256 



ARTICULATION. 



an angle with each other, the motion is that of 

 flexion, the restoration to the direct line is ex- 

 tension. These two motions belong to what 

 Bichat calls limited opposition ; the flexion and 

 extension of the fore-arm on the arm illustrate 

 it. Sometimes a motion of this kind takes 

 place in four directions, indicated by two lines 

 which cut at right angles. This is best under- 

 stood by a reference to the motions which take 

 place at the hip-joint : there it will be seen 

 that the thigh-bone may be brought forward so 

 as to form an angle with the trunk, flexion or 

 it may be restored, extension ; it may be sepa- 

 rated from the middle line of the body so as to 

 form an angle with the lateral surface of the 

 trunk, abduction or it may be restored and 

 made to approximate the middle line, adduc- 

 tion. Bichat terms this " opposition vague." It 

 is evident that a joint, which is suscepti- 

 ble of these four motions, may also move in 

 directions intermediate to them. When these 

 motions are performed rapidly, one after the 

 other, it appears as one continuous motion, in 

 which the distal extremity of the bone describes 

 a circle indicating the base of a cone whose 

 apex is the articular extremity moving in the 

 joint; this motion is called circwnditction. 



Rotation is simply the revolving of a bone 

 round its axis. It is important to bear this 

 definition in mind : through losing sight of it 

 many anatomists have attributed rotation to a 

 joint which really does not possess it. 



The varieties of the diarthrodial joint are as 

 follows : 



a. Arlhrodia (articulatio plana or plani- 

 formis.J In this species the surfaces are plane 

 or one is slightly concave, and the other slightly 

 convex : the motion is that of gliding, limited in 

 extent and direction only by the ligaments of 

 the joint or by some process or processes con- 

 nected with the bones. The examples in man 

 are, the articular processes of the vertebra, the 

 radio-carpal, carpal, carpo-metacarpal, infe- 

 rior radio-ulnar, superior tibio-fibular, tarsal 

 and tarso-metatarsal, temporo-maxillary, acro- 

 mio-clavicular and sterno-clavicular joints. 

 This last articulation and the wrist-joint possess 

 a greater latitude of motion than the others; 

 the former, in consequence of the shape of its 

 articular surfaces : each surface is convex in 

 one diameter and concave in the other, so that 

 the gliding that takes place in this joint is in 

 the direction of the long and short diameters, 

 which intersect each other at right angles. It 

 is capable, therefore, of vague opposition in 

 those lines, but certainly not in the interme- 

 diate directions, the nature of the surfaces being 

 calculated to prevent this. The wrist owes 

 its mobility to the laxity of its ligaments, which 

 permit it to move as well in its transverse as in 

 its antero-posterior diameters, as also in the in- 

 termediate directions; it consequently admits 

 of vague opposition and circumduction. The 

 articulation of the metacarpal bone of the thumb 

 with the trapezium, is also an arthrodia very 

 similar to the sterno-clavicular, but with a 

 greater degree of motion. Arthrodial joints are 

 generally provided with ligaments, placed at the 



extremities of the lines in the direction of 

 which the gliding motion takes place. 



b. Enarthrosis (diarthrosis orbicular is ball- 

 and-socket joint.) This is a highly developed 

 arthrodia. The convex surface assumes a glo- 

 bular shape, and the concavity is so much 

 deepened as to be cup-like, hence the appella- 

 tion ball and socket. The ball is kept in appo- 

 sition with the socket by means of a capsular 

 ligament, which is sometimes strengthened by 

 accessory fibres at certain parts that are likely 

 to be much pressed upon. The best example 

 of enarthrosis is the hip-joint, and next to it the 

 shoulder : in the latter the cavity is but imper- 

 fectly developed. All the quadrupeds have 

 their shoulder and hip joints on this construc- 

 tion, and the same common plan is observed in 

 the vertebrata generally whose extremities are 

 developed. In birds and reptiles the bodies of 

 the vertebrae are articulated by enarthrosis, and 

 the solid calcareous spines on the external 

 surface of the shells of echinida are adapted to 

 round tubercles on which they move, thus ex- 

 hibiting a very complete form of enarthrosis.* 



This species of joint is capable of motion of 

 all kinds, opposition and circumduction being 

 the most perfect, but rotation limited. Indeed 

 what is called rotation at the hip-joint, is 

 effected by a gliding of the head of the femur 

 from before backwards, and vice versa in the 

 acetabulum ; it is not a rotation of the head 

 and neck, but of the shaft of the femur. 



c. Ginglymus (yiyy'h.vp.os, cardo, articulatio 

 card in i for mis, articulation en charnicre, cnge- 

 nou, hinge-joint.) The articular surfaces in 

 the hinge-joint are marked with elevations 

 and depressions which exactly fit into each 

 other, so as to restrict motion in all but one 

 line of direction. They are always provided 

 with strong lateral ligaments, which are the 

 chief bonds of union of the articular surfaces. 



The elbow and ankle joints in man are per- 

 fect ginglymi ; the knee also belongs to this 

 class, but is by no means a perfect specimen, 

 for in a certain position of the bones of this 

 joint, the ligaments are so relaxed as to allow a 

 slight rotation to take place. The phalangeal 

 articulations, both of the fingers and toes, are 

 ginglymi. This form of joint is most exten- 

 sively employed among the lower animals. In 

 quadrupeds, most of the joints of the extremi- 

 ties come under this head. In amphibia and 

 reptiles, too, there are many examples of the 

 hinge-joint. The bivalve shells of conchiferous 

 mollusca are united by a very perfect hinge, 

 and a great number of the joints of Crustacea 

 and insects are of this form. 



The true ginglymus is only susceptible of 

 limited opposition : hence the knee-joint can- 

 not be regarded as a perfect example ; in fact, 

 in the perfect ginglymus there is every possible 

 provision against lateral motion. 



d. Diart/irosis rotatorius (commissura tro- 

 choidcs.) A pivot and a ring constitute the 

 mechanism of this form of joint. The ring is 



* Vide fig. 9 in Grant's Comp. Anat. p. 21. See 

 also the article ECHINODERMATA. 



