730 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



only ones with which we are concerned. Four different varieties of this 

 form of joint have been described, according to the kind of motion 

 permitted in each. 



a. The Rotatory Joint, or Diartlirosis Eotatoria. In this class of 

 joint the movement is limited to rotation, the joint being formed by a 

 pivot-like process turning within a ring or the ring on the pivot, the ring 

 being formed partly of bone and partly of ligament. The articulation 

 of the atlas and the occiput is an example of such a joint. In the elbow- 

 joint a similar rotatory articulation is met with, where in the radio-ulnar 

 articulation the ring is formed by the lesser sigmoid cavity and the orbicular 

 ligament while the head of the radius rotates within the ring. Only in 

 animals in whom pronation and supination of the hand are possible does 

 this movement occur ; it is, therefore, absent in the horse and ox. In 

 general, it may be said that in animals provided with the clavicle this 

 motion of supination and pronation is usually present. 



b. The Ball and Socket Joint, or the Enarthrosis. In this joint motion 

 in all directions is possible, and it is formed by the reception of the 

 globular head of a long bone, into a deep, cup-like cavity, hence called 

 ball and socket, the parts being kept in apposition by a capsular ligament 

 and accessory ligamentous bands. The hip- and shoulder- joints are 

 examples of this class. 



c. The Hinged or Ginglymous Joint. In this form motion is only 

 possible in one plane and only in two directions, forward and backward, 

 the articular surfaces being moulded to each other in such a way that a 

 solid cylinder moves within a greater or lesser segment of a hollow 

 cylinder. The joint between the ulna and the humerus is a most perfect 

 example of a ginglymous joint, while the joints between the phalanges 

 and between the inferior and superior maxillary bones of the carnivora 

 are other examples. The pastern-joint of the horse is a modified form 

 of this joint and is often spoken of as a screw-joint. 



d. The Gliding Joints, or the Arthrodia. In this class motion of a 

 gliding character takes place. Such joints are formed by the approxima- 

 tion of plane surfaces, or one slightly concave, the other slightly convex, 

 movement between them being limited by the ligaments or the osseous 

 processes surrounding the articulation. Such articulations are seen 

 between the vertebra, metatarsal and tar sal bones, and others. 



The forces which move the joints are found in the contraction of 

 striped muscular fibres. The extent of contraction for which the muscle 

 is capable depends upon its length, and therefore we speak of long and 

 short muscles. A muscle whose function it is to bring its points of 

 origin and insertion nearer to each other must necessarily be a long 

 muscle, while the muscles whose contraction only leads to slight change 

 of place are usually short muscles. It will always be found that the 



