116 



OSTEOLOGY. 



Fig. 75. A Central Rib of Right 

 Side. Inner Surface. 



the larger ; the second piece has seldom more than one ; the third, fourth, and fifth 

 pieces are often formed from two centres placed laterally, the irregular union of 



which will serve to explain the occasional 

 occurrence of the sternal foramen (fig. 74), 

 or of the vertical fissure which some- 

 times intersects this part of the bone. 

 Union of the various centres commences 

 from below, and proceeds upwards, taking 

 place in the following order (fig. 72). 

 The fifth piece is joined to the fourth 

 soon after puberty ; the fourth to the third, 

 between the twentieth and twenty-fifth 

 years; the third to the second, between 

 the thirty-fifth and fortieth years; the 

 second is rarely joined to the first except 

 in very advanced age. 



Articulations. With the clavicles, and 

 seven costal cartilages on each side. 



Attachment of Muscles. The Pectora- 

 lis major, Sterno-cleido-mastoid, Sterno- 

 hyoid, Sterno-thyroid, Triangularis sterni, 

 aponeurosis of the Obliquus externus, 

 Obliquus interims, and Transversalis 

 muscles, Rectus and Diaphragm. 



THE RIBS. 



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 vertebra- 

 sternal or true ribs. The remaining five 

 are false ribs ; of these the first three, 

 being connected behind with the spine, 

 and in front with the costal cartilages, 

 are called the vertebro-costal ribs: the 

 last two are connected with the vertebras 

 only, being free at their anterior ex- 

 tremities; they are termed vertebral or 

 floating ribs. The ribs vary in their di- 

 rection, the upper ones being placed nearly 

 at right angles with the spine ; the lower 

 ones are placed obliquely, so that the 

 anterior extremity is lower than the 

 posterior. The extent of obliquity reaches 

 its maximum at the ninth rib, gradually 

 decreasing from that point towards the 

 twelfth. The ribs are situated one be- 

 neath the other in such a manner that 

 spaces are left between them ; these are 

 called intercostal spaces. Their length 

 corresponds to the length of the ribs; 



