COMPARATIVE ARTHROLOGY 209 



superior interdigital ligaments to the third phalanges of the small digits, and blend below with 

 the outer part of the inferior interdigital hgament. 



Dog. — Each joint has a capsule and two lateral ligaments. The distal joints 

 have also two elastic dorsal ligaments (Ligamenta dorsalia), which extend from the 

 proximal end of the second phalanx to the ridge at the base of the third phalanx 

 (Fig. 133). They produce dorsal flexion of the joint, and thus raise or retract the 

 claws when the flexor muscles relax. The distal sesamoids are represented by 

 complementary cartilages attached to the volar margins of the articular surfaces 

 of the third phalanges. 



Three interdigital ligaments restrict the spreading apart of the digits. Two 

 of these cross the volar surface of the proximal parts of the chief digits, i. c, one 

 for the second and third, the other for the fourth and fifth; they blend with the 

 annular ligaments on either side. The third ligament is attached on either side to 

 the foregoing ligaments and the annular ligaments of the third and fourth digits, 

 and curves downward centrally, ending in the large pad on the paw. 



Articulations of the Pelvic Limb 

 sacro-iliac joint 



This joint and the pelvic ligaments present no very striking differences in the 

 other animals. The sacro-sciatic ligament in the dog is a narrow but strong 

 band which extends from the posterior part of the lateral margin of the sacrum to 

 the tuber ischii; it is the homologue of the ligamentum sacro-tuberosum of man. 



HIP JOINT 



Ox. — The shallowness of the acetabulum is compensated by the greater size 

 of the marginal cartilage. The head of the femur has a smaller radius of curvature 

 than that of the horse, and the articular surface extends a considerable distance 

 outward on the upper surface of the neck. The round ligament is entirely intra- 

 articular; it is small, and sometimes absent. The pubo-femoral or accessory 

 ligament is absent. 



There are no important differences in the other animals. 



STIFLE JOINT 



Ox. — There is a considerable communication between the femoro-patellar 

 and inner femoro-tibial joint cavities; this is situated as in the horse, but is wider. 

 A small communication with the external femoro-tibial capsule sometimes occurs. 

 The two femoro-tibial capsules usually communicate. The middle patellar liga- 

 ment is not sunken, as there is no groove on the tuberosity of the tibia where it is 

 attached. The external patellar ligament fuses completely with the tendon of 

 insertion of the biceps femoris, and a synovial bursa is interposed between them 

 and the external condyle of the femur. 



In the other animals there is a single ligamentum patellae, and the synovial 

 sacs communicate so freely as to constitute a common joint cavity. In the dog 

 the semilunar cartilages are united anteriorly by a transverse ligament, and the 

 posterior part of the capsule contains the two Vesalian sesamoids, which articulate 

 with the condyles of the femur. 



TIBIO-FIBULAR JOINTS 

 Ox. — The proximal end of the fibula fuses with the external condyle of the 

 tibia. The distal end remains separate, and forms an arthrosis with the distal end 

 14 



