OF COLYMBUS TORQUATUS. 151 



it articulates, its distal extremity describes a greater arc of a circle than does that of the 

 ulna ; so that, although the plane of the circle in which these bones move is oblique to the 

 axis of the humerus, yet during extension this obliquity becomes gradually less and less, 

 until at full extension it quite disappears, and the bones of the arm and forearm lie in the 

 same plane. 



It is exceedingly interesting to compare this articulation with the elbow-joint in the 

 human species. In the latter, the connection between the ulna and humerus is most strictly 

 ginglymoid, admitting of only backward and forward motion ; while pronation and supina- 

 tion are perfect, by the very free rotation of the radius upon the outer tuberosity of the 

 humerus, and around the distal extremity of the ulna; but the radius also has, in common 

 with the ulna, the ordinary ginglymoid movements of flexion and extension. Thus the 

 elbow-joint, taken as a whole, and considered with reference to the movement of the fore- 

 arm upon the arm, is strictly a ginglymus, notwithstanding the element of the diarthrosis 

 rotatorius of the head of the radius. In birds, although pronator and supinator muscles 

 exist, yet they seem to serve rather to steady the radius than to move it, and the prona- 

 tion and supination of the forearm is exceedingly limited, if indeed it exist at all. While, 

 therefore, the axes of the two bones of the forearm scarcely change their relations to each 

 other, the common plane of their two axes is continually changing its relation to the 

 axis of the humerus, in flexion and extension of the forearm. This peculiar character of 

 the movements of the forearm is arrived at by the complete obliteration of the element 

 of rotation in the humero-radial articulation, and the conversion of the direct antero- 

 posterior ginglymoid motion into an oblique ginglymus. As the ulna must of course 

 accompany the radius in this obliquity of its flexion and extension, the deep sigmoid cavity 

 is converted into a shallow circular depression, which receives the small hemispherical 

 articulating facet of the humerus. By this means it can adapt its motions to any degree 

 of obliquity which may be impressed upon it by the position of the radial articulating 

 surface. 



Another interesting feature in this joint is that it is not the head of the radius alone 

 which articulates with the radial facet of the humerus. That part of the head of the ulna 

 which lies immediately above and posterior to the radio-ulnar also plays over this outer 

 oblique tuberosity of the humerus, forming with the rounded head of the radius an 

 elongated depression, which accommodates itself to this elongated radial facet. In this 

 feature of the articulation of the head of the principal bone of a limb with both the 

 condyles of the bone above it, is seen an indication of what exists in the femoro-tibial 

 articulation of the human species ; and since it seems to denote a tendency of the element 

 of stability to preponderate over that of mobility, it is what we might expect to find when 

 we remember how fixed the radius is to the ulna, causing the motion of the forearm to be 

 limited to flexion and extension. 



The posterior aspect of the end of the bone j>resents two well-marked longitudinal 

 grooves, completely separated from each other by a median ridge. These grooves give 

 passage to the tendons of the two extensor muscles of the forearm, proceeding to be 

 inserted respectively into the outer and inner sides of the olecranon. The olecranonoid 

 depression is hardly recognizable as such ; and the coronoid is but slightly marked. 



Ulna. As usual, much larger throughout its whole extent than the radius, and quite as 

 large at its distal as at its proximal extremity. It is a little less than six inches long. Its 

 shaft is considerably curved in its whole length, subcircular on section, being flattened 



MEMOIRS BOST. SOC. SAT. HIST. Vol. I. Ft. 2. 39 



