DIARTHROSES 171 



one might suppose, and that the capsule is often very irregular in form, i. c, forms a variety of 

 sacculations. 



The foregoing are constant and necessary features in all diarthroses. Other 

 structures which enter into the formation of these joints are ligaments, articular 

 discs or menisci, and marginal cartilages. 



4. Ligaments. — These are strong bands or membranes, usually composed 

 of white, fibrous tissue, which bind the bones together. They are pliable, but 

 practically inelastic. In a few cases, however, e. g., ligamenta flava, ligamentum 

 nuchas, they are composed of elastic tissue. They may be subdivided, according 

 to position, into periarticular and intraarticular. Periarticular ligaments are fre- 

 quently blended with or form part of the fibrous capsule; in other cases they are 

 quite distinct. Strictly speaking, intraarticular ligaments, though within the 

 fibrous capsule, are not in the joint cavity; the synovial membrane is reflected 

 over them. The term seems justifiable, however, on practical grounds. Liga- 

 ments which connect directly opposed surfaces of bones are termed interosseous. 

 The special names are based usually on their position, form, and attachments, e. g., 

 lateral or collateral, cruciate, sacro-iliac, etc. In many places muscles, tendons, 

 and thickenings of the fasciae function as ligaments and increase the security of the 

 joint. Atmospheric pressure and cohesion play a considerable part in keeping the 

 joint surfaces in apposition. 



5. Articular discs or menisci are plates of fibro-cartilage or dense fibrous tissue 

 placed between the articular cartilages, and divide the joint cavity partially or 

 completely into two compartments. They render certain surfaces congruent, 

 e. g., femoro-tibial joint, allow greater range or varietj' of movement, and diminish 

 concussion. 



6. A marginal cartilage (Labrum glenoidale) is a ring of fibro-cartilage which 

 encircles the rim of an articular cavity. It enlarges the cavity and tends to pre- 

 vent fracture of the margin. 



Vessels and Nerves. — The arteries form anastomoses around the larger joints, 

 and give off branches to the extremities of the bones and to the joint capsule. The 

 synovial membrane has a close-meshed network of capillaries; the latter form loops 

 around the margins of the articular cartilages, but do not usually enter them. 

 The veins form plexuses. The synovial membrane is also well supplied with lymph- 

 atics. Nerve-fibers are especially numerous in and around the synovial membrane 

 and there are special nerve-endings, e. g., Pacinian bodies and the articular end- 

 bull)s described l)y Krause. 



Movements. — The movements of a joint are determined chiefly by the form 

 and extent of the joint surfaces and the arrangement of the ligaments. They are 

 usually classified as follows: 



1. Gliding. — This refers to the sliding of one practically plane surface on 

 another, as in the joints between the articular processes of the cervical vertebrae. 



2. Angular Movements. — In these cases there is movement around one or 

 more axes. Motion which diminishes the angle included by the segments forming 

 the joint is termed flexion, while that which tends to bring the segments into line 

 with each other is called extension. 



With reference to the joints of the lower parts of the limbs, it seems advisable 

 to employ the terms dorsal and volar or plantar flexion, since these joints can be 

 "overextended." Similarly, the terms dorsal and ventral flexion should be applied 

 to the corresponding movements of the spinal column. The meaning of the term 

 lateral flexion is evident. These movements are all rotations around axes which 

 are approximately either transverse or vertical. Depression, elevation, and lateral 

 movement of the lower jaw fall in this category. 



3. Circumduction. — This designates movements in which the distal part of 

 the limb describes a circle or a segment of one. In man such movement is easily 



