232 THE AETICULATIONS OF THE HORSE 



The accessory ligament (Lig. accessorium)'^ does not occur in the domestic 

 animals other than the equidae. It is a strong band detached from the prepubic 

 tendon of the abdominal muscles (Fig. 581). It is directed outward, backward, 

 and upward, passes through the acetabular notch dorsal to the transverse ligament, 

 and ends behind the round ligament in the notch on the head of the femur. The 

 origin of the pectineus muscle is perforated by the ligament, which furnishes at- 

 tachment to many fibers of the muscle. 



The synovial membrane is reflected over the intracapsular parts of these lig- 

 aments and covers the fossa acetabuli. A pouch also extends from the acetabular 

 notch for a variable distance along the subpubic groove above the accessory liga- 

 ment. 



Movements. — This joint is capable of all the movements of a ball-and-socket 

 joint, viz., flexion, extension, abduction, adduction, rotation, and circumduction. 

 The greatest range of movement is displayed in flexion and extension. When 

 standing at rest, the joint is partially flexed, the articular angle (in front) being 

 about 100° to 115°. The other movements occur to a very limited extent in norma] 

 action. Abduction appears to be checked by tension of the round and accessory 

 ligaments. The accessory ligament is tensed most promptly by inward rotation of 

 the thigh. 



THE STIFLE JOINT 



This joint (Articulatio genu), which corresponds to the knee-joint of man, 

 is the largest and most elaborate of all the articulations. Taken as a whole, it 

 may be classed as a ginglymus, although it is not a typical example of the group. 

 In realitjr it consists of two joints — the femoro-patellar and the femoro-tibial. 



The femoro-patellar articulation (Articulatio femoro-patellaris) is formed 

 between the trochlea of the femur and the articular surface of the patella. 



Articular Surfaces. — The trochlea consists of two slightly oblique ridges, 

 with a wide and deep groove between them. The medial ridge is much the larger 

 of the two, especially at its proximal part, which is wide and rounded. The lateral 

 ridge is much narrower, and is more regularly curved; its proximal part lies about 

 an inch behind a frontal plane tangent to the medial one. The articular surface 

 of the patella is much smaller than that of the trochlea. It is completed medially 

 l)y a supplementary plate of fibro-cartilage (Fibrocartilago patellae), which curves 

 over the medial ridge of the trochlea. A narrow strip of cartilage is found along 

 the lateral border also. The articular cartilage on the trochlea completely covers 

 both surfaces of the medial ridge, but extends only a short distance on the lateral 

 surface of the outer ridge. 



Joint Capsule. — This is thin and is very capacious. On the patella it is 

 attached around the margin of the articular surface, but on the femur the line of 

 attachment is at a varying distance from the articular surface. On the medial side 

 it is an inch or more from the articular cartilage; on the lateral side and above, 

 about half an inch. It pouches upward under the quadriceps femoris for a distance 

 of two or three inches, a pad of fat separating the capsule from the muscle. Be- 

 low the patella it is separated from the patellar ligaments by a thick pad of fat, 

 but inferiorly it is in contact with the femoro-tibial capsules. The joint cavity is 

 the most extensive in the body. It usually communicates with the medial sac of 

 the femoro-tibial joint cavity by a slit-like opening situated at the lowest part of 

 the medial ridge of the trochlea. A similar, usually smaller, communication with 

 the lateral sac of the femoro-tibial capsule is often found at the lowest part of the 

 lateral ridge. 



The medial communication is rarely absent in adult horses, but is liable to be overlooked 

 on account of the fact that it is covered by a valvular fold of the synovial membrane. It is about 



^ This is also commonly called the pubo-femoral ligament. 



