) 



212 THE ARTICULATIONS 



and knee-joints, there is usually none within the shoulder-joint. Sometimes 

 these fringes become villous and pedunculated, and cause pain on movement of 

 the joints. They contain fibro-fatty tissue, with an isolated cartilage cell or two. 

 The synovial membrane is well supplied with blood, especially near the margins of 

 the articular cartilages and in the fringes. It secretes a thick, glairy fluid like 

 white of egg, called synovia, which lubricates the joint. Another variety of 

 synovial membrane is seen in the bursae, which are interposed between various 

 moving surfaces. In some instances bursse in tlie neighbourhood of a joint may 

 communicate with the synovial cavity of that joint. 



CLASSIFICATION OF ARTICULATIONS 



Joints may be classified: — (a) From an anatomical point of view, with regard 

 to the substances and the arrangement of the substances by which the constituent 

 parts are united, (b) From a physiological standpoint, with regard to the greater 

 or smaller mobility at the seat of union, (c) From a physical standpoint, either 

 the shapes of the portions in contact being mainly considered or the axes round 

 which movement can occur. Or again (d) a combination of the preceding methods 

 may be adopted, and this is the plan most generally followed. None of the classi- 

 fications hitherto used is quite satisfactory, but perhaps, on the whole, that 

 suggested by Prof. Alex. Macalister is the least open to objection, and therefore 

 with slight modification it is utilised here. 



There are three chief groups of joints: — 



1. Synarthroses. In joints of this class the bones are united by fibrous tissue. 



2. Synchondroses. Or joints in which the uniting substance intervening be- 

 tween the bones is cartilage. 



3. Diarthroses. The constituent parts of joints of this class are (a) two or more 

 bones each covered by articular hyaline cartilage; (6) a fibrous capsule uniting the 

 bones, and (c) a synovial membrane which lines the fibrous capsule and covers 

 any part of bone enclosed in the capsule and not covered with articular cartilage. 

 An interarticular plate of cartilage may or may not be present. 



Synarthroses. — 



(a) Sutures or immovable joints, in which the fibrous tissue between the bones is too 

 small in amount to allow movement. 



(1) Harmonic. The edges of the bones are comparatively smooth and are in even 

 apposition, e. g., vertical plate of palate and maxilla. 



(2) Squamous. The margin of one bone overlaps the other, e. g., temporal and 

 parietal. 



(3) Serrate. The opposed edges interlock by processes tapering to a point. 



(4) Dentate. The opposed eclges are dovetailed, e. g., occipital and parietal. 



(5) Limbous. The opposed edges alternately overlap, v.. g., parietal and frontal. 



(6) Schindylesis. A ridge or flattened process is received into a corresponding 

 socket, <>. g., rostrum of sphenoid and vomer. 



(7) (iomphosis. A peg-like process is lodged in a corresponding socket, e. g., the 

 fangs of the teeth. 



(6) Syndesmoses. Movable joints in which the fibrous tissue between bones or carti- 

 lages is sufficiently lax to allow movement between the connected parts, c. g., 

 thyreo-hyoid membrane. Interosseous membranes of forearm and leg. 

 2. Synchondroses. — -In all syiichoiidrosos a certain amount of movement is possible, and 

 the}' are often called aniphiarthroses. 



(1) True synchondroses. The cartilage connecting the bones is the remains of the bar 

 in which the bones were ossified, e. g., occipito-sphenoidal joint. 



(2) False synchondroses. The plate of cartilage intervening between and connecting 

 the bones is fibro-cartilage and is not part of the; (lartilage in which the bones were 

 o.ssified, but is develoi)ed separately, (!. g., intervertebral joint and pubic sym- 

 physi.s. Th(! arti(!ular end of ea(;h bone may be covered with hyaline cartilage 

 and there may be a more or less well-marked cavity in the intervening plate of 

 filjfo-ciirtilagc. 



;'.. Diarthroses. — In diarthrodial joints the surfaces in contact may be equal and similar 

 or unequal and dissimilar. In the former case the joints are homomorphic; in the 

 latter, heteromorphic. 

 (A) Homomorphic. 



(a) Plane or arlhrodial. Flat surfaces, admitting gliding movement, e. g., 



intercarpal and ;i(!romi()-(;lavicular joints. 

 {h) Ephippial. S;iddle-shaped surfaces placed at right angles to each other, ad- 

 mitting free movement in all directions, e. g., metacarpo-phalangeal joint 

 of thumb. 



