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HUMAN ANATOMY. 



Synchondrosis is the union of two bones by an intervening strip of cartilage, 

 which usually ultimately becomes replaced by bone. Such is the union between 

 the pieces of the body of the sternum and between certain bones of the base of the 

 skull. The term is also applied to the union of the shaft and the epiphyses of long 

 bones. 



Half-Joints, including Symphysis and Syndesmosis. From the stand-point of 

 development, there is no funclamental difference between symphyses and the true 

 joints. In both cases a small cavity appears withm the intervenmg mesoblastic tissue 

 connecting the ends of the embryonal bones. This small cavity, in the case of the 

 true joints, rapidly increases, and later is lined by the flattened mesoblastic cells 

 investing the subsequently differentiated synovial membrane. When, on the con- 

 trary, the bones are to become united by dense fibrous and fibro-cartilaginous tissue, 

 as in the case of a symphysis, the interarticular space is always a mere cleft sur- 

 rounded by the interlacing and robust bundles of the dense tissue forming the union 

 in the mature joint. 



A symphysis implies great strength and very limited and indefinite motion, 

 there being no arrangement of surfaces to determine its nature. The chief function 

 of this form of union seems rather to be to break shocks. The central cavity is not 

 always found. The symphysis pubis (Fig. 361) is a typical half -joint. Those con- 

 necting the bodies of the vertebrae are usually so classed^ but it is not certain that 

 they quite agree either in structure or development with the description A transi- 



FiG. 134. 



Wormian 

 bones 



Diagrams of various forms of suture. A, serrated ; B, squamous ; C, harmonic ; Z>, grooved. 



tional form leading from the symphysis to the true joint is one in which the limited 

 synovial cavity, instead of being in the centre of a mass of fibro-cartilage, lies between 

 two cartilaginous surfaces, somewhat like that of a true joint, but so interlocked and 

 surrounded by short, tense fibres as to preclude more than very slight motion. This 

 arrangement is often seen in the articulation between the sacrum and ihum, some- 

 times improperly called the sacro-iliac synchondrosis. 



Syndesmosis is to be included among the half-joints. It is the binding 

 together of bones by fibres, either in bundles or ■\?, a membrane, without any inter- 

 vening cartilage ; an example of this arrangement is seen in the union effected by 

 the interosseous ligament in the lower tibio-fibular articulation. 



True Joints. — These articulations develop in a similar manner to the half- 

 joints, except that the opposed ends of the developing bones are of hyaline carti- 

 lage, fibro-cartilage being present only at the sides, except in the case of a compound 

 joint, where 't forms the intervening plate. The tissue at the sides of the articular 

 cleft differentiates into two layers, — the inner, which is the synovial membrane, consist- 

 ing of a layer of cells continuous with the superficial layer of the cartilage-cells and 

 secreting a viscid fluid, the synovia, which lubricates the joint ; and the outer part, 

 which becomes a fibrous bag called the capS7ilar h'gamenf. The latter, in its simplest 

 form, consists of only enough fibrous tissue to support the synovial membrane. The 

 capsular ligament is strengthened by accessory /i(raments developing in or around it, 

 the arrangement of which depends on the needs of the joint. During development. 



