276 THE ARTICULATIONS 



The movements are chiefly flexion and extension, very little side to side movement being 

 permitted, and that only when the joint is slightly bent. Thus this joint more nearly approaches 

 the simple hinge character than the corresponding articulations of the fingers. The thumb 

 gets its freedom of motion at the carpo-metacarpal joint; the fingers get theirs at the meta- 

 carpo-phalangeal, but they are not endowed with so much freedom as the thumb enjoys. 



Muscles which act upon the metacarpo-phalangeal joint of the thumb. — Flexors. — Flexor 

 pollicis brevis, flexor poUicis longus. Extensors. — Extensor poUicis brevis, extensor poUicis 

 longus. 



11. THE INTERPHALANGEAL ARTICULATIONS 

 Class. — Diarthrosis. Subdivision.— -Ginglymus . 



The ligaments which unite the phalanges of the thumb and of the fingers are 

 (in addition to the articular capsule) : — 



Accessory volar. Collateral. 



The accessory volar (or glenoid) ligament (fig. 305), sometimes called the sesamoid body, 

 is very firmly connected with the base of the distal bone, and loosely, by means of fibro-areolar 

 tissue, with the head of the proximal one. It blends with the collateral ligaments at the sides, 

 and over it pass the flexor tendons. Occasionally a sesamoid bone is developed in the cartilage 

 of the interphalangeal joint of the thumb. 



The collateral ligaments (figs. 304 and 305) are strong bands which are attached to the rough 

 depressions on the sides of the upper phalanx, and to the projecting margins of the lower phalanx 

 of each joint. They are tense in every position, and entirely prevent any side to side motion; 

 they are connected posteriorly with the expansion of the extensor tendon. 



Dorsally (fig. 305) the joint is covered in by the deep surface of the extensor tendon, and a 

 little fibro-areolar tissue extends from the tendon, and thickens the posterior portion of the 

 synovial sac, completing the articular capsule. 



The synovial membrane is loose and ample, and extends upward a little way along the shaft 

 of the proximal bone. 



The arteries and nerves come from their respective digital branches. 



The relations of the interphalangeal joints are the flexor and extensor tendons and the 

 digital vessels and nerves. 



The movements are limited to flexion and extension. Flexion is more free, and can be 

 continued till one bone is at a right angle to the other, and is most free at the junction of the 

 first and second bones; the second phalanx can be flexed on the first through 110° to 115° when 

 the latter is not flexed. The greater freedom of flexion is due to the greater extent of the articu- 

 lar surface in front of the heads of the proximal bones, and to the direction of the fibres of the 

 collateral ligaments, which pass a little forward to their insertion into the distal bone. 



The muscles which act upon the interphalangeal joints are the extensors and flexors of 

 the digits. 



THE ARTICULATIONS OF THE LOWER LIMB 



The articulations of the lower limb are the following: — 



1. The hip-joint. 



2. The knee-joint. 



3. The tibio-fibular union. 



4. The ankle-joint. 



5. The tarsal joints. 



(3. The tarso-metatarsal joints. 



7. The intermetatarsal joints. 



8. The metatarso-phalangeal joints. 



9. The interphalangeal joints. 



1. THE HIP-JOINT 

 Class. — Diarthrosis, Subdivision. — Enarthrodia. 



The hip is the most typical example of a ball-and-socket joint in the body, the 

 round ho.-id of the femur being received into the cup-sluipcd cavity of the acetab- 

 ulum. Hoth articidar surfaces are coattul with cartilage, that covering the head 

 of the femur being thicker above where it lias to bear the weight of the body, and 

 thinning out to a mere edge; Ijelow; the pit for the ligamentum teres is the only 

 part uncoated, but the cartilage is somewhat heaped up around its margin. 

 Covering the acetabulum, the cartilage is horseshoe-shaped, and thicker above 

 than l)elow, being deficient over the dei)ression at the Ijottom of the acetabulum, 



