U4 OSTEOLOGY. 



origin of the Sterno-cleido-mastoid muscles. In well-marked bones, ridges limiting 

 the attachment of these muscles are very distinct. Its posterior surface, concave 

 and smooth, affords attachment on each side to the Sterno-hyoid and Sterno-thyroid 

 muscles. The superior border, the thickest, presents at its centre the interclavicular 

 notch; and, on each side, an oval articular surface, directed upwards, backwards, 

 and outwards, for articulation with the sternal end of the clavicle. The inferior 

 border presents an oval rough surface, covered in the recent state with a thin layer 

 of cartilage, for articulation with the second portion of the bone. The lateral 

 borders are marked above by an articular depression for the first costal cartilage, 

 and below by a small facet, which, with a similar facet on the upper angle of the 

 middle portion of the bone, forms a notch for the reception of the costal cartilage 

 of the second rib. These articular surfaces are separated by a narrow curved 

 edge which slopes from above downwards and inwards. 



The Second Piece of the sternum, the gladiolus, considerably longer, narrower, 

 and thinner than the superior, is broader below than above. Its anterior surface 

 is nearly flat, directed upwards and forwards, and marked by three transverse lines 

 which cross the bone opposite the third, fourth, and fifth articular depressions. 

 These lines indicate the point of union of the four separate pieces of which this 

 part of the bone consists at an early period of life. At the junction of the third 

 and fourth pieces is occasionally seen an orifice, the sternal foramen ; it varies in 

 size and form in different individuals, and pierces the bone from before backwards. 

 This surface affords attachment on each side to the sternal origin of the Pectoralis 

 major. The posterior surface, slightly concave, is also marked by three transverse 

 lines, but they are less distinct than those in front; this surface affords attachment 

 below, on each side, to the Triangularis sterni muscle," and occasionally presents 

 the posterior opening of the sternal foramen. The superior border presents an 

 oval surface for articulation with the manubrium. The inferior border is narrow, 

 and articulates' with the ensiform appendix. Each lateral border presents at each 

 superior angle a small facet, which, with a similar facet on the manubrium, forms 

 a cavity for the cartilage of the second rib ; the four succeeding angular depressions 

 receive the cartilages of the third, fourth, fifth, and sixth ribs, whilst each inferior 

 angle presents a small facet, which, with a corresponding one on the ensiform 

 appendix, forms a notch for the cartilage of the seventh rib. These articular 

 depressions are separated by a series of curved interarticular intervals, which 

 diminish in length from above downwards, and correspond to the intercostal spaces. 

 The costal cartilage of each true rib, excepting the first, is thus seen to articulate 

 with the sternum at the line of junction of two of its primitive component 

 segments. This is well seen in many of the lower animals, where the separate 

 parts of the bone remain ununited longer than in man. In this respect a striking 

 analogy exists between the mode of connection of the ribs with the vertebral 

 column, and the connection of their cartilages with the sternal column. 



The Third Piece of the sternum, the ensiform or xiphoid appendix, is the 

 smallest of the three; it is thin and elongated in form, cartilaginous in structure 

 in youth, but more or less ossified at its upper part in the adult. Its anterior 

 surface affords attachment to the costo-xiphoid ligaments. Its posterior surface, 

 to some of the fibres of the Diaphragm and Triangularis sterni muscles. Its 

 lateral borders, to the aponeurosis of the abdominal muscles. Above, it is con- 

 tinuous with the lower end of the gladiolus ; below, by its pointed extremity, it 

 gives attachment to the linea alba, and at each superior angle presents a facet for 

 the lower half of the cartilage of the seventh rib. This portion of the sternum is 

 very various in appearance, being sometimes pointed, broad and thin, sometimes 

 bifid, or perforated by a round hole, occasionally curved, or deflected considerably 

 to one or the other side. 



Structure. This bone is composed of delicate cancellated texture, covered by 

 a thin .layer of compact tissue, which is thickest in the manubrium, between the 

 articular facets for the clavicles. 



Development. The sternum, including the ensiform appendix, is developed by 



