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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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 =V=, 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.