S6 OSTEOLOGY. 



inferior surface is narrow anteriorly and convex, the centre pre- 

 senting a prominent ridge coated with cartilage, materially 

 increasing the lateral surfaces, which give origin to the pectoral 

 muscles ; the sides are flat and irregular, and present between 

 the segments depressions, in which the cartilages of the true ribs 

 articulate, another depression being entirely formed in the first, 

 and an eighth in the last segment. These depressions are 

 oval in shape, and are nearer to each other as they proceed 

 backwards. 



The Cariniform or keel-shaped cartilage presents a convex 

 border looking forwards and upwards, and elevated so as to sur- 

 mount the articular depressions for the first pair of costal 

 cartilages, to which it is attached by ligaments ; laterally it is 

 flattened to increase the surface for muscular attachment ; and 

 its inferior border, prolonged over the first segments of the sternum, 

 terminates in the cartilaginous ridge. The prominent convexity 

 which this cartilage forms may often be plainly distinguished in 

 the living animal. 



The Xiphoid or Ensiform cartilage is somewhat heart-shaped. 

 The apex being directed downwards and backwards. Its superior 

 surface, broad and cup-shaped, occupies a space bounded by the 

 cartilages of the false ribs, and supports part of the abdominal 

 viscera. Its inferior surface is convex, and roughened for 

 muscular attachment, and its free borders are thin. It affords 

 attachment for muscles, and for a fibrous cord called the linea 

 alba. 



The bony framework of the thoracic cavity bears some resem- 

 blance to a truncated cone, with its apex or anterior extremity 

 compressed laterally. The anterior aperture is a triangular space, 

 having its apex directed downwards and slightly forwards, formed 

 by the sternum, the first pair of ribs, and the first dorsal vertebra; 

 it gives passage to the oesophagus, trachea, and various important 

 nerves and blood-vessels. The base or posterior aperture is oval 

 in outline, and slopes obliquely downwards and forwards from 

 the vertebrae ; it is formed by the last dorsal vertebra, the last 

 pair of ribs, the cartilages of the false ribs, and the Xiphoid 

 cartilage, and its boundary gives attachment to the diaphragm, 

 a large muscular curtain which separates the thorax from the 

 abdomen. 



