OF DIDELPHYS VIRGINIANA. 



77 



twisting of the shaft upon its axis. The curve and twist are hoth greatest at the sixth — 

 eighth bones ; whence thej diminish in both directions with successive ribs. The differ- 

 ence in the direction of the axes of the neck and shaft of a rib decreases from the first 

 to the last; &nd, pari passu, the distinction between shaft and neck becomes less and less, 

 till it is scarcely recognizable on the eleventh — thirteenth. The tubercle, very prominent 

 upon the upper ribs, vanishes before the last is reached. The tubercles do not bear perfect 

 articulating facets beyond the ninth, the following ribs only jointing with the vertebrae by 

 their heads ; and the fifth — ninth ribs bear the second focet upon their neck, rather than at 

 the most prominent point of the tubercle. The intercostal artery only distinctly grooves 

 three or four of the longest ribs. The sternal extremity of the first rib is a stout knob ; 

 that of each of the others is flattened and expanded — most so with the longest bones. 



The sternum is composed of six pieces, the general shape of which is shown in tJie 

 accompanying figure. The lowest piece is thin and narrow, almost linear, with a 

 slightly enlarged base for articulation with the fifth ; it is supi^lemented by an extremely 

 large, broad, flat, " xiphoid " cartilage, with a rounded free border. The fifth is subquad- 

 rate, nearly as broad as long, and abruptlj- thicker and stouter than the sixth. The next 

 three are successively narrowed and elongated ; the fourth and third preserving their 

 squarish shape, and remaining flattened ; but the second, Avith a depressed lower part, is 

 rapidly narrowed and compressed above, to fit it to the manubrium. The latter is cruci- 

 form, being sub-cylindrical below, widening on either side to bear the arms of the cross, 

 and developing from its outer or under surface a median longitudinal crest, that is further 

 prolonged above the cross-pieces to form an elongated process. The clavicles are loosely 

 joined to the apex of this process through the intermedium of oval cartilages ; the heads 

 of the cross-pieces bear focets for the articulation of the first ribs ; the other ribs articulate 

 at the sternebral interspaces ; each sterneber bearing at both ends a demi-facet for this 

 purpose. 



4. The Anterior Extremity. ■ 



Observations upon the general characteristics of the fore limb will be found elsewhere. 

 There are forty-four bones in the scapular arch and its appendage, twelve of which are 

 sesamoid. 



Scapula.— {Fig. 19, A,B.) The gen- 

 eral shape is quadrilateral, with three 

 corners rounded off, and the fourth pro- 

 duced to support the glenoid fossa and 

 coracoid process; the accompanying fig- 

 ure shows the details of contour. The 

 posterior edge is thickened for muscular 

 attachment; the upper and under bor- 

 ders are thin. The thoracic surface, with- 

 out concavity, is smooth and slightly 

 undulating. The outer surface is like- 



wise smooth and flat, or with 



MEMOIRS noST. SOO. NAT. HIST. VOL. II. 



slight 



2(1 



Fig. 19. — Scapula, from above and in profile. 



