The upper part of the body is built around a bony cage called the thorax, conical in shape, and flattened in front. The walls of this cage are the ribs, twelve on each side, fastening to the spine behind and to the sternum or breast bone in front. The upper ribs are quite short and make a small circle; they grow longer until the seventh, which is the
longest and the last to fasten to the breast bone. The next three grow shorter and shorter, and reach the sternum only through a long costal cartilage, which with the projecting end of the sternum (ensiform cartilage) form the abdominal arch. The last two ribs are quite short and are free at their front ends. The first seven are called true ribs, the next three false, and the last two floating ribs.
To the breast bone at the top of this cone the collar bones are attached, lifting the whole mass away from the cone and making it a flat surface, wedging downward. The inner ends of the S-shaped bone are curved around the apex of the
cone, but the outer ends move forward again, bringing the mass of the shoulders with them to form the flat front surface.
Thus, without the shoulders, the cage of the chest is a cone with the apex upward. Under the muscles this form may easily be seen.
With the shoulders, it is a wedge with the apex downward. The profile of the sides forms a wide wedge, buttressed by a mass of lateral muscles over the iliac crest.
The front surface, formed mainly by the pectoral and rectus abdominis muscles, forms a much more slender wedge. Its upper third bevels more sharply in, as far as the lower border of the breast muscle, paralleling the edge of that muscle, bounded by a line across the bottom of the breast muscle, just below the nipple, through the epigastric pit. Its lower two-thirds bevels more sharply down, following the edge of the rectus muscle; its lines almost meet at
the symphysis pubis below.
A vertical central groove divides the symmetrical halves of the front, running its full length. It begins in the pit of the neck, between the collar bones. Over the breast it marks the breast bone, and is deepened by the bulge of the pectoral muscles on either side. At the end ol this, its upper third, is a pit (epigastric pit) marking the divergence of the ribs.
At the end of its middle third is another pit, the umbilicus or navel. At the end of its lower third is the mound of the symphysis. This line is useful for placing the masses of the chest, the epigastrium (over the stomach) and the abdomen.
The masses of the torse are the chest, the abdomen or pelvis, and between them the epigastrium; the first two comparatively stable, the middle one quite movable.
A straight line marking the collar bones defines the top of the first mass; and paralleling it, a line through the base of the breast muscles and pit of the epigastrium forms its base.
Although the shoulders are freely movable, changing the lines of the first mass, and bulging the pectoral muscles, yet the mass itself changes little except the slight change in respiration. Even in respiration the upper portion, as far as the level of the epigastric pit, changes little; the lower ribs perform most of the respiratory movement.
Centering on this pit is the abdominal arch, made of the cartilages of the false ribs. At its centre, the end of the breast bone (ensiform cartilage) hangs pendent; on either side the arch descends diagonally, variously curved, separating thorax from abdomen.
Below this arch is the abdomen, the most movable part of the mobile portion. It is bounded below by a line passing approximately through the anterior points of the iliac crests. Its profile shows the lines of the cone of the thorax diverging downward, the lines of the wedge of the chest and shoulders converging downward, and the buttressing of the lateral muscles.
In the bending or turning of the body the central line of this portion bends always to the convex side, always paralleled by the borders of the rectus muscle.
By this movement the straight wedge of the front is broken. It becomes not a bent wedge, but two wedges; one the upper half of the original wedge, prolonged but not completed downward; the other the lower half, prolonged upward to meet the one above.
More unchanging than either of the above is the mass of the abdomen. The central groove is here shallow and may lose itself below. The long wedge ends in the symphysis pubis.
Muscles of the Trunk front view:
1 Pectoralis major.
3 Rectus abdominis.
4 Serratus magnus.
5 External oblique.
Rectus Abdominis: From symphysis pubis to cartilages of ribs, from fifth to seventh. Action: Flexes thorax.
Serratus Magnus: From eight upper ribs to scapvila -- spinal edge, under surface. Action: Draws shoulder blade forward, raises ribs.
External Oblique: From eight lower ribs to iliac crest and ligament to pubis. Action: Flexes thorax.
SKELETON OF THE TRUNK
Muscles Covering Upper Portion, front view:
1 Pectoralis minor.
2 Pectoralis major.
Pectoralis Minor: From third, fourth and fifth ribs to coracoid process. Action: Depresses point of shoulder.
Pectoralis Major: From inner half of clavicle, sternum, costal cartilages as far as sixth and seventh ribs to humerus. Action: Draws arm downward and forward.
Trunk, front view
Armpit and Shoulder
The erect torse presents in profile the long curve of the front, broken by depressions at the border of the breast muscle and at the umbilicus or navel into three lesser curves, almost equal in length. The back presents the sharp anterior curve of the waist, opposite the umbilicus, bending into the long posterior curve of the chest, and the shorter curve of the buttocks. The former, that of the chest, is broken by the almost vertical shoulder blade and
the slight bulge of the latissimus below it.
In profile the torse presents three masses: that of the chest, that of the waist, and that of the pelvis and abdomen. The first and last are comparatively unchanging.
Above, the mass of the chest is bounded by the line of the collar bones; below, by a line following the cartilages ot the ribs, being perpendicular to the long diameter of the chest.
This mass is widened by the expansion of the chest in breathing, and the shoulder moves freely over it, carrying the shoulder blade, collar bone, and muscles.
It is marked by the ridge of costal cartilages that forms its border, sloping up and forward, and by the ribs themselves, sloping down and forward, and by the "digitations" (finger marks) of the serratus magnus (big saw-toothed) muscle, little triangles in a row from the corner of the breast muscle, paralleling the cartilages of the ribs, disappearing under the latissimus.
Below, the mass of the pelvis and abdomen slopes up and forward. It is marked by the iliac crest and hip, described later. In front it may be flattened by contrAction of the abdominal muscles. Over its surface the hip moves freely, changing the tilt of the pelvis.
Between these the central mass contains the waist vertebrae, and is very changeable. Practically all of the movement af flexion and extension for the whole spine occurs here, and much of the side-bending.
This mass is marked by a buttress of lateral muscles, slightly overhanging the pelvic brim and bearing inward against the side above. It changes greatly in different positions of the trunk.
TORSE - BACK VIEW
The back presents numerous depressions and prominences. This is due not only to its bony structure, but to the crossing and recrossing of a number of thin layers of muscles. It should be borne in mind that the superficial or outside layers manifest themselves only when in Action. For this reason, under all changes of position, the spine, the shoulder-
blade with its acromion process, and the crest of the ilium, must be regarded as the landmarks of this region.
The spine is composed of twenty- four vertebrae. It extends the full length of the back, and its course is marked by a furrow. The vertebrae are known as the cervical, dorsal and lumbar. The cervical vertebrae are seven in number, and the seventh is the most prominent in the whole of the spine. It is known as vertebra prominens. In the dorsal region the furrow is not so deep as below. Here there are twelve vertebrae. When the body is bent forward, the processes of the vertebrae in this section are plainly indicated.
The spinal furrow becomes deeper as it reaches the lumbar vertebras, where it is marked by dimples and depressions. It widens out, too, in this part of the body, and as it passes over the surface of the sacrum to the coccyx it becomes flattened. The average length of the spine is about two feet three inches.
The outer corner of the shoulder girdle is the acromion process, which is the high outer extremity of a ridge rising from the shoulder blade. The shoulder blade or scapula (spade) is a flat plaque of bone fitting snugly against the cage of the thorax, having a long inner vertical edge, parallel to the spine; a sharp lower point; a long outer edge point-
ing to the arm pit; and a short upper edge parallel with the slope of the shoulder. The ridge, or spine of the scapula, starts at the spinal edge, about a third of the way down, in a triangular thickening, and rises until it passes high over the outer upper corner, where the shoulder joint ties, then turns forward to join with the collar bone at the acromion. The prominent portions are this ridge and the spinal edge and the lower corner. The upper outer
corner is thickened to form the socket for the head of the humerus, forming the shoulder joint proper.
Movement of flexion and extension occurs almost entirely in the waist or lumbar vertebrae. Movement of side-bending occurs throughout the whole length. Movement of rotation occurs in the lumbar vertebras when the spine is erect, in the middle vertebrae when it is half flexed, in the upper vertebrae when the spine is fully bent. In the lumbar vertebrae, the axis of this rotation is behind the spine; in the middle vertebrae it is neutral; in the upper dorsals it is in front of the spine.
Each vertebra moves a little, and the whole movement is the aggregate of the many little movements.
The shoulder blade slides against the surface of the cage of the thorax, in any direction, and may be lifted from it so that its point or its spinal edge become prominent under the skin. It produces easily fifty per cent, of the whole movement of the shoulder.
MASSES AND MARKINGS
From the rear the mass of the torse presents a great wedge, with apex downward, marked by a complex of lesser wedges and diamonds, and the shoulder blades.
The profile of the sides presents a wide incomplete wedge, whose lines if prolonged would form an apex well below the buttocks. The surface proper of the back presents a great wedge, with base at the corners of the shoulders, with apex driven between the buttocks, buttressed on the sides by the lateral masses of waist muscle. With the addition of the
neck, this becomes a diamond with a very blunt top.
From end to end vertically runs the dividing line of the spine; when bent, a series of knobs (tips of vertebral spines); when erect a groove except at the root of the neck, the spine of the seventh cervical vertebra. This serves as a sort of ridge pole for muscular tendons for neck and shoulders; and around it therefore is a flat unbroken fascia without muscular fibres, forming a lesser diamond nestling below the upper apex.
The Trunk, side view:
1 Latissimus dorsi.
2 External oblique.
Latissimus dorsi: From spine, sixth dorsal, to sacrum and iliac crest; passes inside of humerus to fasten to front side near head. Action: Draws arm backward and inward.
External Oblique: From eight lower ribs to iliac crest and ligament to pubis. Action: Flexes thorax.
Muscles of the Trunk, back view:
3 Latissimus dorsi.
Trapezius: From occipital bone, nape ligament and spine as far as twelfth dorsal, to clavicle, acromion and ridge of shoulder blade. Action: Extends head, elevates shoulder and rotates shoulder blade.
Trunk back view
The trapezius is a diamond-shaped muscle; with upper apex at the base of the skull, lower apex well below the shoulder blades, and corners at the shoulder girdle opposite the deltoid, as though it were a continuation of that muscle.
From the sacrum the muscles diverge upward, while the lower ribs and lower corner of the shoulder blade diverge downward, making lesser diamonds of various definiteness of outline.
The ridge of the shoulder blade is always conspicuous, pointing diagonally toward the corner of the shoulder. It sets at a fixed angle with the spinal edge (more than a right angle) and at a right angle with the lower turned-out corner.
In relaxation, both ridge and blade are ridges under the skin, and are converted into grooves by the muscles bulging in contrAction.
Of these muscles, those on either side of the ridge are easily recognizable -- the deltoid, below and outside, and trapezius, above and inside, but the trapezius also spreads from the inner end of the ridge to well down the spine. Under this, helping to form the bulge, are the rhomboidei, extending from the blade diagonally upward to the spine, and the levator angula scapulae, from its upper corner almost vertically to the top of the neck.