176 THE ARTICULATIONS. 



2. Flexion, which brings two bony pieces nearer each other, by closing more 



or less their angle of union. 



3. Extension, the inverse movement, during which the bones are straightened 



on each other. 



4. Adduction, which brings the inferior extremity of the movable bone towards 



the median line. 



5. Abduction, the contrary movement to the preceding. 



6. Circumduction, or the sling movement, during which the bone passes 



successively through the last four positions. 



7. Rotation, in which one bone pivots on another. 



Classification of the Diarthroses. — The basis of this classification is 

 founded on the configuration of the articular surfaces and the nature of the 

 movements they permit. This double base serves to establish five kinds of 

 diathrodial articulation : 



1. Enr/rfhrosis, characterized by the reception of an articular head within a 

 cavity of appropriate form. This articulation, the surfaces of which are derived 

 from a sphere, may be the seat of the most extensive and varied movements — 

 flexion, extension, abduction, adduction, circumduction, and rotation. Example : 

 the coxo-femoral articulation. 



2. The trochlea?!, angular ginglymoid, or perfect hinge articulation, when the 

 articular surfaces are formed into trochlea, reciprocally fitting into each other, 

 and the movements of which — flexion and extension only — are executed, from 

 before to behind, with the precision of a hinge. Example : the tibio-tarsal 

 articulation. 



3. The piiwt, trochoid, or lateral ginglymoid articulation, is a diarthrosis formed 

 by a pivot which turns in a semi-cylindrical cavity. Rotation is the only move- 

 ment. Example : the atlo-axoid articulation. • 



4. The condyloid, or imperfect hinge articulation, which permits, like the 

 preceding, the two principal movements of extension and flexion, and the acces- 

 sory movements of rotation or lateral inclination. The articular surfaces, though 

 very diversely shaped, nevertheless exhibit in all the articulations one or more 

 condyles opposed to an equal number of oval excavations. Example : the femoro- 

 tibial articulation. 



5. Arthrodia, or 'plamform diarthrosis, is constituted by plane or nearly plane 

 iacets. Gliding is the only possible movement. Example : the carpo-metacarpal 

 articulation. 



Nomenclature. — The names of the articulations are usually those of the 

 bones which form them. For instance, the scapulo-humeral articulation is the 

 joint between the scapula and humerus ; the intervertehrnl articulations join to 

 each other the various bones constituting the spine. When the qualifying name 

 of an articulation is composed of two elements, as in the first instance, it is well 

 to place first the word which indicates the bone usually most fixed. 



General Characters of the Synarthroses. 



Sutures (Fig. 1 1 5, a) are the temporary articulations which exist only at an early 

 period of life. They nearly all disappear in the adult animal, in consequence of 

 the bones forming them becoming consolidated. They belong almost exclusively 

 to the bones of the head. 



Articular Surfaces. — The bones forming these come in contact by their 



