194 THE SKELETON. 



costal cartilages, in the following order (Fig 138). In the first piece, between 

 the fifth and sixth months ; in the second and third, between the sixth and 

 seventh months; in the fourth piece, at the ninth month; in the fifth, within 

 the first year, or between the first and second years after birth ; and in the 

 ensiform appendix, between the second and the seventeenth or eighteenth 

 years, by a single centre which makes its appearance at the upper part, and 

 proceeds gradually downwards. To these may be added the occasional exist- 

 ence, as described by Breschet, of two small episternal centres, which make 

 their appearance one on each side of the interclavicular notch. These are re- 

 garded by him as the anterior rudiments of a rib, of which the posterior rudi- 

 ment is the anterior lamina of the transverse process of the seventh cervical 

 vertebra. It occasionally happens that some of the segments are formed from 

 more than one centre, the number and position of which vary (Fig. 140). Thus 

 the first piece may have two, three, or even six centres. When two are pre- 

 sent, they are generally situated one above the other, the upper one being 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. 141), or of the vertical fissure which occasionally intersects this 

 part of the bone. Union of the various centres commences from below, and 

 proceeds upwards, taking place in the following order (Fig. 139). The fifth 

 piece is joined to the fourth soon after puberty ; the fourth to the third, be- 

 tween 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 Pectoralis Major, Sterno-cleido-rnastoid, Sterno- 

 hyoid, Sterno-thyroid, Triangularis Sterni, aponeurosis of the Obliquus Ex- 

 ternus, Obliquus Internus, and Transversalis muscles, Eectus Abdominis and 

 Diaphragm. 



THE EIBS. 



The Eibs 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 in- 

 creased 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 vertebrae only, being free at their anterior extremities; they are termed 

 vertebral or floating ribs. The ribs vary in their direction, the upper ones being 

 placed nearly at right angles with the spine, the lower ones obliquely, so that 

 the anterior extremity is lower than the posterior. 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 beneath the other in such a manner 

 that spaces are left between them, which are called intercostal spaces. Their 

 length corresponds to the length of the ribs, their breadth is more considerable 

 in front than behind, and between the upper than between the lower ribs. The 

 ribs increase in length from the first to the seventh, when they again diminish 

 to the twelfth. In breadth they decrease from above downwards ; in each rib 

 the greatest breadth is at the sternal extremity. 



Common Characters of the Ribs (Fig. 142). A rib from the middle of the 

 series should be taken in order to study the common characters of the ribs. 



Each rib presents two extremities, a posterior or vertebral, an interior or 

 sternal, and an intervening portion, the body or shaft. The posterior or vertebral 

 extremity presents for examination a head, neck, and tuberosity. The head 



