OF COLYMBUS TORQUATUS. 149 



superimposed upon the dorsal aspect of the ribs, nearly parallel with the vertebra?, and 

 situate somewhat more than an inch from them. It measures 3.60 inches in length, by 

 about one fourth of an inch at its lower, and three fourths at its upper or broadest end. It 

 is concavo-convex in two planes : i. c, slightly so in the horizontal plane, its convex edge 

 being presented towards the vertebras ; and much more decidedly so in a vertical plane, the 

 convexity presenting upwards. In the greater portion of its length the transverse section 

 of the bone is elliptical or oval; but towards the distal extremity it becomes depressed 

 into a simple thin flat lamina. This lamina is nearly straight and very thin, and forms 

 the termination of the scapula without curving in any direction. The proximal extremity 

 of the bone is much thicker and stronger than the rest. It bends downwards with a very 

 considerable, and inwards with a slighter, curve. The thick head of the bone, thus con- 

 stituted, articulates with three bones, — the clavicle, coracoid, and humerus. 



The claviculo-scapular articulation is a mere apposition of the extreme apex of the clav- 

 icle with a very small space just at the internal edge of the head of the scapula, where 

 that edge projects inwards upon the expanded lateral process of the coracoid, which forms 

 most of the second or superior coraco-clavicular articulation. 



The coraco- scapular articulation is an exceedingly strong and firm apposition of the 

 whole head of the scapula with the rough depression on the superior surface of the shaft 

 of the bone, just internal and posterior to the glenoid cavity. The articulation seems to 

 be essentially almost a synchondrosis. The rough inequalities of the head of one bone are 

 perfectly coaptated to those of the other ; interarticular fibro-cartilage is interposed ; and 

 the whole articulation is so firm and unyielding as to permit of but exceedingly limited 

 motion, and that chiefly in one direction. That part which the scapula takes in the forma- 

 tion of the glenoid cavity constitutes the humero-scapular articulation. The glenoid cavity 

 merits special notice. 



When the glenoid facets of the coracoid and scapula are deprived of the dense layer of 

 fibro-cartilage which chiefly forms the concavity in which the head of the humerus plays, 

 they are found to be perfectly smooth and plane. Both are tolerably regularly oval in 

 shape, the coracoid the larger, and quite oval, the scapular smaller, and more of an 

 elongated and subtriangular shape. The coracoid facet is horizontal antero-posteriorly, 

 with transversely an obliquity of about 45° upwards and outwards ; the scapular is about 

 perpendicular to it, with transversely 45° of obliquity forwards and outwards. Upon this 

 osseous basis a very dense and thick layer of unyielding fibro-cartilage is superimposed, to 

 form the slight concavity which exists for the reception of the head of the humerus. This 

 cartilage is laid over the whole of both facets, but is much the thickest at their margins, 

 where it rises with a well-defined edge, leaving a central depression which constitutes the 

 true glenoid "cavity." This cavity is very broadly semilunar in shape, the cornua of the 

 semilune having quite well-defined, raised borders, its convexity being but slightly, and 

 its concavity scarcely at all, elevated. But the depression is so slight that the entire cavity 

 is able to contain but a comparatively small portion of the articular head of the humerus. 

 This disposition of the elements of the articulation, as well as the shape of the head of the 

 humerus, is plainly indicative of the design of the joint, — the attainment of great free- 

 dom of motion in all directions by some sacrifice of strength. A comparison of the 

 glenoid with the cotyloid cavities of this bird furnishes a striking illustration of the pre- 

 ponderance of mobility over stability which usually obtains in the anterior extremities of the 

 higher vertebrates. 



