulletins de la Societe
Anthropologique_, Paris, 1893).
Muscles of the Forearm
Before commencing the special examination of each of the muscles of this
region, it is absolutely indispensable to consider their general
arrangement, and to determine very clearly how we should study them. We
are too well convinced of the importance of this preliminary examination
to dismiss it without entering rather fully into it. Indeed, the region
on the myological study of which we are now entering is, unquestionably,
one of the most complicated with which we have to deal. We know besides,
in regard to the study of the forearm in man, how much a definite method
is necessary in order that the arrangement of the muscles of this region
be fixed in the memory, and that we are unable to obtain this result
otherwise than by grouping the twenty muscles which constitute it in
clearly defined regions.
We also know that these muscles are first studied with the forearm in
the position of supination, and that it is only when they are well known
after having considered them in this position that we are able to
analyze and comprehend their forms when it is in pronation.
Now, as we have pointed out in the section on osteology (see p. 34), the
forearm in quadrupeds is always in the position of pronation. Should we,
then, in order to maintain the symmetry with human anatomy, first study
the forearm in the position of supination? Evidently not. Besides the
fact that this would in some cases be impossible since--as in the horse,
for example--the radius and ulna are fused together, we should not gain
any advantage; this position being never completely realizable even in
those quadrupeds which have the radius relatively movable--as, for
example, in the cat.
Accordingly, it is pronation which here, in connection with animals,
becomes the standard attitude from the point of view of description.
This is why, supposing that the reader knows well the muscles of the
human forearm in the position of supination, we should recall what is
the general arrangement occupied by these muscles when it is in
pronation.
The fore-limb, being viewed on its anterior surface, presents above the
anterior aspect of the region of the elbow; but below, it is the
posterior surface of the wrist which is seen. Consequently, in the
superior part, we see the external and anterior muscles limiting the
hollow in front of the elbow; interiorly are found the posterior
muscles.
The long supinator, passing obliquely downwards and inwards, divides, in
fact, the forearm into two parts: one supero-internal, the other
infero-external. In the first we see, but to an extent less and less
considerable, the pronator teres, the flexor carpi radialis, the
palmaris longus, and the flexor ulnaris; as to the flexors of the
digits, on account of the rotation of the radius, they are only visible
on the opposite surface--that is to say, on the surface of the wrist,
which is now posterior. In the second part we see the two radial
extensors, the common extensor of the fingers, the proper extensor of
the little finger, and the ulnar extensor which, inferiorly, remains
behind, by reason of the position of the ulna being unchanged, whilst
the anconeus is wholly posterior, since the direction of the elbow is
not modified. We also find, in this region, the long abductor of the
thumb, the short extensor of the thumb, the long extensor of the thumb,
and the special extensor of the index-finger, in the region where these
deep muscles become superficial.
So that, to summarize, the external and posterior muscles occupy the
anterior and external regions of the forearm, whilst the anterior
muscles occupy rather the internal and posterior. It is in regarding
them after this manner--that is to say, arranged in these two
regions--that we proceed to study these muscles in quadrupeds.
Anterior and External Region
=Supinator Longus.=--We know that this muscle, which is especially a
flexor of the forearm on the arm, plays, notwithstanding the name which
has been given it, a part of but little importance in the movement of
supination.
It acts slightly, however, as a supinator, for, being very oblique
downwards and inwards at the time of pronation, it is able, while
tending to resume its vertical direction, to carry the radius outwards;
it places, in fact, the forearm in a position midway between pronation
and supination.
We have just recalled these details, in order that it may be more easy
to understand why it does not exist in animals in which the radius and
ulna are fused together (horse, ox); and why, on the other hand, we find
traces of it in the cat and the dog, in which the radius--to a slight
extent, it is true--is able to rotate on the ulna. This displacement
being a little more considerable in the felide, the long supinator is a
little further developed than it is in the canine species; but,
notwithstanding, it is only rudimentary.
The long supinator arises, above, from the external border of the
humerus; thence, in the form of a very narrow fleshy band, it passes
obliquely downwards and inwards, to be inserted into the inferior part
of the internal surface of the radius.
It assists in turning the radius outwards and placing it in front of the
ulna, the movement of supination being capable of being but little
further extended.
=First and Second External Radial Muscles=: _Extensor carpi radialis
longior and brevior_ (Fig. 73, 8; Fig. 74, 8, 9; Fig. 75, 8, 9).--Fused
together, these muscles form by their union what veterinary anatomists
call _the anterior extensor of the metacarpus_. But we should add that
these two muscles are united so much the more intimately as we examine
them in passing successively from the cat to the dog, pig, ox, and
horse. Thus, in the cat they are often distinct; in the dog, they unite
only at the level of the middle third of the radius, and interiorly they
have two tendons; in the pig, the ox, and the horse they are completely
united, and there exists but a single tendon.
The _anterior extensor of the metacarpus_, which is situated behind the
long supinator when the latter exists, occupies the external aspect of
the forearm; its well-defined form absolutely recalls the prominence on
the superior part of the external margin of the human forearm.
It arises superiorly from the portion of the external border of the
humerus which is situated above the epicondyle and behind the
musculo-spiral groove. Its fleshy mass appears in the angular space
bounded by the brachialis anticus and the triceps. The superior portion
is covered by the external head of the triceps; yet, in the dog, the
superior portion of its humeral attachment is the only part so covered.
This muscle is directed forward and downwards; it is also inclined a
little inwards in such manner as to proceed to occupy the anterior
aspect of the forearm.
Its fleshy belly is narrowed below, and, towards the inferior part of
the forearm, is continued by a tendinous portion which is situated on
the anterior surface of the carpus, after having traversed the median
groove of the inferior extremity of the radius.
In the cat and the dog, in which the union of the two radial extensors
is incomplete, the two tendons are inserted into the front of the base
of the second and third metacarpal bones; consequently, as in man, into
the metacarpals of the index and middle fingers.
In the ox, the tendon, which is single, is inserted into the internal
and anterior half of the superior extremity of the principal metacarpal.
In the pig, this tendon is attached to the base of the large internal
metacarpal.
In the horse, the corresponding tendon is attached to a tubercle which
is situated on the anterior surface of the base of the principal
metacarpal, a little internal to the median plane of the latter.
[Illustration: FIG. 73.--MYOLOGY OF THE DOG: LEFT ANTERIOR LIMB,
EXTERNAL ASPECT.
1, Mastoido-humeralis; 2, biceps; 3, brachialis anticus; 4, triceps,
long portion; 5, triceps, external head; 6, olecranon process; 7,
epicondyle; 8, radialis muscles (anterior extensor of the metacarpus);
9, extensor communis digitorum (anterior extensor of the phalanges); 10,
extensor minimi digiti (lateral extensor of the phalanges, or common
extensor of the three external digits); 11, posterior ulnar (external
flexor of the metacarpus); 12, pisiform bone; 13, anconeus; 14, extensor
ossis metacarpi pollicis and extensor primi internodii pollicis (oblique
extensor of the metacarpus); 15, radius; 16, anterior ulnar (oblique
flexor of the metacarpus); 17, external border of the hypothenar
eminence (abductor of the little finger).]
In order to properly understand and remember the respective positions
occupied by these inferior insertions, it must be remembered that the
human forearm being in the position of pronation, the tendons of the
radials are attached to the bases of the metacarpals nearest to the
thumb--that is to say, those occupying an internal position as regards
the fourth and fifth metacarpals.
As its name indicates, this muscle extends the metacarpus. Consequently
it is, in the horse, an extensor of the canon-bone.
It is also an adductor of the hand in those animals (cat, dog) in which
the radio-carpal articulation, analogous in form to the corresponding
articulation in man, permits lateral movements of the hand on the
forearm. The union of the fleshy bodies of the two radials is sometimes
found in the human species.
=Supinator Brevis.=--As in the case of the long supinator, the short
supinator is found only in animals in which the radius can be rotated to
a greater or less extent around the ulna; therefore this muscle is not
found in the pig, the ox, or the horse; but it forms part of the forearm
of the cat and the dog.
Deeply situated at the region of the elbow, the short supinator has
little interest for us. All that we will say of it is that it goes from
the external part of the inferior extremity of the humerus to the
superior part of the radius; and that it is, in carnivora, the essential
agent in the production of the movement of supination.
=Extensor Communis Digitorum= (Fig. 73, 9, 10, 11; Fig. 74, 10, 11,
12).--Also named in veterinary anatomy the _anterior extensor of the
phalanges_, this muscle is situated external to and behind the anterior
extensor of the metacarpus already described.
In the human being, the common extensor of the fingers springs, in its
superior part, from the bottom of a depression, situated on the outer
side of and behind the elbow, and limited in front by the muscular
prominence which the long supinator and the first radial extensor form
at that level. At the bottom of this hollow or fossette is found the
epicondyle, which gives origin, amongst other muscles, to the common
extensor of the fingers. It is necessary to add that it is most
prominently visible during supination, and that it tends to be effaced
during pronation.
[Illustration: FIG. 74.--MYOLOGY OF THE OX: LEFT ANTERIOR LIMB, EXTERNAL
ASPECT.
1, Mastoido-humeralis; 2, pectoralis major; 3, deltoid; 4, brachialis
anticus; 5, triceps; 6, triceps, external head; 7, olecranon; 8, radial
extensors (anterior extensor of the metacarpus); 9, insertion of the
tendon of the anterior extensor of the metacarpus to the tubercle of the
superior extremity of the principal metacarpal; 10, 11, extensor
communis digitorum (10, proper extensor of the inner digits; 11, common
extensor of the two digits); 12, tendon of the common extensor of the
two digits; 13, band of reinforcement from the suspensory ligament of
the fetlock; 14, external tuberosity of the superior extremity of the
radius; 15, extensor minimi digiti (proper extensor of the external
digit); 16, tendon of the proper extensor of the external digit; 17,
posterior ulnar (external flexor of the metacarpus); 18, pisiform; 19,
extensor ossis metacarpi pollicis and extensor primi internodii pollicis
(oblique extensor of the metacarpus); 20, ulnar portion of the deep
flexor of the toes; 21, tendon of the superficial flexor of the toes
(superficial flexor of the phalanges); 22, tendon of the deep flexor of
the toes (deep flexor of the phalanges); 23, suspensory ligament of the
fetlock.]
An analogous arrangement is met with in animals. But the muscular
prominence is formed by the united radial extensors, and the fossette,
because of the permanent pronation of the forearm, is scarcely
recognisable. Likewise, with regard to the dog, we may say that it does
not exist, on account of the prominence which the epicondyle forms in
that animal (Fig. 73, 7).
In connection with this prominence of the epicondyle, it is interesting
to add that this detail recalls the relief which the same process
produces on the external aspect of the human elbow when the forearm is
flexed on the arm. We know that, in this case, the epicondyle is
exposed, because the muscles which mask it in supination (long supinator
and long radial extensor) are displaced and set it free during flexion.
But, in the dog, as in other quadrupeds besides, the forearm is, in the
normal state, flexed on the arm; the latter being oblique downwards and
backwards, and the former being vertical. Further, the epicondyle is
well developed.
The muscle with which we are now occupied, long and vertical in
direction, arises from the inferior part of the external border of the
humerus (there it is covered by the anterior extensor of the metacarpus,
from which it is freed a little lower down) and from the external and
superior tuberosity of the radius. In the carnivora, it arises from the
epicondyle. Its fleshy body is fusiform in shape, becomes tendinous in
the lower half of the forearm, and then divides into a number of slips,
varying in number according to the species; this division is correlated
to that of the hand--that is to say, with the number of the digits.
Before reaching this latter, the common extensor of the digits passes
through the most external groove on the anterior surface of the inferior
extremity of the radius.
[Illustration: FIG. 75.--MYOLOGY OF THE HORSE: LEFT ANTERIOR LIMB,
EXTERNAL ASPECT.
1, Mastoido-humeral; 2, pectoralis major; 3, deltoid; 4, brachialis
anticus; 5, triceps, long head; 6, triceps, external head; 7, olecranon;
8, radial extensors (anterior extensor of the metacarpus); 9, insertion
of the tendon of the anterior extensor of the metacarpus into the
tubercle of the superior extremity of the principal metacarpal; 10,
extensor communis digitorum (anterior extensor of the phalanges); 11,
tendon of the anterior extensor of the phalanges; 12, reinforcing band
arising from the suspensory ligament of the fetlock; 13, external
tuberosity of the superior extremity of the radius; 14, extensor minimi
digiti (lateral extensor of the phalanges); 15, tendon of the lateral
extensor of the phalanges; 16, fibrous band which this latter receives
from the tendon of the anterior extensor of the phalanges; 17, fibrous
band which the same tendon receives from the carpal region; 18,
posterior ulnar (external flexor of the metacarpus); 19, pisiform; 20,
extensor ossis metacarpi pollicis and extensor primi internodii pollicis
(oblique flexor of the metacarpus); 21, ulnar portion of the deep flexor
of the digits; 22, flexor digitorum profundus; 23, 23, tendon of the
flexor digitorum sublimis (superficial flexor of the phalanges); 24, 24,
tendon of the flexor digitorum profundus (deep flexor of the phalanges);
25, sesamoid prominence; 26, suspensory ligament of the fetlock; 27,
external rudimentary metacarpal.]
In the cat and the dog, the four tendons which result from the division
of the principal tendon go to the four last digits, and each of them is
inserted, as in the human species, to the second and third phalanges.
In the pig, the anterior extensor of the phalanges is rather complicated
in its arrangement. Its fleshy body is divided into four bundles
terminated by tendons, which in turn divide and join certain digits;
whence the special names given to each of these fasciculi, commencing
with the most internal, of: _proper extensor of the great inner toe_;
_common extensor of the two inner toes_; _common extensor of the two
outer toes_; and _proper extensor of the great outer toe_.
In the ox, the same muscle is divided into two bundles: the internal
proceeds to the internal toe, the external is common to the two toes.
In the horse, the tendon of the anterior extensor of the phalanges is
divided into two parts of unequal bulk. The smaller of these tendinous
slips, which is the more external, unites at the level of the superior
part of the metacarpus with the tendon of the muscle which we are about
to study in the following paragraph (Fig. 75, 16). The larger, after
having reached the anterior surface of the digit, is attached to the
anterior aspect of the first and second phalanges, and then forms a
terminal expansion which is inserted into the pyramidal eminence of the
third.
At the level of the first phalanx this tendon receives on each of its
lateral aspects a strengthening band, which proceeds from the terminal
extremity of _the suspensory ligament of the fetlock_,[26] and crosses
obliquely downwards and forwards over the surface of the first phalanx
to join the extensor tendon (Fig. 75, 12).
[26] See p. 200 for a description of this ligament.
A similar arrangement is found in the ox.
This band is noticeable under the skin which covers the lateral aspects
of the ham.
As the name indicates, this muscle extends the phalanges, one upon the
other. It also contributes to the extension of the hand, as a whole, on
the forearm.
=Extensor Minimi Digiti= (Fig. 73, 10; Fig. 74, 15, 16; Fig. 75, 14,
15).--This muscle, _the lateral extensor of the phalanges_ of veterinary
anatomy, situated on the external surface of the forearm, behind the
common extensor of the digits, arises, as a rule, from the epicondyle
(dog, cat), or from the external surface of the superior extremity of
the radius (horse). The tendon succeeding to the fleshy body appears
towards the lower third of the forearm, and at the level of the wrist
lies in a groove analogous to that which in man is hollowed out for the
passage of the corresponding tendon at the level of the inferior
radio-ulnar articulation. This groove corresponds to the same
articulation in animals in which the ulna is well developed, such as the
dog and the cat; but it belongs to the radius when the inferior
extremity of the ulna does not exist--for example, in the horse. Indeed,
in this animal the groove in question is found on the external surface
of the carpal extremity of the radius.
In the dog, the tendon is divided into three parts, which, crossing
obliquely the tendons of the common extensor of the digits, pass to the
three external digits, to be inserted by blending with the corresponding
tendons of the latter into the third phalanges of those digits.
Thus is explained the name of _common extensor of the three external
digits_ which is sometimes given to this muscle.
In the cat, there is a fourth tendon, which passes to the index-finger,
so that the name _common extensor of the four external digits_ is in
this case legitimate, and the lateral extensor of the phalanges is also
a common extensor, as is the anterior extensor of the phalanges, or
common extensor of the digits.
In the pig, the tendon, which is single, is inserted into the external
digit, for which reason it has received the name of the _proper extensor
of the small external digit_. This muscle is, then, really the homologue
of that which exists in the human species.
In the ox, it is called the _proper extensor of the external digit_; it
is as thick as the common extensor.
Finally, in the horse, the muscle is little developed. Its fleshy body,
thin and flattened from before backwards, becomes distinctly visible
only below the middle of the forearm. Above, it is enclosed in a limited
space, bounded in front by the common extensor of the digits, and
behind by the posterior ulnar; there these two muscles approach each
other so closely that from the point of view of external form they seem
to be nearly in contact.
The tendon, after receiving the small fasciculus from the common
extensor (Fig. 75, 16), as well as a fibrous band emanating from the
external surface of the carpus (Fig. 75, 17), is situated at the
external side of the tendon of the anterior extensor of the phalanges,
and is inserted into the anterior surface of the superior extremity of
the first phalanx.
This muscle extends the digit or digits into which it is inserted. It
also assists in the movement of extension of the hand as a whole.
=Posterior Ulnar= (_Extensor carpi ulnaris_) (Fig. 73, 11; Fig. 74, 17;
Fig. 75, 18).--Designated by veterinary anatomists as the _external
flexor of the metacarpus_,[27] or _external cubital_, this muscle is
situated in the posterior region of the external surface of the forearm,
behind the lateral extensor of the phalanges.
[27] Certain authors give it the name of _ulnar extensor of the
wrist_. It is true that in the human being this is its action;
but in quadrupeds, owing to its insertion into the pisiform, it
draws the hand into the position of flexion.
It arises from the epicondyle; its fleshy body, thick but flattened, is
directed vertically towards the carpus, and its tendon is inserted into
the external part of the superior extremity of the metacarpus, after
having given off a fibrous band, which takes its attachment on the
pisiform.
It is inserted, in the cat and the dog, into the superior extremity of
the fifth metacarpal; in the pig to the external metacarpal; in the ox
to the external side of the canon-bone; in the horse to the superior
extremity of the external rudimentary metacarpal.
This muscle flexes the hand on the forearm, and in animals in which the
radio-carpal articulation permits, by its formation, it inclines the
hand slightly outwards--that is, abducts it.
=Anconeus= (Fig. 72, 17; Fig. 73, 13).--We have already stated (p. 174)
that the anconeus is included with the triceps brachialis in zoological
anatomy, and that veterinary anatomists give it the name of _small
extensor of the forearm_.[28]
[28] It is also called by some authors, the _small anconeus_.
In the dog it recalls, as to position, the human anconeus, but with this
difference--that, in the latter, the anconeus, triangular in outline,
has one of its angles turned outwards (the epicondyloid attachment) and
one of its sides turned towards the olecranon. Here it is entirely the
opposite. The anconeus, similarly triangular, is broader externally. At
this level it takes its origin from the external border of the humerus,
the epicondyle, and the external lateral ligament of the articulation of
the elbow; thence its fibres converge towards the external surface of
the olecranon, to be there inserted.
It is in relation, anteriorly and inferiorly, with the posterior ulnar
muscle. It is covered superiorly by the external head of the triceps. In
the cat the disposition of the anconeus is analogous. But in the other
quadrupeds with which we are here concerned it is completely covered by
the external head of the triceps. It really participates in the
formation of the triceps; and seeing that it takes origin from the
posterior surface of the humerus at the margin of the olecranon fossa
(Fig. 72), and proceeds thence towards the olecranon to be inserted, we
can understand why veterinary anatomists have connected its study with
that of the posterior muscular mass of the arm.
This muscle is an extensor of the forearm on the arm.
We proceed now to inquire what the deep muscles of the posterior region
of the human forearm become in quadrupeds: the long abductor of the
thumb, the short extensor of the thumb, the long extensor of the thumb,
the proper extensor of the index. We know that in every instance these
muscles, which are deeply seated at their origin, become superficial
afterwards.
In quadrupeds, on account of the position in which the forearm is
placed--viz., pronation--the corresponding muscles occupy the anterior
aspect of this region.