so A MANUAL OF ANATOMV 



direction, and form. It may be entirely osseous, or osseous above 

 and cartilaginous below. Its typical direction is downwards 

 between the seventh pair of costal cartilages, but it may have an 

 inclination forwards, backwards, or even to one side. It is narrow 

 from side to side, and compressed from before backwards. It 

 may terminate in a thin transverse border, in a sharp point, or in 

 a bifurcated extremity. The anterior surface lies at the bottom 

 of the infrasternal depression. The posterior surface gives origin 

 at either side to a portion of the triangularis sterni, and inferiorly 

 it gives origin to a portion of the diaphragm, usually in the form of 

 two fleshy slips. The superior border articulates with the meso- 

 sternum, and the inferior border gives attachment to the linea alba. 

 Each lateral border usually presents superiorly a demi-facet for a 

 portion of the seventh costal cartilage, but this may be transferred 

 to the fourth segment of the mesosternum. In rare cases there may 

 be an entire facet for the eighth costal cartilage, this being constant 

 in early life. The lateral border gives insertion at either side to 

 some of the fibres of the internal oblique aponeurosis, and occasion- 

 ally, at its upper part, to a portion of the rectus abdominis. 



The sternum derives its blood-supply from branches of the 

 internal mammary artery. 



Articulations. — With the clavicle and first seven costal cartilages, 

 at either side. 



Structure. — The sternum is composed of cancellated tissue covered 

 by a thin layer of compact bone. 



Varieties. — (i) The sternum is sometimes characterized by its shortness, 

 breadth, and great depression in its lower part. This condition is Uable to be 

 met with in cobblers. (2) A sternal foramen may be present in the mesosternum, 

 usually in the third or fourth segment. (3) A sternal foramen may be present 

 in the metasternum. (4) In very rare cases the sternum may be intersected 

 from end to end by a sternal fissure, in which cases the heart and pericardium 

 are left uncovered (ectopia cordis). (5) The costal cartilages may articulate 

 with the sternum asymmetrically. 



The Sternum of the Female. — The bone is usually shorter than 

 in the male, the shortness affecting the mesosternum. 



Ossification. — The sternum ossifies in cartilage from a variable number of 

 centres. There is usually one centre for the presternum, which appears in 

 the sixth month of intra-uterine life. Sometimes there are two, placed one 



above the other, and there may be as many as six, placed thus • • • The 



first segment of the mesosternum usually ossifies from one centre, appearing 

 in the seventh month, though there may be two, disposed laterally. The 

 second, third, and fourth segments of the mesosternum usually ossify from 

 two centres each, which are disposed laterally and remain separate for some 

 time, but subsequently unite as a rule. There may, however, be only one 

 mesial centre for each of these segments. In the second segment they appear 

 in the eighth month, in the third just before birth, and in the fourth during 

 the first year. The metasternum ossifies from one centre, which appears in 

 its upper part from the third to the sixth year, though it may be delayed to a 

 later period. The lower three segments of the mesosternum unite in order 

 from below upwards, the union commencing about puberty and being com- 

 pleted shortly afterwards. The first segment of the mesosternum joins the 

 remainder about twenty-five. The metasternum unites with the mesosternum 



