LOCOMOTIVE ORGANS. THE JOINTS. 155 



other concave, Fig. 3. These ends, or articular surfaces of the bones, 

 are moreover covered by a thin layer of closely adherent cartilage, 

 which serves to deaden shock, and to facilitate, to the utmost degree, 

 the movements of one bone upon the other. Surrounding the joint 

 closely at all sides, and attached to the opposed bones, near the bor- 

 ders of their cartilaginous articular surfaces, is a membranous sac or 

 closed tube, called the synovial capsule, which limits the joint, and 

 secretes a viscid fluid named synovia, which serves to lubricate the 

 articular surfaces, and so diminishes friction and prevents the percep- 

 tion of grating, or noise, to the individual. This ropy fluid escapes 

 when a joint is opened, and is vulgarly called joint-oil, though it is not 

 of an oily or greasy nature, but is an albuminous liquid, which has 

 an alkaline reaction and a slightly saline taste ; it resembles the white 

 of egg ; hence its name (from GUV, sun, like, d>ov, an egg). Outside the 

 synovial capsule, and more or less blended with it, are the proper con- 

 necting tissues between the bones, or the special ties of the joint, 

 called the ligaments (from ligare, to tie). These ligaments are com- 

 posed of white fibrous connective tissue. In certain parts, the fibrous 

 bands of which they consist, are spread out, and merely strengthen 

 slightly the loose synovial capsule ; at other parts, they are collected 

 into dense bands of various shapes, tying the bones very firmly to- 

 gether. Besides permitting motion between the bones, the ligaments 

 are generally so inserted around each articulation, as to restrain the 

 movements in certain directions, or at some determinate point. Some- 

 times the muscles, or else the tendons of muscles, exercise what 

 might be called a ligament-like protection around a joint ; as, for 

 example, the muscles around the shoulder, and the tendons around the 

 ankle-joint. The articular surfaces of the movable joints are also 

 held together by the atmospheric pressure which acts on the whole 

 body. This is best exemplified, experimentally, in the ball and socket 

 joints, and especially in the hip joint, as will be presently mentioned. 

 The movable joints, or diarthroses, are classed in three divisions, ac- 

 cording to the shape of their articular surfaces, and to the character 

 of the movements 'performed at them. In the simplest form, the sur- 

 faces are more or less plane, and the movements gliding ; these are 

 the planiform joints, sometimes called arthrodia. They are met with 

 chiefly, in the tarsal and metatarsal articulations of the foot, and in 

 the carpal and metacarpal articulations of the hand ; also in the 

 articulations of the collar-bone with the scapula and with the ster- 

 num, in the articulations of the lower jawbone, in the junction of the 

 upper end of the fibula with the tibia, in the joints between the oc- 

 ciput and atlas, and between the articular processes of the several 

 cervical arid dorsal vertebrae, in the junctions of the tubercles of the 

 ribs with the vertebrae, and in those of the costal cartilages with the 

 sternum. The ligaments of such joints are usually short and strong, 

 especially in the foot and hand, and, together with the neighboring 

 processes of bone, serve to check the movements of the bones. These 

 joints allow of limited motion in many directions, deaden shock, and 

 impart elasticity and slight flexibility, without impairing the strength 

 of the part in which they are found. 



