) 



214 THE ARTICULATIONS 



(fig. 255), the result being a syndesmosis. (3) Or, finally, the central portion of the inter- 

 mediate tissue may degenerate, so that an articular cavity is produced, the peripheral portions 

 being converted into connective tissue, forming a sleeve-like capsule surrounding the cavity, 

 continuous at either extremity with the periosteum of the articulating bones (fig. 255). This 

 is the articular capsule, and the connective-tissue cells arranging themselves in a layer upon 

 its inner surface give rise to a synovial membrane. As the result of these processes a diarthrosis 

 is produced, and from its mode of formation it is clear that the cavity of such an articulation is 

 completely closed. 



In a tj'pical diarthrosis there is therefore a ligamentous capsule which entirely encloses the 

 joint cavity, which is continuous with the periosteum of the bones entering into the articulation 

 but which is not attached to nor reflected onto the cartilaginous ends of the bones which consti- 

 tute the articulating surfaces. Such a capsule constitutes the primitive bond between the articu- 

 lating bones and furnishes a complete lubricating bag in which these smooth cartilaginous 

 ends glide over one another. This primitive capsule, however, becomes modified in most adult 

 joints, (1) by unequal development of various parts of the capsule; and (2) by the more or less 

 complete incorporation of other structures which are developmentally separate from the capsule, 

 Under the first heading come specially thickened bands which may be so distinctly marked off 

 from the rest of the capsule as to be named as separate ligaments (e. g., the temporo-mandibular 

 ligament of the mandibular joint). Again certain thickened bands of capsule may, with 

 alteration of joint contour, take up anatomical positions which are apparently separated from 

 the rest of the capsule; advanced examples of this process are, in all probability, seen in the 

 ligamentum teres of the hip-joint and the crucial ligaments of the knee. Under the second 

 heading comes a series of ligaments derived from a great variety of sources; the most common 

 origin being from the divorced or rearranged tendons of the muscles around the joint. 



Muscles arising from, or inserted into, bones in the immediate vicinity of a joint tend to 

 become metamorphosed into tendon near their attachments, and a comprehensive study of 

 myology in low vertebrate forms indicates that there is associated with this tissue-change a 

 tendency for the muscle to alter its point of attachment; hence a muscle originally inserted below 

 a joint may eventually come to have its insertion above the joint. In the same way, a muscle 

 arising above a joint may, as a result of altered environment, shift its origin to some point below 

 the joint. To this change of position the term migration of muscles has been applied. In 

 many instances a portion of the muscle equivalent to the distance between the original and the 

 acquired attachment persists as a fibrous band and fulfils the function of a ligament. This is well 

 seen in the knee-joint, where the tibial collateral ligament is derived from the adductor magnus, 

 this muscle having shifted its insertion from the tibia to the femur. In the same way the 

 jVndar collateral hgament represents the tendon of the peroneus longus, which has migrated 

 from the femur to the head of the fibula. 



Among other ligaments derived in a similar way from muscles may be mentioned the sacro- 

 tuberous ligament. This was originally the tendon of origin of the biceps femoris. (H. Morris, 

 Med. Times and Gazette, 1877, p. 361.) The sacro-spinous is derived from the fibrous retro- 

 gression of portions of the coccygeus. The sacro-coccygeal ligaments represent the muscles 

 which lift, depress, and wag the tail in those mammals furnished with such an appendage; 

 indeed, these ligaments are occasionally replaced by muscle-tissue. 



The coraco-Jiumeral ligament is derived from the original tendon of insertion of the pectoralis 

 minor, and not unfrequently the muscle is inserted into the lesser tuberosity of the humerus, the 

 ligament being then replaced by the tendon of the muscle. The coraco-clavicular, rhomboid, and 

 gleno-humeral ligaments are probably derived from modifications of the subclavius muscle. 



Other anatomical structures besides muscles may, when degenerated or functionally 

 altered, form the basis of ligaments in connection with joints. The spheno-mandibular ligament 

 is the fibrous remnant of the cartilaginous mandibular bar. 



/The pulpy substance in the centre of each intervertebral disc is derived from the notochord; 

 the apical ligament passing from the tip of the dens to the anterior margin of the foramen 

 magnum is a remnant of the sheath of the notochord, and indicates its position as it passed 

 from the vertebral column into the base of the cranium. The transverse ligament of the atlas 

 (as pointed out by Professor Cleland) is a persistent and functional form of the posterior conjugal 

 ligament uniting the rib-heads in seals and many other mammals, whilst the interosseous 

 ligament of the head of a rib in man is the feeble representative of this structure in the thoracic 

 region of the spine. The ligameyitum conjugale costarum was described by Mayer in 1834 

 I (MuUer's Archiv fiir Anatomic). According to Luschka's account of this ligament it would 

 1 seem as though the posterior superior fibres of the capsule of the costo-central joint represented 

 \it in man, rather than the interosseous ligament. 



THE MOVEMENTS OF JOINTS 



The movements wliich may take place at a joint are either gliding, angular, 

 rotatory, or circumductory. 



The gliding motion is the simplest, and is common to all diarthrodial joints; it consists of a 

 8im{)lc sliding of t he ap[)OHod surfafics of t he boiuis upon one another, without angular or rotator}' 

 motion. It is the only kind of motion permitted in the carpal and tarsal joints, and in those 

 between the articular proces.ses of the vortcbrir. 



The angular motion is more elaborate, and increases or diminishes the angle between difi^er- 

 ent parts. There are four varieties, viz., flexion and extension, which bend or straighten the 

 various joints, and take place in a forward and l)a"kward direction (in a perfect hinge-joint this 

 is the only motion permittedj; and adduction and abduction, which, extiept in the case of the fin- 

 gers and toes, signifies an approach to, or deviation from, the median plane of the body. In the 



