THE ARTICULATIONS IN GENERAL. 177 



borders or angles, which, for this purpose, generally present very uneven 

 surfaces. 



Sometimes they arc cut perpendicularly and simply roughened ; at other times 

 they are bevelled, and joined by means of fine laminse or trifling inequalities ; 

 again, they are notched into deep and sinuous dentations ; and lastly, one bone 

 is fixed into a groove cut in the other. It will be understood that such formations 

 of the articular surfaces will limit their movements, and assure the solidity of 

 their union. 



Modes of Union, — A fibrous tissue interposed between these synarthrodia! 

 surfaces, unites them closely to each other. It has absolutely the same texture as 

 the primary cartilage of the bones, and, like it, possesses the property of becoming 

 ossified after having been vascularized. This ossification, which causes the 

 disapppearance of the sutures, occurs earlier inwards than outwards. The 

 periosteum, in passing from one bone to another, also concurs in bringing about 

 a more complete synarthrosis. It should, therefore, be included in their means 

 of union. 



Movements. — These are very obscure, and only noticeable in young animals, 

 by the elasticity they communicate to the bony walls of the cranium or face. In 

 the adult, they may be said to be null. 



Classification. — Tliere are four principal descriptions of sutures .- 



1. When two wide bones correspond by means of denticulations fitting into 

 each other, the suture is named frup, Umhosa, serrated, or dentated. Example : 

 the articulations unitmg the three portions of the parietal bone. 2. If the opposite 

 borders of two bones in contact are widely bevelled, one inwards, the other out- 

 wards, it forms a scaly or squamous suture {squamosa). Example : the parieto- 

 temporal articulations. 3. When the union of bones takes place by plane or 

 roughened surfaces, cut perpendicularly on their borders or angles ; this constitutes 

 the harmoaia suture, or suture hij juxtaposition (or apposition). Example : the 

 occipito-temporal articulations. 4. The schiruii/Iesis, mortised suture, synchron- 

 drosis, or gomphosis, results from the reception of a bony plate into a groove more 

 or less deep in another bone. Examples : the spheno-frontal and supermaxillo-nasal 

 articulations ; the teeth in the alveolar cavities. 



General Characters of the Amphiarthroses or Symphyses. 



Articulae Surfaces. — They are frequently smooth, and formed almost on 

 the same model as the diarthrodial surfaces. They are covered by a thin layer of 

 cartilage ; but, instead of being smooth and polished, they are more or less rugged, 

 without, however, presenting the anfractuous disposition of the majority of synar- 

 throdia! surfaces. 



Modes of Union. — The organs which perform this office are : 1. Fibro- 

 cartilage, which establishes continuity between the articular surfaces. 2. Ribbon- 

 shaped and peripheral ligaments (Fig. 115, b a). These latter do not differ from 

 the analogous bands attaching the diarthrodial articulations. With regard to the 

 fibro-cartilage, it is distinguished from the complementary discs of these same 

 articulations, by a less intimate mixture of the cartilaginous and fibrous elements 

 entering into its composition. The last may be sometimes absent, as well as the 

 peripheral bands ; and then the articulation only differs from the synarthroses by 

 the extent of motion it permits. Occasionally, the interarticular fibro-cartilages 

 are excavated by one or two little narrow cavities (Fig. 115, b b, c) ; but these 

 are never lined by a synovial membrane, like the diarthrodial cavities. 



