Thorax 



47 



The inner surface, covered by pleura in its greater extent, presents below a costal 

 groove (S) for intercostal vessels, best marked behind : the intercostal muscles are 

 attached to the margins of the grove. 



The anterior end of the shaft has an oval hollow (CC) which receives the rounded 

 end of the costal cartilage without any intervening synovial cavity. 



A slight bend in the shaft may be noticed a little distance from its anterior 

 extremity : this is the anterior angle. The rib is bent in three directions : (i) a general 

 curve forwards and then inwards ; (2) a curve upwards of the head and neck and 



FIG. 37. A middle left rib. A . head ; B. ridge for interarticular ligament ; C. crista superior ; 

 .D.Jshaft. Upper figure shows the inner or pleural surface of shaft, and the subcostal groove S. 

 Lower drawing shows the posterior part of the rib, between head and angle, which has inter- 

 costal structures attached to it and is completely covered by post-vertebral muscles over 

 these ; the most superficial of these last reach the bone at the angle and make the secondary 

 lines here. The head articulates with two vertebra and the tubercle rests on the transverse 

 process of the lower one. 



adjoining portion of shaft, connected with a downward turn at the front end in many 

 ribs ; (3) a twist on its longitudinal axis that begins at the angle, so that the outer 

 surface of the bone shows a tendency to look upwards as well as outwards as it is 



followed forward. 



COSTAL CARTILAGES. 



These are bars of hyaline cartilage rather more rounded than the ends of the ribs 

 with which they are joined. Each is covered by a thick perichondrium continuous 

 with the periosteum of the rib, and this is sometimes partly ossified in old age.* The 

 cartilages that reach the sternum, with the exception of the first, articulate with that 

 bone by diarthrodial joints. There are also synovial cavities at the junction of the 

 eighth with the seventh, the ninth with the eighth and the tenth with the ninth, and 



* The perichondrium of the first rib becomes calcined much earlier. 



