OSTEOLOGY. 25 



THE RIBS. 



The ribs are 36 in number, 18 on either side, forming the lat- 

 eral boundaries of the thorax, terminating below by cartilaginous 

 prolongations, the costal cartilages. They articulate above with 

 the dorsal vertebrae. They are divided into sternal or true, eight 

 in number, and asternal or false, the posterior ten. 



General Characteristics. Each rib has an external convex 

 and internal concave smooth surface and an anterior or convex 

 border, a posterior border, showing a groove for the intercostal 

 vessels and nerves, a shaft and two extremities. 



The superior extremity articulates with the vertebral column, 

 and shows a head with two demi-facets for the bodies of the ver- 

 tebrae in front and behind, a neck below the head, and a tuber- 

 osity for articulation with a dorsal transverse process. 



The inferior extremity is excavated for the cartilage. 



The costal cartilages are flattened laterally, the first eight lying 

 on the sternum, the remainder articulating with the' cartilage in 

 front. 



The length of the ribs increases from the first to the gth, then 

 decreases; their width from the ist to the 6th, then decreases. 



The ist rib has no outer groove and no notch on its head. Its 

 cartilage is short and thick, and has an articular facet below for 

 the opposite. 



The iSth rib has no external channel, and the facet on its tuber - 

 osity is confounded with that of its head. 



For thorax see Lungs. 



THE ANTERIOR EXTREMITY. 



This is divided into the shoulder, arm, forearm and forefoot or 

 hand. 



THE SHOULDER. 



In solipedes this consists of the scapula only, the clavicle being 

 absent. 



THE SCAPULA. 



The scapula or shoulder blade is a flat, triangular bone, lying at 

 the anterior and inferior part of the outer thoracic wall, articula- 

 ting below with the head of the humerus. Its general direction is 

 downward and forward. 



It has two surfaces, external and internal, three angles, anterior 

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