The ribs extend from the dorsal portion of the vertebral column to the sternum, forming arches, which enclose the lateral parts of the thorax. They are usually twelve in number at each side, but cases occasionally occur in which the number is augmented by the addition of a cervical or a lumbar rib, to which reference has already been made in describing the vertebras of those regions. The number may also be diminished to eleven. I have lately seen an instance in which this diminution was accompanied with the absence of a dorsal vertebra. The seven superior pairs, which are united by means of cartilaginous prolongations to the sternum, are called sternal or true ribs; the remaining five, which are not prolonged to the sternum, being denominated asternal or false ribs.
The ribs do not arch uniformly from the vertebral column towards the sternum; the greater number consists of parts of two circles or arches of different diameters, the anterior being much the larger.
Thus, the rib, directed at first backward from its connection with the bodies of the vertebrae, reaches and is supported by the transverse process (which in the dorsal region is inclined backward); after leaving the extremity of this process, it turns abruptly outwards and finally is directed forward towards the sternum.
In their length, breadth, and direction, these bones present several varieties. From the first to the eighth their length successively increases, whilst from the ninth to the twelfth they gradually decrease, so that the last is very little longer than the first.
Their breadth, considered in the whole series, diminishes gradually from the first to the twelfth; but in each rib, it is greatest towards its sternal extremity.
As to their direction in reference to the vertebral column, the first forms almost a right angle with it, and the succeeding ones gradually incline downwards, so that their anterior extremity is lower than the posterior. The body of all the ribs, except the first, is, as it were, twisted on itself, so that their two extremities cannot be made to rest at the same time on a plane surface.
These bones present two surfaces, two borders, and two extremities.
The external surface is convex and smooth. The internal surface is concave and corresponds with the pleura.
The superior border, smooth and rounded, gives attachment to the intercostal muscles; the inferior border is marked on its inner aspect by a groove (sulcus costalis), which is for the lodgment of the intercostal vessels, and also gives attachment to the intercostal muscles [at its external and internal margin].
The posterior extremity, somewhat rounded and thicker than the other parts of the rib, and hence denominated its head, (capitulum costae,) presents (except in the instances to be presently stated) two articular faces (separated by a slight ridge) which articulate with the corresponding small surfaces on the bodies of the dorsal vertebrae; the head is supported by a narrow round part, or neck, terminated externally by a tubercle, (tuberosity, tuberculum costae,) which is smooth in one part for its articulation with the transverse process of the lower of the two vertebras to which the head is connected, and rough in the other for the insertion of the posterior costo-transverse ligament.
The anterior extremity is broad, flat, and hollowed at its tip into an oval pit, into which is implanted the costal cartilage.
Between the tuberosity and the most convex part of the body of each rib is a rough line, marking what is termed its angle. The distance of the angle from the tuberosity increases gradually from the second to the eleventh inclusive. In the last it is not perceptible: in the first, it is not distinguishable from the tuberosity.
The first two and the last two ribs
The first two and the last two ribs present some peculiarities deserving of notice.
The first rib is shorter and broader than either of the succeeding ones; its direction is nearly horizontal; its body not being twisted, as is the case in other ribs, it lies evenly when placed on a plane surface.
One surface looks upwards and is marked by two very slight depressions (over which pass the subclavian vessels), and an intervening roughness, sometimes well marked, which indicates the attachment of the scalenus anticus muscle. The other surface looks downwards, towards the cavity of the thorax. The external border, convex and rounded, is surmounted by the tuberosity; 9 the internal is thin and forms the margin of the superior aperture of the thorax. The anterior extremity is broad and thick; the head, which articulates with the first dorsal vertebra, is small, presents an undivided articular surface, and is supported by a slender neck.
The second rib is longer than the first, and presents externally a prominent line for the attachment of the serratus magnus; its internal surface is somewhat grooved posteriorly.
The eleventh rib has no groove on its inferior border, nor a tubercle, as it is not articulated with the transverse process; its angle is scarcely perceptible, and the head has but one articulating surface. The cartilage by which its anterior extremity ends is unconnected with those of the other ribs.
The twelfth rib differs little from the preceding one, except in being shorter; it has neither angle, tubercle, nor groove, and as its anterior extremity, which is pointed, seems loose and unattached, it is called a floating rib. The same designation has likewise been applied to the eleventh rib.
Ossification of the ribs
Ossification begins in the ribs at a very early period — somewhat before it has made its appearance in the vertebrae, and the deposit of osseous matter extends rapidly over them. Each rib (with exceptions to be presently noticed) is formed from one principal piece and two epiphyses.
The epiphyses are small and thin, and one of them, which in most ribs is slightly angular in shape, belongs to the head, the other, to the tubercle. Their growth commences between the sixteenth and the twentieth year of age, and they have united to the rest of the bone a few years after, — about the twenty-fifth. The last two or three ribs want the epiphysis of the tubercle.