ARTICULATIONS OF THE ANTERIOR LIMBS. 147 



inferior moiety a fibre-cartilaginous cap which is fixed on the ulna and radius, and united 

 in front to the annular ligament of the superior radio-ulnar joint. This cap, with the last- 

 named ligament, completes the osteo-fibrous ring in which the superior extremity of the 

 radius turns. The internal lateral ligament is inserted by two very short fasciculi into 

 the ulna and inner side of the head of the radius. A third fasciculus, deeper and median, 

 much more developed than the first, and covered by the inferior insertion of the flexors 

 of the fore-arm, descends between the radius and ulna to the posterior face of the 

 former, and is there inserted near the inferior attachment of the external ligament, 

 which it appears as if about to join. 



In Man, the elbow articulation is formed nearly on the same plan as that of the Dog 

 and Cat. The radius and ulna move together when the fore-arm is flexed and extended 

 on the humerus. 



3. ^Radio-ulnar Articulation. 



Articular surfaces. The two bones of the fore-arm correspond by 

 diarthrodial and synarthrodial surfaces. 



a. The diarthrodial surfaces consist of four undulated, transversely 

 elongated facets, two of which are radial and two ulnar. The first border, 

 posteriorly, the great articular surface forming the elbow joint ; the second 

 are situated beneath the sigmoid notch. 



b. The synarthrodial surfaces are plane and roughened, and are also two 

 on each bone : one, superior, extends below thai diarthrodial facets to the 

 radio-ulnar arch ; the other, inferior, more extensive, occupies all the 

 anterior face of the ulna from this arch ; on the radius it forms a very 

 elongated triangular imprint which descends to the lower fourth of the bone. 

 See pages 75, 76. 



Mode of union. Two iuterosseous and two peripheral ligaments. 



a. The interosseous ligaments, interposed between the synarthrodial 

 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 circumstance 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. 



b. 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 fore- arm. 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. Very obscure in youth ; nearly null when consolidation of 

 the two bones takes place. 



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

 age. 



In the Dog and Cat, we have already seen (p. 87) that the radius and ulna 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 shortness, they are 

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



Superior radio-ulnar artit ulation, This is a trochoid articulation, which only allows 

 movements of rotation or pivoting. 



