ARTICULATIONS. 51 



by means of the processes which continue its tissue into the vascular channels 

 within the bone. In addition to being the most important source of nutrition, 

 on account of its blood-vessels, the periosteum responds to demands for the 

 production of new bone, whether for renewed growth or for repair, and, 

 when occasion requires, again becomes active as the chief bone-forming 

 tissue, its cells reassuming the role of osteoblasts. 



THE ARTICULATIONS. 



Broadly considered, the individual pieces composing the skeleton are 

 united by articulations of two kinds: (i) the continuous joint {synarthrosis) , 

 in which the union is effected by uninterrupted masses of tissue and the bones 

 have no, or only very slight, play; and (2) the discontinuous joint (diarthro- 

 sis), in which the bones are joined by tissue containing definite joint-cavities 

 and, therefore, are free to move on each other. 



Synarthrosis may be: (a) by dense connective tissue (sutura)^ as in 

 the immovable articulations of the skull, where the intervening periosteum is 

 intimately connected with the bones by 

 penetrating processes composed of white 

 and elastic fibres; (<) by ligamentous tissue 

 (jsyndesmosis) arranged in dense fibrous 

 bands, which stretch between the adjacent 

 bones and permit of slight movement, as 

 between the lower ends of the tibia and 

 fibula; and (c) by cartilaginous tissue (syn- 

 chondrosis), which affords a rigid or flexible 

 joint according to the proportions of the 

 hyaline or fibrous varieties of the tissue. 

 Thus, where the bond of union consists 

 exclusively of hyaline cartilage, as between 

 the component pieces of a young bone, 

 immobility results; where fibrous cartilage 

 predominates, as in the massive interverte- 

 bral disks, the union provides great strength 

 and some flexibility. Outside the spongy 



substance or nucleus pulposus, which occupies the centre of the disk and is 

 regarded as the modified remains of the chorda dorsalis of foetal life, the inter- 

 vertebral disk consists of interwoven bundles of fibrous cartilage. Towards 

 the surface the more typical cartilaginous tissue is replaced by a peripheral 

 layer resembling tendon in structure. 



Diarthrosis or the true joint includes, as its essential parts, the artic- 

 ular cartilage and the capsule; interarticular and adaptation cartilages and 

 synovial fringes are secondary structures which may or may not be present. 



Capsule 



Synovial membrane 



Articular cartilage 



Joint cavity 



Reflection of 

 synovial membrane ~ 

 Epiphyseal bone 



FIG. 63. Diagram showing essential 

 parts of a typical joint. 



deeper, the cells are more spherical in form and disposed in groups, while in 

 the layers still nearer the underlying bone, the cartilage-cells often show a 

 characteristic columnar arrangement, in which the rows of cells lie in a gen- 

 eral way perpendicular to the surface of the bone. The matrix immediately 

 overlying the bone is commonly the seat of more or less marked calcifica- 

 tion, a zone of calcified matrix thus forming the union between the cartilage 

 and the bone. 



