Thorax 



57 



fascia passes down the rhomboid ligament to enclose the muscle, carrying down the 

 layer of scalene fascia that covers the vein, so that both these are attached to the inner 

 part of the cartilage below the Sterno-thyroid. 



Development. 



Ribs are laid down in cartilage : chondrification appears in the blastemal costal 

 processes of the vertebrae (that extend ventrally into the body wall) and is not con- 

 tinuous with the cartilage of the vertebrae, but connected with it by the continuous 

 blastema. 



The ends of the cartilaginous ribs are connected by the sternal plate of that side 

 and the ribs only extend at first partly round the body : the sternal plates have partly 

 met and fused in the middle line when ossification begins in the ribs. 



Bony centres appear near the angles : first in the middle ribs, in the eighth week, 

 and other ribs rapidly following, that for the first usually appearing before that for 

 the twelfth. Ossification extends in both directions in each rib. At birth the head, 

 tubercle, and part of neck are cartilaginous (see Fig. 8). At puberty epiphysial 

 centres appear for (i) the head and crista, (2) the articular, and (3) the non-articular 

 tubercle : these all join the main bone after twenty, the head epiphysis joining last 

 about twenty-four. 



The ribs below the sixth do not have a non-articular tubercular epiphysis, and 

 the last two bones have no tubercular epiphysis at all. 



STERNUM. 



The sternum is a long flat bone, evidently composed of several fused segments, 

 that lies in the front wall of the chest, having the true costal cartilages attached to 

 its sides (Fig. 47). 



The bone consists, as already mentioned, of several parts, of which the first piece 

 is the largest and is termed the manubrium, presternum, or episternum. The succeeding 

 pieces that carry the remaining cartilages are taken together as constituting the body 

 of the bone, mesosternum or gladiolus. 



The last piece is an irregular prolongation of the bone, usually bifid at its extremity 

 and lying below and between the seventh cartilages, and is termed the xiphisternum, 

 ensiform process, xiphoid cartilage, or metasternum.* 



The manubrium is the thickest and strongest portion. It articulates with the 

 next piece by a layer of fibre-cartilage that does not ossify until late in life, and the 

 two pieces do not lie quite in the same plane, but form an angle, so that the line of 

 their junction is prominent, and is known as the angle of the sternum, or angle of 

 Louis. Below this the body is composed of four segments, frequently termed 

 sternebra, and, although these segments are firmly united in the full-grown bone, 

 the lines of junction are usually apparent as three transverse ridges on the front aspect 

 of the body. They are not usually visible behind, though as a rule some indication 

 of the division is to be found near the margins. Each sternebra is flat, or very slightly 

 concave, on its front aspect, and flattened behind. The sternum as a whole, however, 

 shows a slight forward convexity. The metasternum, partly bone and partly carti- 

 laginous, is set in a plane deeper than that of the body, but its lower end may project 

 forward : its upper part may be partly hidden by the cartilages of the seventh ribs. 



* The older names, manubrium, gladiolus and xiphoid, were given with reference to the fancied resem- 

 blance to a gladiator's sword, viz., handle, blade, and point. The other terms are better, in that they refer 

 to the situation of the parts, although the term " episternum " is open to some objection on grounds of 

 comparative anatomy. 



