55 



sternal bone. Each of the four angles of the external process is obliquely trun- 

 cated to form the shallow concave articular surfaces for portions of the terminal 

 articulations of the second and third sternal ribs. The angles of the posterior 

 expanded plate are similarly truncated, forming smaller articular surfaces, to 

 which the corresponding subterminal surfaces at the posterior part of the second 

 and third sternal ribs are adapted. The anterior and posterior articulations, 

 with the manubrium and third sternal bone, are of the kind called synchondro- 

 sis ; but the eight glenoid articulations for the sternal ribs were evidently pro- 

 vided with smooth cartilage and synovial membranes. 



The third sternal bone* is more distinctly divided than the preceding into its 

 posterior plate and anterior or external cuboid process. The surface of the plate 

 is nearly smooth and flat towards the chest, and presents an oblong quadrilate- 

 ral figure, broadest below, and shghtly concave at the lateral margins : on the 

 anterior part of each angle is the articular surface for the corresponding half of 

 the subterminal articulations of the sternal ribsf. The anterior subcompressed 

 portion of this sternal bone presents the same number of articular surfaces as at 

 a, a', fig. 4. for the convex terminal joints of the sternal ribs, and it has nearly 

 the same form as that of the preceding bone. 



The fourth sternal bone differs from the preceding in the greater breadth and 

 flatness of its posterior expanded portion ; this gives to the anterior division the 

 character rather of a subordinate epiphysis ; but the number and form of the ar- 

 ticular surfaces are the same. 



In the fifth sternal bone the expanse of the flat posterior plate has reached its 

 maximum, and it is broader than it is long: the anterior portion or process 

 having diminished in antero-posterior extent, without having increased in breadth, 

 gives to this singular bone a resemblance to a common form of paper-weight, 

 the handle of which is represented by the anterior process, here reduced by the 

 approximation of the four articular surfaces to the form of a cube J. 



In the sternal bone, which appears to be the eighth of the series, the propor- 



* Plate IX. fig. 1. III. t Fig. 4. b, b'. 



I The anterior or external surface of the fifth sternal bone is shown at fig. 3. v. ; the heads of the 

 fifth and sixth sternal ribs being introduced in the right side to show their mode of application to the 

 median external process. The flat inner or upper surface of thesame sternal bone is shown at fig. 1 . V. 

 A side view of the fourth sternal bone is given at fig. 4. iv. 



