MOVEMEXTS OF JOINTS 179 



THE VARIOUS 310 YEMENIS OF JOINTS 



The movements which 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 simple sliding of the apposed surfaces of the Ijones upon one another, 

 without angular or rotatory motion. It is the only kind of motion permitted in the 

 carpal and tarsal joints, and in those between the articular processes of the vertel^ra?. 



The angular motion is more elaborate, and increases or diminishes the angle 

 between diti'erent parts. There are four varieties, viz. : flexion and extension, which 

 bend or straighten the various joints, and take place in a forAvard and backward 

 direction (in a perfect hinge-joint this is the only motion permitted); and adduction 

 and abduction, which, except in the case of the fingers and toes, signifies an approach 

 to, or deviation from, the centre line of the boch'. In the case of the hand, the line 

 to or from which adduction and abduction are made is drawn through the middle 

 finger, while in the foot it is through the second toe. 



Rotation is the revolution of a bone al^out its own axis without much change 

 of position. It is only seen in enarthrodial and trochoidal joints. The knee also 

 permits of slight rotation in certain positions, Avhich is a distinctive feature of this 

 articulation. 



Circumduction is the movement compounded of the four angular movements 

 in quick succession, by which the moving bone describes a cone, the proximal end 

 of the l^one forming the apex, while the distal end describes the base of the cone. 

 It is seen in the hip and shoulder, as well as in the carpo-metacarpal joint of the 

 thumb, which thus approximates to the ball-and-socket joint. 



In some situations where a variety of motion is required, strength, security, and 

 celerity are obtained by the combination of two or more joints, each allowing a 

 different class of action, as in the case of the Avrist, the ankle, and the head with 

 the spine. Many of the long muscles, which pass over two or more joints, act on 

 all, so tending to co-ordinate their movements and enabling them to be produced 

 with the least expenditure of power. Muscles also act as elastic ligaments to the 

 joints; and when acting as such, are diff users and combiners, not })roducers of 

 movement; the short muscles producing the movement, the long dift'using it, and 

 thus allowing the short muscles to act on more than one joint. 



^Muscles are so disposed at their attachments near the joints as never to strain 

 the ligaments by tending to pull the bones apart, but, on the contrary, they add to 

 the security of the joint by bracing the bones firmly together during their action. 



The articulations may be divided for convenience of description into those: 1. 

 of the Skull; 2. of the Trunk; 3. of the Upper Limb; and 4. of the Lower Limb. 



1. THE ARTICULATIONS OF THE SKULL 



The articulations of the skull comprise (1) the temporo-mandibular; and (2) 

 those between the skull and the spinal column, namely (a) betAveen the occiput 

 and atlas; (6) betAveen the atlas and axis; and (c) the ligaments Avhich connect the 

 occiput ancl axis. 



The union of the atlas and axis is described in this section because, firstly, there 

 is often a direct communication betAveen the synovial cavity of the transverse 

 axoidean and the occipito-atlantal joints; secondly, the rotatory movements of the 

 head take place around the odontoid process; and, thirdly, important ligaments from 

 the odontoid process pass over the atlas to the occiput. 



