THE RIBS. 159 



descend to the ninth, because of the greater breadth of the sacro- 

 lumbalis muscle. 



The ribs are commonly divided into true and false. 



The true ribs are the seven uppermost of each side. Their 

 cartilages are all gradually longer as they descend, and are joined 

 to the breast bone : so that, being pressed constantly between 

 two bones, they are flattened at both ends ; and are thicker, 

 harder, and more liable to ossify than the other cartilages that 

 are not subject to so much pressure. These bones include the 

 heart and lungs ; and therefore are called true ribs. 



The five inferior ribs of each side are the false, whose carti- 

 lages do not reach to the sternum ; but on this account having 

 less pressure, their substance is softer. To these five ribs the 

 circular edge of the diaphragm is connected. 



The first rib of each side is so situated, that the flat sides are 

 above and below, while one edge is placed inwards, and the 

 other outwards, or nearly so ; therefore sufficient space is left 

 above it for the subclavian vessels and the muscles ; and the 

 broad concave surface is opposed to the lungs. But in conse- 

 quence of this situation, the channel for the intercostal vessels is 

 not to be found. The head of this rib is not divided into two 

 plane surfaces by a middle ridge, because it is only articulated 

 with the first vertebra of the thorax. Its cartilage is frequently 

 ossified in adults, and is united to the sternum at right angles. 

 This first rib frequently has a ridge rising near the middle of its 

 posterior edge, where one of the heads of the scalenii muscles 

 rises. Farther forward it is flattened, or sometimes depressed 

 by the clavicle. 



The position of the second rib is such that its two broad 

 surfaces have oblique aspects, inward and downwards, outwards, 

 and upwards, so as to make the surface of the thorax uniform : 

 and it may be observed of all the ribs, that the aspect of their 

 surfaces is varied upon this principle, according to their situation 

 in the thorax. 



The sixth, seventh, and eighth ribs have their cartilages 

 nearly contiguous. They are frequently joined to each other 

 by cross cartilages ; and frequently the cartilages of the eighth, 



