THE RIBS 



161 



Angle 



Non-articular part of tubercle 



Articular part of tubercle 



Shaft 



FIG. 129. A central rib of the left 

 side. Inferior aspect. 



third, fourth, and fifth pieces are often formed from t\vo 

 centres placed laterally, the irregular union of which will 

 serve to explain the occasional occurrence of the sternal fora- 

 men (Fig. 126), or of the vertical sternal fissure, which occa- 

 sionally intersects this part of the bone (Fig. 126), and 

 which is further explained by the manner in which the 

 cartilaginous matrix, in which ossification takes place, is 

 formed. Union of the various centres of the gladiolus com- 

 mences about puberty, from below, and proceeds upward, 

 so that by the age of twenty-five they are all united, and 

 this portion of bone consists of one piece. The ensiform 

 cartilage becomes joined to the gladiolus about forty. The 

 manuorium is occasionally but seldom joined to the gladiolus 

 in advanced life by bone. When this union takes place, 

 however, it is generally only superficial, a portion of the 

 centre of the sutural cartilage remaining unossified. 



Articulations. With the clavicles and seven costal carti- 

 lages on each side. 



Attachment of Muscles. To nine pairs and one single 

 muscle the Pectoralis major, Sternomastoid, Sternohyoid, 

 Sternothyroid, Triangularis sterni, aponeuroses of the 

 Obliquus externus abdominis, Obliquus internus abdominis. 

 Transversalis, Rectus abdominis muscles, and Diaphragm. 



The Ribs (Costae). 



The ribs are elastic arches of bone, which form 

 the chief part of the thoracic walls. They are twelve 

 in number on each side; but this number may be 

 increased by the development of a cervical or lumbar 

 rib, or may be diminished to eleven. The first seven 

 are connected behind with the spine and in front 

 with the sternum, through the intervention of the 

 costal cartilages; they are called true (vertebrosternal) 

 ribs (costae verae). 1 The remaining five are false 

 ribs (costae spuriae); of these, the first three have 

 their cartilages attached to the cartilage of the rib 

 above, the vertebrochondral ribs; the last two are free 

 at their anterior extremities, the floating or vertebral 

 ribs. The ribs vary in their direction, the upper ones 

 being less oblique than the lower. The extent of 

 obliquity reaches its maximum at the ninth rib, and 

 gradually decreases from that rib to the twelfth. The 

 ribs are situated one below the other in such a 

 manner that spaces are left between them. Each 



1 Sometimes the eighth rib cartilage articulates with the sternum; this condition occurs more frequently on the 

 right than on the left side. 



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