MUSCLES AND MOVEMENTS.
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.
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.
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.
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.
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.
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).
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.
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.
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.