CHAPTER IV.
THE THORAX.
The Sternum: its three portions—manubrium, gladiolus, xiphoid appendage; position and direction of the sternum; its dimensions, absolute and relative.—The ribs; the true ribs, the false and floating ribs; the obliquity and curvature of the ribs.—Of the thorax in general; its posterior aspect, anterior aspect, and base.
We have already seen that that portion of the vertebral column which is formed by the seven cervical vertebræ is free, and forms of itself the bony structure of the neck. It is the same in the lumbar region, where the five vertebræ alone form the bony structure of the abdomen. The twelve thoracic vertebræ, however, corresponding to the upper two-thirds of the trunk, are in connection with the ribs and sternum, and constitute with these bones the osseous frame-work of the thorax.
The Sternum.—In the front part of the thorax is the sternum, a bone in the middle line, single and symmetrical (Fig. 11). This bone is, in quadrupeds, formed of a considerable number of separate bones jointed together in a linear series. In the human subject it consists of three separate parts, one superior, one in the middle, and one inferior, known respectively as the pre-sternum, meso-sternum, and meta-sternum. The whole bone has been compared in shape to a short Roman sword, of which the pre-sternum represents the handle, or manubrium; the meso-sternum, the longest piece, is the body, or gladiolus; and the meta-sternum, the pointed extremity of the sword, and usually tipped with cartilage, is the ensiform or xiphoid cartilage. Thus constituted, the sternum presents for our consideration an anterior surface, a posterior surface, two lateral borders, an upper and a lower extremity.
Fig. 11.
Thorax (anterior view).—1, pre-sternum, or manubrium of sternum;—2, meso-sternum, or body;—3, meta-sternum, or xiphoid appendage;—4, body of first thoracic vertebra;—5, twelfth thoracic vertebra;—6 and 7, first and second ribs;—8, 8, the true or sternal ribs;—9, 10, the floating ribs;—11, costal cartilages.
The anterior surface is smooth, but the union of the manubrium with the body of the sternum is marked by a prominent transverse ridge (sternal angle), due to the difference in direction of the two pieces at their junction. This projecting angle is very remarkable in some subjects, and gives a clearly marked convex shape to the superior portion of the anterior surface of the thorax. The posterior surface of the bone, which it is not necessary for artists to study, is generally flat, and presents a returning angle corresponding to the projecting angle of the anterior surface.
The superior extremity of the sternum, forming the broader portion of the bone, is marked by three notches, or depressions: two lateral, one on each side, articulating with the inner end of the clavicle, and one in the middle called the suprasternal, or episternal, notch. This notch, which is easily discerned on the living model, forms the inferior border of the deep depression situated at the lower part of the front of the neck. Its depth is still further increased by the inner ends of the clavicles and by the sterno-cleido-mastoid muscles on either side.
The inferior extremity of the sternum is formed by the meta-sternum, or xiphoid appendage, which remains very frequently in the cartilaginous state, in the form of a plate, thin and tapering. In shape and direction it is very variable, being sometimes pointed, rounded, or bifurcated. It may be situated in a plane corresponding to that of the body of the sternum, or it may be placed obliquely or project forwards or backwards. In a case where it projects in front it may cause a slight elevation of the skin of the region of the pit of the stomach, or epigastrium; but it is a detail of form so irregular that it is not worth reproducing, except in the representation of violent muscular exertion or extreme attenuation.
The lateral borders of the sternum are not vertical, but concave. The sternum is narrowest at the manubrio-sternal junction, the manubrium increasing in size towards its upper end, and the gladiolus, or body of the bone, enlarging towards its inferior part. Each lateral border is marked by seven small notches, or depressions, for the reception of the anterior extremity of each of the cartilages of the first seven ribs. The highest of these depressions is situated on the border of the manubrium just below the clavicular articular surface; the second depression is situated opposite the manubrio-sternal junction, partly on the pre-sternum, partly on the meso-sternum; those following are situated on the edge of the body of the bone, or meso-sternum, and the spaces between the depressions become smaller as they approach its lower extremity, so that the last depressions for the sixth and seventh costal cartilages are almost fused into one. The seventh costal cartilage is usually attached opposite the sterno-xiphoid junction, and is thus connected with both meso-sternum and meta-sternum.
It is necessary also to determine the exact position and direction of the sternum in relation to the other parts of the thorax, in the complete skeleton. The direction of the sternum is not vertical, but very oblique; it forms an angle of fifteen to twenty degrees, with a vertical line passing through the inferior extremity (Fig. 12), and an angle of severity to seventy-five degrees, with a horizontal line passing through the same extremity (Fig. 12). The manubrium is more oblique than the body of the sternum, and the body of the sternum than the xiphoid cartilage. Such is the direction of the sternum in the male; in the female it is less oblique, and approaches the vertical—a disposition which artists are prone to exaggerate by giving a more rounded form to the superior portion of the thorax in the female.
Fig. 12.
Relations of the Sternum to the Vertebral Column.
To compare the relations of the sternum with the rest of the thorax, it is necessary further to determine the level of the parts corresponding to the two extremities in relation to the vertebral column. The upper end of the sternum does not correspond to the first thoracic vertebra, but rather to the disc which separates the second and third, so that the horizontal plane passing through the superior extremity of the sternum strikes the second thoracic vertebra at its lower part (Fig. 12). The horizontal plane passing through the lower end of the sternum strikes the tenth thoracic vertebra; so that, viewing the thorax in profile, the sternum is seen to project between the second and eleventh vertebræ. The exact level of the bone varies with the movements of the chest wall in respiration.
The average length of the sternum in the adult man is eight inches. The pre-sternum, or manubrium, is usually about half as long as the meso-sternum, or body of the bone.
The most important measurement, however, is the length of the sternum without the xiphoid appendage. A measurement equivalent to the length of the sternum is found in various parts of the skeleton, which for the most part are adjacent to the sternum, and the sternal length may be taken as a common measure for constructing a correctly proportioned torso.
As a fact, this measure of the length of the manubrium and body of the sternum is equal to (1) the clavicle, to (2) the vertebral border of the shoulder-blade, and to (3) the distance which separates the two shoulder-blades in the figure when the arms are hanging by the side; further, the length of the sternum is equal to (4) the length of the hand without the third phalanx of the middle finger.
The Ribs.—The thoracic part of the vertebral column and the sternum being known, it is easy to understand the arrangement of the parts which complete the thorax. These parts are the ribs and costal cartilages, arranged somewhat like the hoops of a cask, proceeding from the vertebral column to the sides of the sternum; the ribs articulate posteriorly with the vertebral column, and are connected anteriorly to the sternum or to one another by the costal cartilages. The ribs are twelve in number on each side. They are known as first, second, and third ribs, etc., counting from above downwards; the first seven are the true ribs, or sternal ribs, which have their costal cartilages directly joined to the sternum; the next three (eighth, ninth, and tenth) ribs are the vertebro-costal ribs, as the costal cartilage of each articulates with the cartilage of the preceding rib; the last two, the eleventh and twelfth, are the false, floating or vertebral ribs: they are remarkable for their shortness; they are provided at their extremities with only rudimentary cartilages, which are pointed, and project by free extremities among the muscles of the walls of the abdomen.
In a general sense the ribs are long bones, presenting an external surface and an internal surface, a superior border and an inferior border. They are not horizontal, but oblique, from above downwards and from behind forwards: so that the anterior extremity of a rib is always placed on a lower level than its posterior extremity.
A typical rib possesses three curves. It is bent from behind forwards in a downward direction; it is bent like the hoop of a cask in order to surround the thorax, and presents, therefore, a curve similar to that of a scroll, of which the convexity is turned outwards and the concavity inwards; and, again, it is twisted upon itself as if the anterior extremity had been forcibly carried inwards by a movement of rotation upon its own axis. This curvature of torsion makes the surface, which is really external in the central portion of the rib, become a superior surface in the anterior portion. In order to have a good idea of the torsion of the ribs it is necessary to take a single rib and place it on a horizontal surface, such as a table; it will be then seen that, instead of its being in contact through its entire extent with the flat surface, it touches it only at two points, as if it formed a half-hoop of a cask to which a slight spiral twist had been given.
The ribs vary much in length, in order to correspond to the ovoid shape of the thorax; their length increases from the first to the eighth, which is the longest, and corresponds to the largest part of the thorax; and it gradually diminishes from the eighth to the twelfth.
A typical rib (Fig. 13) consists of the following portions, an indication of which is useful for the study of the shape of the thorax. The posterior extremity is slightly raised, and constitutes the head of the rib. It is shaped like a wedge, and articulates with the bodies of two vertebræ as a rule, and it is received, precisely like a wedge, into the space which separates the bodies of these vertebræ; so that it is in contact by the summit of the wedge with the intervertebral disc, and by the surfaces of the wedge with the vertebra which is situated above and that which is situated below the disc. Beyond the head, the rib presents a portion thin and compressed from before backwards, called the neck of the rib, which is placed in front of the transverse process of the vertebra corresponding to it.
At the external extremity of the neck is a slight enlargement called the tubercle, which corresponds to the level of the external extremity of the transverse process of the vertebra, and which articulates with the corresponding transverse process (Fig. 7, F G). By means of the connections of the head with the bodies of the vertebræ, and of the tubercle of the rib with the transverse process of the thoracic vertebra which corresponds to it, the posterior end of the rib moves on these joints as on a fulcrum, in the act of respiration; the chest cavity being enlarged by the uplifting of the shaft of the rib and by the eversion of the rib simultaneously.
Fig. 13.—A Typical Rib.
Passing on from the tubercle, the shaft of the rib is formed of a bar of bone, which at first is directed outwards and backwards (Fig. 13); then, after travelling some distance, it bends abruptly, so as to be directed forward, describing the characteristic curve of the rib. We give to this bend the name of the angle of the rib. The series of the angles of the ribs shows, upon the posterior aspect of the thorax, a line plainly visible, curved, with its convexity outwards, and having its summit at the level of the eighth rib, which is the longest, and upon which a relatively greater distance separates the angle from the tubercle.
Fig. 14.
Thorax (posterior view).—1, 1, spinous processes of the thoracic vertebræ;—2, 2, vertebral laminæ;—3, 3, series of transverse processes;—4, 4, the parts of the ribs included between the tuberosities and the angles of the ribs;—5, 5, angles of the ribs, becoming more distant from the vertebral column as the rib becomes more inferior.
Such are the characters of ribs in general. For the peculiar characters of the several ribs, after we have spoken of the last two ribs, it will suffice to note the shortness of the upper ribs, and principally of the first, which is flattened from above downward. In other words, it is curved along the borders, and not along the surfaces, and it does not present any twist. The last two ribs, besides being the shortest as a rule (excepting the first rib), are peculiar in their straightness and in the rudimentary nature of the angles; they further have no articulation with the transverse processes of the corresponding vertebræ.
The costal cartilages are attached to the extremities of the ribs in front: these cartilages, in proceeding to join the sternum, follow a course more or less oblique, so that the cartilage of the first rib is oblique from above downwards, and from without inwards; and those following present the same obliquity (Fig. 11), which becomes more accentuated in the cartilages lower down. The spaces which separate these cartilages are wide above, especially between the cartilages of the three first ribs, and become narrower towards the lower part of the chest.
The Thorax as a Whole.—The thorax, the constituent parts of which we have just examined, forms a kind of truncated cone, with its base below and its apex above; but, from an artist’s point of view as to form, it is not necessary to take this into account, as the shape of the summit of the thorax is completely changed by the addition of the osseous girdle constituted by the clavicle and shoulder-blade.
We limit ourselves, then, to a rapid view of the posterior surface, the anterior surface, and the base of the thorax.
The posterior surface (Fig. 14) presents upon the skeleton, in the median line, the series of spinous processes, and on each side, first a row of transverse processes and then the angles of the ribs. As already explained (p. 37), respecting these several details, the summits of the spinous processes, although just under the skin, are scarcely visible, especially in a very muscular subject.
On the anterior surface of the thorax (Figs. 11, 15), in a muscular subject, the osseous details do not show on the external figure, with the exception of the episternal or suprasternal notch (p. 43), and the inner ends of the clavicles, which are more or less visible. The great pectoral muscles form on each side a large fleshy surface, and the median line of separation of these muscles is marked by a narrow depression corresponding to the central portion of the sternum, the only region where this bone becomes subcutaneous; but in less muscular subjects, in the aged and in thin children, all the details of the thoracic frame-work show beneath the skin, and clearly reveal the forms of the costal cartilages with their obliquities, the thin intervals of separation (intercostal spaces) becoming narrower as we get lower down. Moreover, especially in infants, the articulation of the cartilages with the sternum, and the articulation of the cartilages with the anterior extremities of the ribs, are shown by a double row of nodules, as the points of junction of the osseous and cartilaginous portions are slightly elevated. We find therefore a series of chondro-sternal nodules (χόνδρος, cartilage) marking the borders of the sternum, and, on the outer side, a series of chondro-costal nodules, marking the line of junction of the ribs with the cartilages. This chondro-costal line descends obliquely from within outwards; so that, above, it is very near the sternum, owing to the shortness of the cartilage of the first rib, and, below, it is withdrawn from the sternum owing to the greater length of the cartilages of the ninth and tenth ribs.
Fig. 15.
Front View of the Chest and Abdomen.
The base, or inferior circumference, of the thorax is continuous, without distinct limits upon the living subject, with the abdominal wall behind and laterally; but in front this circumference presents a depression in the form of an inverted with the mouth looking downwards (Fig. 11); this depression, limited on both sides by the cartilages of the lower six ribs, and corresponding at its apex to the junction of the body of the sternum with its costo-xiphoid appendage, shows in the living subject a depression of the same form, called the pit of the stomach, or epigastrium (ἐπὶ, upon; γαστήρ, stomach). Upon the dead body, or upon a model in a state of repose, the outlines of the pit may be compared to a pointed arch; but when the model makes a violent effort, as in taking a deep inspiration, the elevation of the ribs spreads the cartilages of the false ribs from the median line, and the pointed arch in question shortens and tends to take a rounded form. On the other hand, in very muscular subjects, the great anterior muscles of the abdomen are sufficiently thick at their superior parts, where they cover the cartilages of the false ribs, to add their shape to that of the cartilages, and to give to the epigastric region a more rounded form. It is this form of a rounded arch that the Greek sculptors have adopted almost exclusively, and this choice we find justified to some extent in the fact that they had for their models very muscular athletes, whom they studied during the wrestling-matches of the gymnasium, when efforts which dilated the thorax most powerfully were to be observed.