The ribs are elastic arches of bone, which are connected behind with the vertebral column, and form a large part of the skeleton of the thorax. They are twelve in number on each side; but this number may be increased by the development of a cervical or a lumbar rib, or may be reduced to eleven. The first seven are connected in front, through the intervention of the costal cartilages, with the sternum (fig. 298) ; they are called true ribs (vertebrosternal ribs). The remaining five are false ribs; of these the cartilages of the eighth, ninth and tenth are each joined to the cartilage of the rib immediately above (vertebrochondral ribs) ; the eleventh and twelfth are free at their anterior ends and are termed floating ribs (vertebral ribs).
General features of the ribs (figs. 303, 304)- A rib from the middle of the series should be taken in order to study the general features of a typical rib.
Each rib has a posterior and an anterior end, and an intervening portion–the body or shaft. The anterior end can be distinguished by the presence of a small cup-shaped depression, which receives the lateral end of the costal cartilage. The shaft is curved with the convexity outwards, and is grooved along the lower part of its inner surface so that the lower border of the shaft is thin and sharp in contrast to the thick, rounded upper border. With this information the student is in a position to assign a typical rib to its correct side of the body.
The posterior or vertebral end possesses a head, a neck and a tubercle.
The head presents two facets, an upper and a lower, separated by a transverse ridge, named the crest. The lower facet, which is the larger, articulates with the body of the numerically corresponding vertebra ; the upper facet articulates with the vertebra above, and the crest of the head is placed opposite the intervertebral disc.
The neck is the flattened portion which succeeds the head; it is about 2.5 cm. long and is placed in front of the transverse process of the numerically corresponding vertebra. It is placed obliquely so that its anterior surface faces forwards and upwards. Its posterior surface is directed backwards and downwards, and is roughened and pierced by numerous foramina. Its upper border is sharp and forms the crest of the neck of the rib: its lower border is rounded.
The tubercle is placed on the outer surface of the posterior part of the rib, at the junction of the neck with the shaft; it is more prominent in the upper than in the lower ribs and consists of a medial articular and a lateral non-articular portion. The articular portion bears a small, oval facet for articulation with the end of the transverse process of the numerically corresponding vertebra ; the non-articular portion is rough for ligamentous attachments.
The shaft is thin and flat, with an external and an internal surface, a superior and an inferior border. It is not only curved but it is also bent, and the angle is situated about 5.6 cm. from the tubercle. In addition, the shaft is somewhat twisted in its long axis, and this may be demonstrated by placing the rib with its lower border in contact with a horizontal surface. The part behind the angle inclines medially and upwards, and its outer surface is directed downwards and backwards; in front of the angle the outer surface is directed slightly upwards. The external surface is convex and smooth. A short distance from the tubercle it is crossed by a rough line, directed downwards and laterally, which marks the position of the angle. The internal surface is smooth and is marked along its lower border by a groove, termed the costal groove, which is bounded below by the inferior border of the shaft. The upper border of the groove is continuous behind with the lower border of the neck, but anteriorly it terminates at the junction of the middle and anterior thirds of the shaft; in front of this point, the groove is absent.
The second rib (fig. 306) is about twice the length of the first, but has a similar curvature. The non-articular portion of the tubercle is often small. The angle is slight, and situated close to the tubercle. The body is not twisted, so that both ends of it touch any plane surface upon which the rib may be laid ; but at the tubercle there is an upward convexity, similar to, but smaller than, that found in the first rib. The external surface of the body is convex, and looks upwards and a little outwards; near its middle it is marked by a rough, muscular impression. The internal surface, smooth and concave, is directed downwards and a little inwards; on its posterior part there is a short costal groove.
The tenth rib has only a single articular facet on its head.
Particular features.-The head of a typical rib gives attachment along its anterior border to the radiate ligament; and its crest to the intra-articular ligament. The anterior surface of the head is related to the sympathetic trunk and the costal pleura. The anterior surface of the neck is divided into an upper and a lower area by a faint ridge, which affords attachment to the posterior intercostal membrane, and is continuous with the inner of the two lips on the superior border of the shaft. The upper area of varying size and more or less triangular in shape, is separated from the posterior intercostal membrane by some fatty tissue ; the lower area is smooth and covered with the costal pleura. The posterior surface of the neck gives attachment to the inferior costotransverse ligament (ligamentum colli costae) and is pierced by numerous vascular foramina. The crest of neck is rough for the attachment of the superior costotransverse (anterior costotransverse) ligament, and it can be traced laterally into the outer lip of the superior border of the shaft. The inferior border of the neck is rounded and can be traced laterally into the upper border of the costal groove; it gives attachment to the posterior intercostal membrane. The articular part of the tubercle conforms to the shape of the articular facet on the transverse process of the corresponding vertebra. In the upper six ribs it is convex and faces backwards and medially ; in the succeeding three or four ribs it is flattened and faces downwards and backwards and slightly medially The non-articular part of the tubercle gives attachment to the lateral costotransverse ligament (ligament of the tubercle of the rib).
The first rib (fig. 305) presents important particular features. The scalene tubercle and the adjoining part of the upper surface give insertion to the scalenus anterior muscle. The groove in front of the tubercle lodges the subclavian vein and the irregularly roughened area which intervenes between it and the first costal cartilage gives attachment to the costoclavicular ligament and more anteriorly to the subclavius muscle. The groove behind the tubercle is occupied by the subclavian artery and as a rule the lower trunk of the brachial plexus. Behind this groove a rough area which extends as far as the tubercle gives insertion to the scalenus medius. The obliquity of the first rib is responsible for the obliquity of the thoracic inlet and accounts for the appearance of the apex of the lung in the root of the neck.
The outer border of the first rib is thin anteriorly but is thicker behind, where it is covered by the scalenus posterior as it descends to the second rib for its insertion. It gives origin to the upper part of the first digitation of the serratus anterior, behind and opposite to the groove for the subclavian artery. The inner border gives attachment to the strong fascia which covers the cervical dome of the pleura.
The second rib (fig. 306) bears a rough tubercle for the serratus anterior muscle on its external surface just behind its midpoint; this tubercle gives origin to the lower part of the first and the whole of the second digitation. The costal groove is very poorly marked on the internal surface and is restricted to its posterior part. The second intercostal nerve lies between the second rib and the pleura in most of its course. The inner and outer lips of the upper border are distinct and are widely separated behind. Just in front of the poorly marked angle the outer lip is roughened to give insertion to the scalenus posterior.
The twelfth rib (fig. 307) although short gives attachment to numerous muscles and ligaments. The lower part of its anterior surface, in its medial half to two-thirds, gives insertion to the quadratus lumborum muscle and its covering fascia. Lateral and superior to these attachments the surface is related to the costodiaphragmatic recess of the pleura. The lower bowler gives attachment to the middle lamella of the lumbar fascia and at the lateral border of the quadratus lumborum, to the lateral arcuate ligament (lateral lumbocostal arch). The lower border of the neck rives attachment to the lumbocostal ligament, by which it is connected to the transverse process of the first lumbar vertebra. The external surface gives attachment to the lowest levator costae, the longissimus thoracis, and the iliocostalis, in its medial half; more laterally, it gives insertion to the serratus posterior inferior, and origin to the latissimus dorsi and the external oblique muscle of the abdomen.
It should be observed that the lower limit of the pleural sac crosses the anterior aspect of the twelfth rib approximately at the point where the rib is crossed by the lateral border of the iliocostalis muscle. The lateral extremity of the rib usually lies below the line of pleural reflection and is therefore not covered with pleura.
Structure.-The ribs consist of highly vascular spongy substance, which is enclosed in a thin layer of compact bone and contains a large proportion of red marrow.
Ossification – Each rib. with the exception of the first and the last two, is ossified from four centers; a primary centre for the shaft and three secondary centers, one for the head and one each for the articular and non-articular parts of the tubercle*. The primary centre appears near the angle, towards the end of the second month of fetal life and is seen first in the sixth and seventh ribs. The secondary centers for the head and tubercle appear between the sixteenth and twentieth years. and unite with the body about the twenty fifth year. The first rib has three centers. viz.: a primary one for the body, a secondary centre for the head, and one for the tubercle. The eleventh and twelfth ribs being destitute of tubercles, have each only two centers.
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