21 BONES OF THE THOEAX. 



broad at the upper part of the manubrium, considerably narrower at the 

 lower end of that portion and in the following segment, somewhat wider near 

 the lower than at the upper end of the body, and finally compressed and 

 narrowed where the body joins the ensiform cartilage. It consists of light 

 cancellated texture, with a thin covering of compact bone, so as to be liable 

 to crushing from violence. 



The manubrium is the thickest part of the sternum. Its superior border 

 is divided into three deep notches ; the middle one is named the incisura, 

 furcula, or interclavicular notch ; the lateral ones form two depressed articular 

 surfaces directed upwards and outwards, for articulation with the clavicles. 

 Each lateral border presents superiorly a rough depression close to the 

 clavicular surface, but looking directly outwards, which receives the cartilage 

 of the first rib ; and, at its inferior angle, a small surface which, with a 

 similar one on the second segment, forms a notch for the cartilage of the 

 second rib. The inferior margin is straight, and united by intervening 

 cartilage to the upper margin of the second segment. 



The body is marked on its anterior surface by three slight transverse 

 elevations, which mark the lines of junction of its four component parts. 

 Its posterior or thoracic surface, together with that of the manubrium, is 

 of more uniform smoothness. The lateral margins present each five notches 

 for the reception of costal cartilages, and a small surface superiorly, which, 

 with the similar depression on the manubrium, forms the notch for the 

 second costal cartilage. 



The notches for the third, fourth, and fifth costal cartilages are opposite 

 the lines of junction of the four segments of the body of the sternum ; 

 those of the sixth and seventh are placed close together on the sides of 

 the inferior segment, that for the seventh being frequently completed by the 

 ensiform cartilage. 



The lower segment of the bone, often called ensiform cartilage, varies 

 considerably in different individuals. In its simplest form it is a thin 

 spatula-like process projecting downwards between the cartilages of the 

 seventh ribs. It is, however, subject to very frequent varieties of form ; 

 being sometimes bent forwards, sometimes backwards, often forked, and 

 sometimes porforated. It projects into the aponeurosis of the abdominal 

 muscles. 



The sternum is subject to many varieties. It is sometimes divided vertically in 

 the whole or a part of its length. It is not unfrequently much shorter than usual, and 

 indented at its lower part, as occurs especially from the pressure of the cobbler's last. 



Two episternal nodules have been found in some rare cases surmounting the 

 incisura of the manubrium. (Breschet, " Annales des Sciences Naturelles," 2d series, 

 v. 10, Zoologie, p. 191 ; and Luschka, "Die Halsrippen und die Ossa suprasternalia," 

 Vienna, 1859.) Their position is indicated by the asterisks (**) in figure 24, E. 

 They derive an additional interest from the occurrence of suprasternal bones in 

 reptiles and monotremata. 



THE KTBS. 



The ribs (costce) are twelve in number on each side. They form a series 

 of arched and highly elastic bones, which extend outwards and forwards 

 from the vertebral column, and form the lateral walls of the thorax. Their 

 anterior extremities give attachment to cartilaginous prolongations the 

 costal cartilages, the first seven pairs of which pass forward to the sternum. 

 On this account the first seven pairs of ribs are called sternal, or true ribs, 

 while the remaining five pairs are called asternal, or false ribs. 



