PECTORAL AND ABDOMINAL MUSCLES.
Of _muscles_ in general.--_Muscular contraction_: changes
of form; movements produced. Composition of muscles; fleshy
belly and tendon; aponeurosis. Nomenclature of muscles, their
classification into long, broad, and short muscles; their
arrangement: superficial and deep. Muscles of the trunk,
anterior region.--_Great pectoral muscle_, its relation to the
armpit, its action.--_The lesser pectoral._--The _breast_ and
_nipple_.--The _external oblique muscle of the abdomen_, its
fleshy part, its abdominal aponeurosis; _linea alba_.--Groove
and space bounded by the prominence of the _external oblique_
and _rectus_.--_Internal oblique_ and _transversalis_
muscles.--_Rectus abdominis_ muscle, its aponeurotic sheath,
its aponeurotic intersections.--_Linea semilunaris_; _lineæ
transversæ_.
The form of the body is produced largely by the muscles. Each
muscle is formed of a peculiar tissue called _muscular tissue_ or
_muscular fibre_, which has the property of changing its form and of
_contracting_ under the influence of nervous action, in most cases
controlled by the will. For example, in contracting the biceps muscle
on the front of the arm, we see that this muscle, which in a state of
repose is fusiform and long, becomes in action (in contraction) short,
thick, and rounded; and as it is attached below to one of the bones
of the forearm, it draws the anterior surface of the forearm towards
the anterior surface of the upper arm and produces flexion of the
elbow-joint. This simple experiment, which is easy to repeat at any
moment, gives a clear idea of the part which muscles play in the animal
economy, and of the part that they take in producing the external form;
for it shows that they are the _active agents_ of movements in which
the bones are the _passive levers_, and, at the same time, that a
muscle in action presents a very different shape from that which it has
in a state of repose, a change which may be stated in a more general
manner by saying that in action a muscle becomes shorter, thicker, and
more prominent.
[Illustration: FIG. 66.
DIAGRAM OF A CONTRACTED MUSCLE (Biceps).]
In general, besides their _fleshy mass_, the only part which contracts
and changes its form, the muscles have extremities more or less slender
called _tendons_, formed of a white fibrous tissue, which are, as it
were, actual cords by which the muscle is attached to the bones; during
the contraction of muscle these tendons do not change in form, but, as
with all tight cords when in a state of tension, they start up, and
become more visible and clearly marked beneath the skin (Fig. 66).
Each muscle is enveloped by a fibrous membrane called its
_aponeurosis_, which frequently takes the place of a tendon, if the
muscle is broad and thin, so that in these cases the attachment of the
muscle, instead of being rounded in the form of a cord, is flattened in
the form of a membrane, and the term aponeurosis is at times applied
to these membranous tendons. Membranous tendons in the contraction of
a muscle give rise to depressions on the surface of the body in the
regions where they occur superficially.
The muscles are variously named according to various ideas; sometimes
from the region they occupy (_pectoral_, _gluteal_, _anterior brachial
muscles_, _&c._) or from the direction of their fibres (_oblique of the
abdomen_), or from their dimensions (_gluteus maximus, gluteus medius,
and peroneus longus, &c._), or again from their form (_trapezius,
rhomboid, serratus_), or from their structure (_semi-membranosus,
semi-tendinosus_). Another mode of nomenclature, which Chaussier
attempted to make general, forms the name of a muscle by the
association of the names of the bones to which it is attached; in this
way we get the _sterno-cleido-mastoid muscle_, and most of the muscles
of the neck. This nomenclature, however, cannot be easily applied to
all muscles, for it would make some names of an inconvenient length,
owing to the complexity of the bony attachments.
Before we terminate this rapid sketch it may be useful, with regard
to plastic anatomy, to point out how the various muscles are classed,
according to their form and situation. 1st. With regard to their form
we distinguish the muscles as _long_, _broad_, and _short_; the _long
muscles_, consisting in general of a fleshy belly fusiform in shape,
and with cord-like tendons, are principally distributed among the great
segments of the limbs (arm, forearm, thigh, leg); the _broad muscles_,
with their fibres arranged in broad and thin layers, and with tendons
membranous and aponeurotic, are almost exclusively found in the trunk
(pectorals, trapezius, latissimus dorsi, &c.); lastly, the _short
muscles_, frequently without tendons, and inserted directly into the
bone, are found principally at the extremities of the limbs (hand and
foot) and in the face.
2nd. With regard to their situation we distinguish the muscles as
_superficial_ and _deep_. The _superficial muscles_ (Fig. 67) are those
which are entirely visible when the subject is stripped of its skin,
and of which, accordingly, the fleshy bellies and tendons are marked in
their principal details on the external form. These superficial muscles
should be carefully studied here with regard to their insertions,
forms, and actions. The _deep muscles_, situated beneath the preceding,
form fleshy masses, which are shown externally by the manner in which
they fill up the external depressions of the skeleton, and raise up the
superficial muscles. It is enough for the artist to know the general
indications of these muscular masses, without studying the insertions
and forms of each of the muscles composing any given mass.
MUSCLES OF THE TRUNK.
Under this title we will now study the anterior muscles of the trunk
(_pectoral and abdominal_), and the muscles of the back and the
posterior surface of the neck (_trapezius_ and _latissimus
dorsi_). The study of the muscle which covers the lateral wall of
the chest (_serratus magnus_) will be taken after that of the
muscles of the shoulder and armpit.
[Illustration: FIG. 67.
GENERAL VIEW OF THE SUPERFICIAL MUSCLE (the _Gladiator_ of
Agasias with the skin removed. Compare with Figure 4, page 24).]
_Great pectoral muscle._--The great pectoral muscle (_pectoralis
major_) forms a large fleshy mass (1, Fig. 68) which covers the
anterior surface of the thorax, on each side of the middle line of the
sternum, and extends outwards as far as the upper part of the arm. This
muscle is attached (1st) to the inner half of the anterior border of
the clavicle (2, Fig. 68); (2nd), to the whole extent of the anterior
surface of the sternum (3, Fig. 68); and (3rd), to the aponeurosis
of the external oblique muscle of the abdomen. It arises, also, more
deeply from the cartilages of the true ribs (except the first) (9,
9, 9, Fig. 68). From these thoracic origins the muscular fibres are
directed towards the arm; the upper fibres pass obliquely from above
downwards and outwards; the middle fibres pass transversely outwards,
and the inferior, obliquely from below upwards. Towards the insertion
of the muscle these different parts cross over each other in a twisted
form, so that the upper (clavicular) fibres are placed in front of
the inferior (sternal) fibres (7, Fig. 68), which correspond to the
anterior margin of the armpit. The muscle is inserted into the outer
lip of the bicipital groove of the humerus, the clavicular fibres,
known as the _portio attollens_, being inserted lower down the arm than
the external fibres (_portio deprimens_). The former fibres raise the
arm upwards, the latter draw it downwards in relation to the trunk.
[Illustration: FIG. 68.
THE MUSCLES OF THE ANTERIOR SURFACE OF THE THORAX (to the right
the superficial muscles; to the left the deep muscles).--1,
the great pectoral muscle;--2, its clavicular fibres;--3,
its costo-sternal fibres;--4, 5, 6, its origin from the
aponeurosis of the abdomen;--7, its external portion formed
by the superposition of the preceding fibres (9, bundles
of fibres arising from the cartilages of the ribs; 10, the
pectoralis minor);--11, the subclavius;--12, the deltoid;--14,
the digitations of the serratus magnus;--15, the digitations of
the external oblique of the abdomen;--16, anterior border of
the latissimus dorsi, and (17) tendon of the same;--18, teres
major muscle;--19, the subscapularis;--20, the long head of the
triceps brachialis; 21, the humeral extremity of the deltoid;
22, 22, the sterno-cleido-mastoid;--23, the sterno-hyoid;--24,
the trapezius muscle.]
When the arm is hanging beside the trunk the great pectoral muscle
presents a four-sided square, in which we can distinguish four borders:
one supero-external (in contact with the anterior border of the
deltoid, 12, Fig. 68), another superior, in the line of the clavicle;
the third, internal or sternal, curved with its convexity inwards; and
the fourth, infero-external or axillary (forming the anterior boundary
of the armpit). But when the arm is horizontal, and especially when it
is raised above the horizontal (Fig. 70), the figure of the muscle is
represented by a triangle with the base inwards, as the clavicular and
supero-external borders then come into line with one another.
The action of the great pectoral muscle is essentially to draw the
arm to the trunk; its shape becomes prominent when we carry the arms
forward and bring them near each other, as in the attitude of prayer.
It also becomes well marked in climbing, for then the muscle takes its
fixed point at the humerus and draws the trunk towards it. Acting in
a similar manner upon the thorax, with the humerus as a fixed point,
this muscle elevates the ribs, and consequently expands the thorax in
respiration. Thus we see that the muscle contracts when the inspiratory
muscular powers are brought into play as in movements like struggling,
anguish, or agony.
[Illustration: FIG. 69.
OUTLINE OF THE FEMALE BREAST.]
_The breast and nipple._--On the surface of the great pectoral muscle
is placed the breast in the female. In the male only the nipple and
surrounding _areola_ are found, usually occupying a position at the
level of the space between the fourth and fifth ribs on each side. The
form of the breast varies in the female with age. In a young woman it
is normally round and firm, with its lower aspect more rounded than its
upper surface, which is slightly flatter (in the erect position) (Fig.
69).
[Illustration: FIG. 70.
THE MUSCLES OF THE ANTERIOR WALL OF THE TRUNK.--1, 2, 3,
the great pectoral;--4, 4, the external oblique of the
abdomen;--5, 5, the serratus magnus;--6, 6, the anterior border
of the latissimus dorsi;--7, 8, the inferior portion of the
sternum;--9, the aponeurosis of the external oblique;--10,
linea alba;--11, umbilicus;--12, 12, 12, the tendinous
intersections of the rectus abdominis;--13, the inguinal
ring;--14, the pyramidalis of the abdomen;--15, the external
border of the rectus abdominis;--16, the sterno-hyoid;--17,
the omo-hyoid;--18, sterno-cleido-mastoid;--19, the
trapezius;--20, the deltoid;--21, the biceps brachialis;--22,
the pectineus;--23, the sartorius;--24, the rectus femoris;--25,
the tensor of the _fascia lata_;--26, the adductors.]
The great pectoral muscle conceals under its middle third a smaller
muscle, the _lesser pectoral_ (10, Fig. 68), which, arising from the
third, fourth, and fifth ribs, is directed upwards and outwards, to
be attached to the coracoid process of the scapula. This muscle serves
to move the scapula by drawing its upper part downwards and forwards.
Its outer border becomes obvious when the arm is raised above the head,
along the axillary border of the great pectoral muscle.
_The external oblique muscle of the abdomen._--The external oblique
muscle of the abdomen (Figs. 68 and 70) forms a large sheet half
fleshy, half aponeurotic, which covers the lateral and anterior
surfaces of the abdomen. The fleshy portion, which forms the outer half
of the muscle, is attached to the external surfaces of the lower eight
ribs, from which it arises by angular slips or digitations, interlacing
with those of the serratus magnus and latissimus dorsi muscles (15,
Fig. 68, and Fig. 74). From these costal origins the fibres are carried
forwards and downwards; the posterior fibres pass vertically downwards
to be attached to the iliac crest (Fig. 74); the others are directed
obliquely downwards and forwards, to give origin to a broad membranous
tendon (9, Fig. 70) called the _aponeurosis of the external oblique
muscle_. The fibres of this aponeurosis, continuing in the original
direction of the muscular fibres, pass in front of the rectus muscle
of the abdomen (12 and 15, Fig. 70), where, joining the subjacent
aponeuroses of the internal oblique and transversalis muscles, and
interlacing with the aponeurotic fibres of the opposite side, it
forms a long median raphé, called the _linea alba of the abdomen_,
proceeding from the xiphoid cartilage to the symphysis pubis (10, Fig.
70).
It is important, with regard to external form, to define the direction
of the line at which the aponeurotic fibres of the external oblique
succeed to the fleshy fibres (Fig. 70). This line descends at first
vertically from its origin at the inferior angle of the great pectoral
muscle, but its lower part inclines abruptly outwards (4, Fig. 70),
towards the anterior superior iliac spine, describing a curve with
its convexity downwards and inwards. This line marks the prominence
of the anterior or internal border of the muscular fibres; and as,
on the other hand, the rectus abdominis muscle forms, by its outer
border, another curved line (_linea semilunaris_), which is at first
vertical but inclines inwards below (15, Fig. 70), it follows that this
part of the anterior surface of the abdomen is traversed by a narrow
vertical groove which opens out below into a large triangular space.
This triangular space, bounded above and on the outer side by the
external oblique muscle, and on the inner side by the semi-lunar line
of the rectus, is limited below by the fold of the groin, or _Poupart’s
ligament_, the line at which the aponeurosis of the external oblique
becomes attached to the fascia lata of the thigh (page 111). In the
male this part of the aponeurosis of the external oblique is perforated
just above the inner end of Poupart’s ligament (13, Fig. 70) by the
spermatic cord, which passes obliquely downwards and inwards in the
inner part of the groin.
[Illustration: FIG. 71.
THE MUSCLES OF THE ABDOMEN (the superficial on the right
side, the deep on the left).--1, the external oblique;--2, 2,
the serratus magnus;--3, 3, the aponeurosis of the external
oblique;--4, the umbilicus;--5, 6, the linea alba;--7, the
crural arch or Poupart’s ligament;--8, the inguinal ring;--9,
pectoralis major;--10, latissimus dorsi;--11, 11, the rectus
abdominis;--13, the anterior portion of its sheath;--14, the
pyramidalis;--15, 15, the internal oblique of the abdomen;--16,
the inferior portion of the aponeurosis of the external oblique
turned downwards;--18, the upper part of the thigh covered by
its aponeurotic envelope;--19, a section of the external oblique
of the abdomen.]
The external oblique muscle draws the ribs downwards and forwards. If
the two muscles of that name (that of the right and that of the left
side) contract at the same time they bend the body forward; but when
one muscle only--_e.g._, that of the right side--contracts it imparts
to the trunk a movement of rotation to the left or opposite side.
Generally speaking, whenever we make any great effort the oblique
muscles of the abdomen contract, and their prominences, especially the
costal digitations and the anterior borders of the muscles, become
clearly marked.
[Illustration: FIG. 72.
THE MUSCLES OF THE BACK.--1, trapezius;--2, its vertebral
origin;--3, acromion process;--4, Latissimus dorsi;--5,
deltoid;--6, infraspinatus;--7, obliquus externus;--8, gluteus
medius;--9, gluteus maximus;--10, levator anguli scapulæ;--11,
rhomboid minor;--12, rhomboid major;--13, splenius;--14,
transversalis cervicis;--15, vertebral aponeurosis;--16,
serratus posticus inferior;--17, supraspinatus;--18,
infraspinatus;--19, teres minor;--20, teres major;--21,
triceps;--22, serratus magnus;--23, obliquus internus.]
The external oblique muscle covers two other muscular layers, which
are, passing from without inwards, the _internal oblique_ (15, Fig.
71) and _transversalis_ muscles. The internal oblique muscle is formed
by fibres which arise from the fascia of the loin, the crest of the
ilium, and Poupart’s ligament. They radiate forwards and for the most
part upwards (Fig. 71) to become attached, the superior fibres to the
lower six ribs, while the middle and inferior fibres are continued in
front as an aponeurosis, or broad flat tendon. The aponeurosis soon
becomes united, partly, with that of the external oblique and that
of the transversalis beneath. This aponeurosis splits to enclose the
rectus abdominis muscle and constitute the _sheath of the rectus_. The
_transversalis_ muscle is formed by fibres directed horizontally, and
it terminates in front in an aponeurosis which passes for the most part
behind the _rectus abdominis_, to blend with the aponeurosis of the
internal oblique muscle and assist in the formation of the sheath of
the rectus abdominis.
The _rectus abdominis_ (11, 11, Fig. 71).--This muscle forms a long,
broad, fleshy band on each side of the _linea alba_, which occupies
the middle line of the abdomen. Extending from the epigastric pit to
the pubis, it is attached above at its base, which is its widest part,
to the cartilages of the filth, sixth, and seventh ribs, and to the
ensiform cartilage, and below, at its apex, or narrowest part, by a
double tendon to the crest and spine of the pubis. The muscle presents,
with regard to form, several remarkable peculiarities. 1st. It is
enclosed in a fibrous sheath, formed in front by the aponeuroses of
the external and internal oblique muscles (16, Fig. 71), and behind
by the aponeuroses of the internal oblique and transversalis; so
that in the subject stripped of its skin its shape is half concealed
by the aponeurotic lamina which passes in front of it (Fig. 70 and
the right half of Fig. 71). 2nd. It is not formed of fleshy fibres
proceeding without interruption from the costal cartilages to the
pubis, but it is crossed by aponeurotic intersections (Figs. 70, 71),
the _lineæ transversæ_, at the level of which the fleshy fibres are
replaced by short tendinous fibres. These aponeurotic intersections
are usually three in number, the most inferior being placed at the
level of the umbilicus (4, Fig. 71), the other two higher up--one at
the level of the ninth rib, the other at the level of the seventh.
These intersections adhere to the anterior wall of the sheath of the
muscle, and as the muscle is thinner at their level each line is marked
on the surface by a transverse groove. 3rd. The portion of the muscle
below the level of the umbilicus does not present any aponeurotic
intersection, but it rapidly diminishes in breadth from the umbilicus
to the pubis, so that the outer border of the muscle slopes from above
downwards and inwards. To this arrangement is due the fact, upon which
we have already dwelt (page 199), that the narrow groove bounded by the
external oblique and the rectus muscles spreads out below the umbilicus
into a broad, triangular surface, which is bounded below by the fold of
the groin.
The rectus muscle flexes the trunk; that is, it depresses the thorax
and brings it nearer the pubis--a movement which it accomplishes by the
flexion of the vertebral column.
The lower part of the rectus muscle is covered by the _pyramidalis_
muscle (14, Fig. 71), of which the contour does not show beneath the
skin in the supra-pubic region, as the skin here is always padded by
a cushion of fat. This pyramidalis muscle, which is only pointed out
here in order that it may be remembered, forms on each side of the
middle line a small fleshy triangle, of which the base is attached to
the pubis, and the apex forms a short tendon which is inserted into the
linea alba, the median fibrous raphé resulting from the interlacing
of the aponeuroses of the oblique and transversalis muscles of the
abdominal wall.