156 SPECIAL PHYSIOLOGY. 



The second kind of movable joints have pulley-like surfaces, and 

 execute hinge-like movements ; hence they are called trochlear (from 

 trochlea, a pulley), or ginglyform (^A^oc, a hinge). In these joints, 

 the end of one bone is modelled so as to present a median groove with 

 two lateral projections ; whilst the end of the other bone has a median 

 projection and two lateral concavities ; or the surfaces are otherwise 

 adapted by opposite curves, so as to admit of free motion in one plane 

 only, though, of course, in two directions, viz., backwards and for- 

 wards. The best example of this kind of articulation, amongst the 

 larger joints, are to be found in the elbow, the ankle, the wrist, and 

 the knee (Fig. 3) ; the knee being the least perfect, because, when 

 flexed, it permits of slight rotation. In the smaller joints, the articu- 

 lations of the phalanges of the fingers and toes with one another, are 

 also examples of hinge-joints. In these joints the ligaments, on one 

 aspect, which may be called the aspect of extension, from which the 

 joint can be most strongly bent, are more or less thin and loose ; whilst 

 at the sides, and on the aspect of flexion, they are, as a rule, very 

 strong : this twofold arrangement gives the necessary strength to a 

 hinge-joint, without impeding, or preventing, its almost complete flex- 

 ion. In the knee, especially, the lateral ligaments and the posterior 

 ones, which project into the back of the joint, and have a crucial form 

 (Figs. 3, 8), serve to check the extension of the leg upon the thigh, 

 when the one is in a line with the other ; and, in the act of standing, 

 when the weight of the body is thrown upon the fully extended knee, 

 these ligaments save the expenditure of much muscular force. In the 

 case of the fingers' and toes, the flexor and extensor tendons act as 

 additional ligaments to the joints. 



The third kind of movable articulations have ball and socket sur- 

 faces, and power of movement in all directions; they are named 

 enarthrodia. In these joints, the one bone presents a cuplike cavity, or 

 socket, either shallow or deep, lined, of course, with cartilage; whilst 

 the other bone presents a rounded extremity, forming, more or less, 

 part of a spheroid, and also covered with its cartilage. When the 

 receiving cavity is shallow, it is called a glenoid, when deep, a cotyloid 

 cavity. Examples of the ball and socket joint are met with in the 

 hip, which is the most perfect of these joints in the body; in the 

 shoulder; in the head of the astragalus, amongst the tarsal bones, 

 where this moves in the cup-shaped cavity of the scaphoid bone; in 

 the head of the os magnum, amongst the carpal bones, where it artic- 

 ulates with the scaphoid and semilunar bones: and, lastly, in the 

 several joints at the bases of the fingers and toes, where these articu- 

 late with the rounded heads of the metacarpal and metatarsal bones. 

 The synovial capsule of the ball and socket joints, is generally loose; 

 but it is fortified by strong ligaments in certain positions, where the 

 motion requires to be restrained. In the larger joints of the shoulder 

 and hip, the cavity of the socket is 'deepened by a fibrous rim or 

 border attached all round its margin. In the shoulder, the tendon of 

 the biceps muscle passes through the joint, and, undoubtedly, exercises 

 a ligamentous control over it, and affords it support; whilst, in the 

 hip, an internal ligament, named ligamentum teres, passes from one 



