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the manubrium or handle; the corpus sterni or body; and the processus xiphoideus (or xiphoid cartilage). Of these the body is formed by the fusion in early life of four segments or sternebræ.
The manubrium or superior part, usually separate throughout life from the rest of the bone, though occasionally fused with it, is of a flattened triangular form. The anterior surface, slightly saddle-shaped, affords attachment to the fibres of the pectoralis major and sterno-mastoid muscles. It is bounded above by a thick border, the lateral parts of which are hollowed out obliquely to form the facets (incisuræ claviculares) for the sternal ends of the clavicles; around the facets, which have an upward, lateral, and slightly backward direction, the bone
is faintly lipped. In the interval between these two facets there is a slight notch (incisura jugularis) which forms the floor of the characteristic hollow seen at the root of the neck anteriorly-the tilage supra-sternal notch. The borders are excavated immediately below the clavicular facets for the reception of the cartilages of the first ribs. Below this, the margin of the bone slopes medially, and is sharp, except inferiorly, where it presents a facet which supports a part of the second costal cartilage. Around this the bone is usually lipped anteriorly. The upper angles correspond to the ridge separating the clavicular facets from the first costal facets: whilst the lower angle, which may be regarded as cut across transversely, forms the surface which is united by cartilage to the body of the sternum. The anterior edge of this surface is usually prominent. The posterior aspect of the manubrium is smoother than the anterior, is pierced by numerous foramina, and IV. Rib cartilage is slightly concave from side to side and | above downwards. Here are attached
some of the fibres of the sterno-hyoid and sterno-thyreoid muscles.
The body or middle part of the sternum, usually twice the length and from half to two-thirds the width of the manubrium, displays evidence of its composite nature. If the planum sternale, (anterior surface) which is slightly convex from above downwards, and faintly concave from side to side, is carefully examined, three ill-marked ridges may be seen crossing it transversely; these correspond to the lines of fusion between the four primitive segments. To this surface of the bone the great pectoral muscles are extensively attached on either side of the median plane. The borders are thick and interrupted at points corresponding to the transverse lines already mentioned by U-shaped hollows, the edges of which are more or less projecting. These are for the reception of the cartilages of the third, fourth, and fifth ribs. The superior border is united to the manubrium above, and forms with it an angle of variable degree-the angulus sterni (sternal angle). A small facet is formed at the expense of the lateral extremity of this
border, and in conjunction with the facet on the inferior edge of the manubrium forms a recess on either side, in line with the articulation between the manubrium and body, into which the cartilage of the second rib fits. The inferior border of the body is curved, and is united in the middle line with the xiphoid process, whilst on either side it is pitted to receive the cartilages of the sixth and seventh ribs, the latter being in part supported by the xiphoid process. The middle line of the body of the sternum anteriorly corresponds to the floor of the medial surface furrow, which runs down the front of the chest in the interval between the two great pectoral muscles. The posterior surface is slightly concave from above downwards, and displays faint indications of three transverse lines in correspondence with those placed anteriorly. It is in relation with the pleura and pericardium, and affords attachment at its inferior extremity, on each side, to a transversus thoracis muscle. The xiphoid process displays many varieties of form and structure. It is a pointed process of cartilage, supported by a core of bone connected above with the inferior end of the body of the sternum, and having its lower extremity, to which the linea alba is attached, free. It lies somewhat posterior to the plane of the anterior surface of the manubrium, and forms a floor to the V-shaped interval between the cartilages of the seventh ribs. In this way a depression is formed, the surface hollow in correspondence with which is called the pit of the stomach or infrasternal depression. To the sides of this process are attached the aponeuroses of the abdominal muscles, whilst posteriorly the fibres of the diaphragm and transversus thoracis muscles derive attachment from it. It remains partly cartilaginous until middle life, at which time it generally undergoes ossification, particularly at its upper part, becoming fused with the body. Of varied form, it may be met with of spatula-shape, bifid, circular, pierced in the centre, or twisted and deflected to one or other side, or turned forward.
The sternum as a whole is broadest above, where the first rib cartilages are attached. It becomes narrow opposite the second rib cartilages, but again expands until the level of the fifth rib cartilage is reached, below which it is rapidly
reduced in width and ends below in the pointed xiphoid process. Its position in the body is oblique from above downwards and forwards; its axis, if prolonged upwards, would touch the vertebral column opposite the third or fourth cervical vertebra. Though liable to changes in position by the rising and falling of the chest wall, its upper extremity corresponds to the level of the lower border of the second thoracic vertebra, whilst the lower end of the xiphoid process usually falls in line with the fibro-cartilage between the tenth and eleventh thoracic vertebræ.
In women the sternum as a whole is usually narrower and shorter than 1. Appears about 5th or 6th month. 2. Appear about 7th month; in men, and its position less oblique. unite from 20 to 25. 3. Appear about 8th or 9th month; third segOn the other hand, Paterson has ment unites with second about puberty; fourth segment unites with shown that the male manubrium is third in early childhood. 4. Appears about 3rd year or later. proportionately wider and shorter than the female, whilst the male body is proportionately longer and narrower than the female.
Ossification. The cartilaginous sternum, developed from the fusion, in the median plane, of two cartilaginous bands uniting the anterior extremities of the cartilages of the first eight ribs, according to the researches of Ruge and more recently of Eggeling, begins
to ossify about the sixth month of fœtal life. About this time a single centre appears in the manubrium; at birth this is well developed. Two centres placed vertically have also been recorded. Secondary epiphyses have been described in connexion with the clavicular facets; these do not unite with the rest of the manubrium till adult life is reached. The body, formed by the fusion of four segments, is ossified from independent centres, either single or double, for each segment. These appear-the highest as early as the sixth month of intrauterine life-in some cases even before the manubrium has begun to ossify (Lambertz), the lowest towards the end of full term. The common arrange
ment met with at birth is a single centre for the first, and double centres for each of the succeeding segments. Union between these segments occurs rather irregularly, and is liable to much variation. The fourth unites with the third segment in early childhood, the third with the second about puberty, whilst the fusion of the second with the first segment may not be complete till the twentieth or twenty-fifth year.
The xiphoid process usually ossifies from a single centre, which may appear as early as the third year, though often very much later. The xiphoid process usually unites with the body about forty or fifty, and in exceptional cases osseous union between the body and manubrium may occur in advanced life.
According to Paterson the manubrium or presternum is developed in association with the shoulder girdle and becomes only secondarily associated with the ventrally growing ribs.
The ribs, of which there are twelve pairs, form a series of curved osseous bands which support the thoracic wall; posteriorly they articulate with the thoracic vertebræ; anteriorly, each rib is provided with a costal cartilage. The first seven ribs articulate with the sternum by means of their cartilages, and are termed the costæ veræ, true or vertebro-sternal ribs. The lower five ribs are not so supported, and are described as the costa spuriæ (false ribs). Of these the eighth, ninth, and tenth are united by their cartilages to the cartilage of the seventh rib, and are called the vertebro-chondral ribs, whilst the last two ribs are free at their anterior extremities, and are named the floating or vertebral ribs.
A typical rib consists of a capitulum costa (head), a collum costa (neck), a tuberculum costa (tubercle), and a corpus costa (shaft), on which, near its posterior end, is the angulus costæ (angle).
The head, placed on the posterior or vertebral end of the bone, is somewhat expanded. Medially, its articular surface is wedge-shaped and divided into two parts, a superior and inferior, by a ridge or crest (crista capituli), to which the interarticular ligament of the head of the rib is attached. Of these two facets the inferior is usually the larger, and articulates with the superior facet on the body of the vertebra in numerical correspondence with it, whilst the superior facet is for the corresponding area on the inferior part of the body of the vertebra above. The head is supported by a more or less constricted bar of bone, the neck, which lies anterior to the transverse process of the lower of the two vertebra with which the rib articulates, and thus assists in the formation of the costo-transverse cleft. The neck is continuous with the shaft laterally, at which point there is a well-marked tubercle on its posterior surface. The anterior surface of the neck is smooth; its posterior aspect is rough, and pierced by numerous small holes for vessels. Here is attached the ligament of the neck of the rib. Not uncommonly the superior border of the neck is lipped and ridged (crista colli costa), especially in the case of the sixth, seventh, and eighth ribs, and affords attachment to the anterior and posterior costotransverse ligament. The inferior border is continuous with the costal groove laterally. The tubercle consists of an articular and a non-articular part; the former is medial and inferior to the latter. Its articular surface, of rounded or oval shape, is directed downwards, posteriorly, and a little medially, and rests upon a facet on the transverse process of the vertebra in numerical correspondence with the rib. The non-articular part, most prominent in the upper ribs, has the fibres of the ligament of the tubercle of the rib attached to it. It is usually separated from the superior border of the neck and shaft by a groove, in which lies the lateral division of the posterior ramus of a thoracic nerve.
The body is thin, flattened, and band-like. Its length varies much;
Head Neck Tubercle
the seventh and eighth, which are usually the longest, are from two and a half to three times the length of the first and twelfth ribs respectively. The bodies are curved so as to adapt them to the form of the thoracic wall. More acute in the upper members of the series, where the shafts are shorter, the curve opens out in the middle and lower parts of the thorax, where the diameters of that cavity are greater. The curve, however, is not uniform. Including the whole length of the bone, it will be seen to be most accentuated towards the posterior part, where, in correspondence with the point at which the bend is most pronounced, there is a rough ridge placed obliquely across the outer surface of the shaft for the attachment of the slips of the ilio-costalis muscle; this bend is the angulus costæ. The distance between the angle and the tubercle is greatest on the eighth rib; above that, the width between these two points gradually decreases until, in the case of the first rib, the two coincide. Below the level of the eighth rib the distance slightly diminishes in conformity with the general narrowing of the thorax below that level. Towards the anterior extremity of the rib where the digitations of the serratus anterior and external oblique muscles are attached to its outer surface the curve of the body is somewhat more pronounced, and is referred to as the anterior angle.
Combined with the curve, there is in many of the ribs a twist. This may best be understood if the student will take a strip of stiff paper and bend it in the form of the curve of the rib. If, after he has done this, he pulls down the anterior end and turns up the posterior end of the strip, he will have imparted to the strip of paper a twist similar to that met with in the rib. This appearance is best seen in the middle members of the series, notably in the seventh and eighth ribs, above and below which it gradually becomes less marked. It is the occurrence of this twist which prevents the extremities of the ribs, together with the body, from resting on the same plane surface. To this rule there are certain notable exceptions, viz., the first and second, the twelfth, and not infrequently the eleventh.
The body has two surfaces, internal and external, and two borders, a superior and an inferior. The external surface, which is smooth, conforms to the general vertical convexity of the thorax, being directed upwards in the first rib, upwards and outwards in the higher ribs, outwards in the middle series, and outwards and slightly downwards in the tenth, eleventh, and twelfth. The internal surfaces are arranged conversely and are covered with the parietal pleura. Towards the sternal end of the middle ribs, at the anterior angle where the downward twist is most marked, there is often an oblique line across the outer surface. The upper border of the body is thick and rounded behind, thinner and sharper in front; to it are attached the FIG. 123.-FIFTH RIGHT Rib fibres of the internal and external intercostal muscles. The lower border is grooved behind at the expense of the inner surface, and is overhung laterally by a sharp margin. Anteriorly this sulcus costalis (costal groove) fades away, and its lips coalesce to form a rounded edge. The intercostal vessels and nerve are lodged in this groove, whilst its lips afford attachment to the
external and internal intercostal muscles respectively. On the floor of the groove may also be seen the openings of the canals for the transmission of the nutrient vessels, which are directed towards the vertebral end of the rib.
The anterior or sternal extremity of the body, often slightly enlarged, displays an elongated oval pit into which the costal cartilage is sunk.
Peculiar Ribs.-The first, second, tenth, eleventh, and twelfth ribs all display characters by which they can be readily recognised.
The first rib can be easily distinguished from the others by its size, curvature, and flattened form, as well as by the great proportionate width of its body. The head, which is of small size, has a single oval or circular facet, which is directed medially and slightly backwards for articulation with the side of the body of the first thoracic vertebra. The neck is flattened fromabove downwards,
and is slightly down-turned towards the end which supports the head. Its anterior border is rounded and smooth; its posterior edge rough for the attachment of ligaments. At the point where the neck joins the body posteriorly, a prominent tubercle curves upwards and backwards.
The inner and lower surface of this process has a small circular facet which rests on a corresponding articular surface on the transverse process of the first thoracic vertebra. The angle coincides with the tubercle, and thus assists in emphasising its prominence. The surfaces of the body of the rib are directed upwards and downwards, its borders inwards and outwards. If the finger is run along the thin inner border, a distinct spine or tubercle can be readily felt about an inch or an inch and a quarter from its anterior extremity. This is the tuberculum scaleni (scalene tubercle of Lisfranc), which also forms an elevation on the upper surface of the shaft and affords an attachment for the scalenus anterior muscle. There is a shallow, oblique groove crossing the superior surface of the shaft in front of this, for the lodgment of the subclavian vein; whilst behind. the tubercle there is another groove, usually better marked and passing obliquely forwards, for the subclavian artery (sulcus subclavia). In this groove, behind and below the artery, is lodged that part of the first thoracic nerve which contributes to the formation of the brachial plexus. According to Wood Jones the development of the groove depends upon the size of the nerve passing over it. The space on the upper surface of the rib between this latter groove and the tubercle posteriorly is somewhat rough, and affords attachment to the fibres of the scalenus medius muscle. In many specimens these features are