ii MECHANICS OF LOCOMOTOE APPAEATUS 99 



instance, articular anchylosis, and after amputations or resections, 

 the mechanical requirements to which the bones naturally conform 

 are changed, the systems of lamellae of the spongy substance alter 

 considerably. 



The enlargements usually presented at the ends of the long 

 bones, the ridges, tuberosities, and spines are for the purpose of 

 giving the muscles large and adequate surfaces of attachment. 



The bones of which the skeleton is made up are united rigidly 

 together, or in such a manner as to permit a more or less extensive 

 displacement and movement on each other. The bones united by 

 sutures (synarthroses), as those which compose the cranium, are 

 perfectly immobile ; those united by means of cartilages (synchon- 

 droses) are semi-mobile, or admit of very limited movements. Such 

 are the symphyses of the pubis and innominate bone and the 

 synchondroses of the ribs and vertebrae. Finally, the bones 

 united by articular capsules are semi -mobile (amphiarthrose), 

 mobile (arthrose), or very mobile (diarthrose), according to the 

 form of the articulation. The articulations of the carpal and 

 tarsal bones belong to the first category ; the elbow, knee, and ankle 

 to the second ; the shoulder and hip-joints to the last. 



In all these true articulations the heads of the bone are covered 

 with a layer of cartilage, to the edges of which the fibrous articular 

 capsule, which connects the two bones and surrounds the articular 

 cavity, is attached. Each capsule is covered internally by a pave- 

 ment epithelium which extends over the joint cartilage, and 

 secretes the synovia, a colourless, transparent, viscous fluid, formed 

 by the mucous metamorphosis of the epithelium, which is destined 

 to lubricate the articular surfaces and enable them to move easily 

 one upon the other. 



Externally, fibro-elastic ligaments strengthen the capsule, and 

 prevent or limit to a greater or less extent the movements of the 

 articular heads. 



From the physiological point of view, articulations can be 

 subdivided into the classes proposed by A. Fick. The first 

 comprises the synchondroses (ribs and vertebrae) and the 

 amphiarthroses (joints between the tarsus and carpus). In these 

 articulations the bony surfaces never change their relations, 

 and can only be fixed or moved to a limited extent by the 

 elasticity of the interpolated fibro-cartilages, or pericapsular liga- 

 ments. The bones thus united are in stable equilibrium, to which 

 they return immediately when any external cause which has 

 displaced them from thejr normal position ceases to act. The 

 arthroses and diarthroses form the second class, as the articular 

 surfaces change their relations while moving. The bones thus 

 articulated are in unstable equilibrium, that is, they remain in 

 whatever position they are placed by external causes, until this is 

 removed by some force working in the opposite direction. A 



