OSTEOLOGY. 59 



sometimes nine or eleven, with tlie inferior extremity of 

 tlie cartilage pointed, and more or less firmly united to the 

 neighbouring cartilages by a small elastic ligament. The 

 union of the posterior cartilages is most lax, and in some 

 cases the last rib is wholly unconnected to the last but one ; 

 it is then said to be floating. The floating rib is generally 

 found when nineteen ribs exist on each side ; sometimes it 

 is -a mere cartilaginous elongated body extending from the 

 transverse process of the first lumbar vertebra imbedded 

 in the abdominal muscles. The external surface of the 

 fourteen posterior cartilages is connected with the external 

 oblique abdominal muscle. To that of the four or five 

 jDOsterior true ribs rectus abdominis is attached, and ante- 

 riorly is blended with lateralis sterni, which is continued 

 obliquely forwards and upwards over the second and third 

 to become attached to the first, from which also sterno- 

 thyro-hyoideus partly derives its inferior attachment. The 

 cartilages decrease in thickness from before backwards ; 

 they increase in length from behind and from before to 

 the ninth and tenth. They are connected together by the 

 inferior thickened portion of the intercostales interni, for 

 the external intercostales terminate opposite the inferior 

 extremity of the rib. The inferior extremity of the 

 cartilages of the false and floating ribs terminates in a 

 point, that of the true ribs is convex, coated with ar- 

 ticular cartilage, articulating with the cartilage of the 

 sternum. 



The first rib is the shortest and strongest, and is also 

 the straightest. Its head articulates with the last cervical 

 and first dorsal vertebrae. Its tuhercle is large and very 

 rough externally for attachment of transversalis costarum, 

 and longissimus dorsi. It is irregularly rounded, to the 

 inner surface is attached the pleura at its reflection to 

 form the mediastinum. And many of the important 

 structures which pass through the entrance of the thorax 

 lie in contact with this bone. The posterior margin 

 affords attachment to the first intercostals. The ante- 

 rior margin presents at the inferior part of the superior 

 third, a prominence to which the short or upper head 

 of scalenius is attached. Below this the bone is smooth 

 for the passage of the axillary plexus of nerves. On 

 the external surface at the superior part of the inferior 

 third the lower head of scalenius is attached, and 



