THE STERNUM. 125 



upward, backward, and outward, 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 a 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 downward and inward. 



The Second Piece of the sternum, or Gladiolus (meso-sternuni), considerably 

 longer, narrower, and thinner than the first piece, is broader below than above. 

 Its anterior surface is nearly flat, directed upward and forward, and marked by 

 three transverse lines which cross the bone opposite the third, fourth, and fifth 

 articular depressions. These lines are produced by the 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 backward. 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 

 downward, and correspond to the intercostal spaces. Most of the cartilages 

 belonging to the true ribs, as will be seen from the foregoing description, articulate 

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

 ments. 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 the costal cartilages with the sternum. 



The Third Piece of the sternum, the Ensiform or Xiphoid Appendix (meta- 

 sternum), is the smallest of the three ; it is thin and elongated in form, cartilagi- 

 nous in structure in youth, but more or less ossified at its upper part in the adult 

 Its anterior surface affords attachment to the chondro-xiphoid ligament ; 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 articulates with the lower end of the gladiolus, and at each superior angle 

 presents a facet for the lower half of the cartilage of the seventh rib ; below, by 

 its pointed extremity it gives attachment to the linea alba. 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. The bone is composed of delicate cancellous structure, covered by 

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

 articular facets for the clavicles. 



Development. The cartilaginous sternum originally consists of two bars, situated 

 one on either side of the mesial plane and connected with the rib cartilages of its 

 own side. These two bars fuse with each other along the middle line, and the bone, 

 including the ensiform appendix, is developed by six centres ; one for the first piece 

 or manubrium, four for the second piece or gladiolus, and one for the ensiform 



