]98 TEE ARTICVLATIO^iS. 



Mode of union. — Two interosseous and two peripheral ligaments. 



a. The interosf^eom ligaments, interposed between the synarthrodia! surfaces, 

 are composed of extremely short white fibres passing from one to the other 

 surface, and which are endowed with a very remarkable power of resistance. The 

 inferior always ossifies a long time before the animal is full grown — a circum- 

 stance which caused the older veterinary anatomists to describe — and with some 

 show of reason — the radius and ulna as a single bone. Ossification of the superior 

 ligament is very rare. 



h. The peripheral bands are bundles of arciform fibres which, from the beak 

 of the olecranon to the radio-ulnar arch, leave the lateral faces of the ulna to 

 pass — some inwards, others outwards — to the posterior face of the radius. The 

 fibres of the external ligament are confounded with the external humero-radial 

 ligament. The internal fibres are united to the internal humero-radial ligament, 

 and to the small ulnar tendon belonging to the short flexor of the forearm. 

 Analogous fibres are found beneath the radio-ulnar arch ; but they are much 

 shorter and less apparent. (This is the external transverse radio-ulnar ligament 

 of Leyh.) 



Movements. — Yerj obscure in youth ; nearly null when the two bones are 

 fused together. 



In the Ox, ossification of the, superior interosseous ligament is constant at adult age. 



In the Dog and Cat, we have already seen (p. 107) that the radius and uliia are not fused 

 to each other, but remain independent during life. They are united in their middle portion 

 by an interosseous ligament, and join by diarthrosis at their two extremities. These animals 

 therefore exhibit; 1. An interosseous ligament. 2. A superior radio-ulnar articulation. 3. 

 An inferior radio-ulnar articulation. 



Interosseous ligament. — It is composed of very resisting white fibres, attached by their 

 extremities to the bodies of the bones. Notwithstanding their sliortness, they are loose enough 

 to allow movements taking place between the radio-ulnar articulations. 



Superior radio-ulnar articulation. — This is a trochoid articulation, which only allows 

 movements of rotation or pivoting. 



The articular surfaces wliich form this articulation are : in the ulna, the small sigmoid 

 cavity — a surface excavated in the lateral sense, and semicircular; in the radius, a cylindrical 

 half-hinge receive<l into the preceding cavity. 



To unite these there is an annular ligament — a kind of fibrous web thrown around the 

 superior extremity of the radius, fixed inwardly on the ulna near the inner extremity of ilie 

 small sigmoid cavity, attached outwardly to the external lateral ligament of the elbow 

 articulation, and confounded superiorly with the anterior ligament of the same articulation. 

 This fibrous web, in uniting with the fibro-cartilaginous cap of tlie external humero-radial 

 ligament, and joining the small sigmoid cavity by its internal extremity, transforms this last 

 into a complete ring, covered with cartilage in its bony portion. The head or superior 

 extremity of the radius is also encrusted, over its entire contour, with a layer of cartilage— a con- 

 dition which permits it to glide not only in the concave face of the small sigmoid cavity, but 

 also on the internal face of tlie two ligaments which complete this cavity. 



Inferior radio-ulnar articidation.—Th\s is also a trociioid articulation analogous to the pre- 

 ceding, but inversely disposed. Thus, the concave articular surface is hollowed on the radius, 

 outside the inferior extremity; tlie convex surface lies within tlie ulna. These two facets an; 

 very small, and are maintained in contact by a diminutive peripheral fibrous capsule. A 

 strong interosseous ligament, situated beneath the artieular facets, also consolidates this 

 diathrosis, and concurs by its inferior border to form the antibrachial surface of the radio-carpal 

 articulation. A small synovial capsule is specially devoted to this articulation. 



Mechanism of the radio-ulnar joints. — The play of these two articulations is simultaneous, 

 and tends to the same end— that is, to the execution of the double rotatory movement which 

 constitutes supination and pronation. 



Supination is when the ulna remains fixed, and the radius pivots on it in such a manner as 

 to carry its anterior face outwards. Its superior extremity then turns from within forwards — 

 and even from before outwards if the movement is exaggerated, in the articular girdle formtd 



