ARTICULATIONS OF lUE ANTERIOR LIMBS. 197 



are even attached to it in a very evident manner. The elhow articulation, closed 

 in front and on the sides by the three ligaments just described, has no particular 

 ligaments posteriorly ; but it is powerfully strengthened there by the olecranian 

 insenioa of the extensor muscles of the forearm, and by the tendons of origin of 

 the five flexor muicles of the metiicarpus or phalanges. 



8i/ii0i'ial membrane. — -This membrane is very extensive, and, stretched out 

 on the internal face of the before-mentioned ligaments, forms, behind, three great 

 culs-de-sac of prolongraent : a superior, occupying the olecranian fossa, and 

 covered by a fatty cushion, as well as by the small extensor muscle of the fore- 

 'irm ; ' two lateral, which descend from eacli side of the ulnar beak, and are 

 distinguished as internal and external — the first lines the tendon of the external 

 flexor of tlie metacarpus, the second facilitates the play on the upper radial 

 extremity of the four flexor muscles of the foot or digits, and which are attached 

 in common to tht. «pitrochlea. This synovial sac also furnishes the radio-ulnar 

 articulation with a diverticulum, which descends between the bones of the fore- 

 arm to below the adjacent diarthrodial facets. 



Movements. — Flexion and extension. 



In fleximi, the two bones do not approach each other directly, the inferior 

 extremity of the radius deviating a little outwards. This is due more to the 

 slight obliquity of the articular grooves, than to the difference in thickness 

 between the external and internal extremities of the humeral surface. 



Extension is limited by the reception of the beak of the olecranon in its fossa, 

 and by the tension of the lateral ligaments ; so that the two bones cannot be 

 straightened on one another in a complete manner, or placed on the same line. 



In the Dog and Cat, the external lateral ligiment is very thick, and forms in its inferior 

 moiety a titiro-cartilaginoiis cap which is fixed o'l the ulii:i and radius, and united in fronc ♦<) 

 the annular ligament of the superior radio-ulnar joint. This cap), with the last-named liga- 

 ment, complttcs the Osteo-fibroiis ring in which tlie superior extremity of the radius turns. 

 The internal lateral lig^iment is inserted, by two very sliort fasciculi, into the ulna an<I inner 

 side of the head of the ladins. A third fasciculus, deeper and median, much more developed 

 than the first, and covered by the inferior insertion of the flexors of the forearm, descends 

 between the radius and uhia to the jwsterior face of the former, and is there inserted near the 

 inferior altacliment of the external ligament, svhich it appears as if about to join. 



In Man, the elbow articulation is fornud marly on the same plan as that of the Dog and 

 Cat. The radiua and ulna move together when the forearm is flexed and extended on the 

 humerus. 



3. Radio-ulnar Articulation. 



jirticular surfaces. — The two bones of the forearm articulate by diarthrodial 

 and synarthrodial surfaces. 



n. T'he 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. 



h. The synarthrodial surfaces are plane and roughened, and are also two on 

 each bone : one, superior, extends below the 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 pp. 104, 105). 



' Some grey elastic fibres which cover this cul-de-sac externally, have been wrongly 

 described as a posterior membranifurm ligament 



