THE PHYSIOLOGY OF THE SKELETON. 45 



modified and adapted to one another in accordance with the character 

 and extent of the movements which there take place. 



2. Hyaline cartilage, which is closely applied to the articulating end of each 



bone. The smoothness of this form of cartilage facilitates the move- 

 ments of the opposing surfaces, while its elasticity diminishes the force 

 of shocks and jars imparted to the bones during various muscular acts. 

 In a number of joints, plates or discs of white fibro-cartilage are inserted 

 between the surfaces of the bones. 



3. A synovial membrane, which is attached to the edge of the hyaline cartilage, 



entirely inclosing the cavity of the joint. This membrane is composed 

 largely of connective tissue, the inner surface of which is lined by endo- 

 thelial cells, which secrete a clear, colorless, viscid fluid the synovia. 

 This fluid not only fills up the joint-cavity, but, flowing over the 

 articulating surfaces, diminishes or prevents friction. 



4. Ligaments tough, inelastic bands, composed of white fibrous tissue 



which pass from bone to bone in various directions on the different 

 aspects of the joint. As white fibrous tissue is inextensible but pliant, 

 ligaments assist in keeping the bones in apposition, and prevent dis- 

 placement while yet permitting of free and easy movements. 

 Classification of Joints. All joints may be divided, according to the 



extent and kind of movements permitted by them, into (i) diarthroses; (2) 



amphiarthroses; (3) synarthroses. 



i. Diarthroses. In this division of the joints are included all those which 

 permit of free movement. In the majority of instances the articulating 

 surfaces are mutually adapted to each other. If the articulating sur- 

 face of one bone is convex, the opposing but corresponding surface is 

 concave. Each surface, therefore, represents a section of a sphere or 

 a cylinder, which latter arises by rotation of a line around an axis in 

 space. According to the number of axes around which the movements 

 take place all diarthrodial joints may be divided into: 



i. Uniaxial Joints. In this group the convex articulating surface is a 

 segment of a cylinder or cone, to which the opposing surface more or 

 less completely corresponds. In such a joint the single axis of rotation, 

 though nearly, is not exactly at right angles to the long axis of the bone, 

 and hence the movements flexion and extension which take place 

 are not confined to one plane. Joints of this character e.g., the elbow, 

 knee, ankle, the phalangeal joints of the fingers and toes are, therefore, 

 termed ginglymi, or hinge- joints. Owing to the obliquity of their 

 articulating surfaces, the elbow and ankle are cochleoid or screw-ginglymi. 

 Inasmuch as the axes of these joints on the opposite sides of the body 

 are not coincident, the right elbow and left ankle are right-handed 

 screws; the left elbow and right ankle, left-handed screws. In the 

 knee-joint the form and arrangement of the articulating surfaces are 

 such as to produce that modification of a simple hinge known as a 

 spiral hinge, or helicoid. As the articulating surfaces of the condyles 

 of the femur increase in convexity from before backward, and as the 

 inner condyle is longer than the outer, and, therefore, represents a 

 spiral surface, the line of translation or the movement of the leg is also 

 a spiral movement. During flexion of the leg there is a simultaneous 



