THE RIBS. 155 



It has a ligamentous lining, which will be described, when an 

 account is given of the fresh bones and their ligaments. 



The Thorax. 



The thorax resembles a flattened cone, cut away obliquely at 

 its basis ; and regularly truncated at its apex. 



It is formed by the dorsal vertebrae behind, the ribs on the 

 sides, and the sternum before. 



The Ribs 



Are long crooked bones, placed in an oblique direction 

 downwards as respects the back-bone. - Their number is gene- 

 rally twelve on each side ; though sometimes eleven or thirteen 

 have been found. 



They are convex externally, and concave internally. They 

 are made smooth by the action of the contained parts, which, on 

 this account, are in no danger of being hurt by them. 



The ribs approach towards a round form at their extremities, 

 near the vertebrae. Farther forwards they are flat and broad, 

 and have an upper and lower edge ; each of which is made 

 rough by the action of the intercostal muscles inserted into 

 them. These muscles being all of nearly equal force, and 

 equally stretched in the interstices of the ribs, prevent the 

 broken ends of these bones, in a fracture, from being removed 

 far out of their natural place, to interrupt the motion of the vital 

 organs. The upper edge of the ribs is more obtuse, and 

 rounder than the lower, which is deepened on its internal side 

 by a long fossa, for lodging the intercostal vessels and nerves : 

 on each side of which there is a ridge, to which the intercostal 

 muscles are fixed. The fossa is not observable at the ends of 

 the ribs ; for, at the posterior, or root, the vessels have not yet 

 reached the bones ; and, at .the fore end, they are split away 

 into branches, to serve the parts between the ribs. 



From this situation of the blood-vessels, has originated the 

 rule adopted by surgeons, that the incision, in cases of empyema, 

 &c., should be made midway between the spine and sternum, 

 and that the lower edge of the upper rib should be avoided. 



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