882 MUSCULAR MOTION. 



in pairs at the roots of the thiu^b and great toe ; between the tendons 

 of the flexor muscles and joints; and, occasionally, at the roots of the 

 fingers and small toes. 



The bones are connected by means of articulations or joints, which 

 differ materially%om each other. To all the varieties, names are ap- 

 propriated, which form a difficult task for the memory of the anatomical 

 student. Technically, every part at which two bones meet, and are 

 connected, is called an articulation, whether any degree of motion 

 exists or not. This, indeed, is the foundation of the division that pre- 

 vails at the present day, the articulations being separable into two 

 classes ; the immovable or synarthroses ; and the movable or diarthroses. 

 Synarthroses are variously termed, according to their shape. When 

 the articular surfaces are dovetailed into each other, the joint is called 

 a suture. This is the articulation that prevails in the bones of the 

 skull. Harmony is when the edges of bones are even, and merely 

 touch ; as in the bones of the head in quadrupeds and birds. When a 

 pit in one bone receives the projecting extremity of another, we have 

 a case of gomphosis. It is exhibited in the union between the teeth 

 and their sockets. Lastly, schindylesis is when the lamina of one bone 

 is received into a groove of another ; as in the articulation of the 

 vomer, which separates the nasal fossae from each other. The movable 

 articulations comprise two orders : amphiarthroses, in which the two 

 bones are intimately united by an intermediate substance, of a soft 

 and flexible character, as in the junction of the vertebrae with each 

 other; and diarthroses, properly so called. The last admit of three 

 subdivisions enarthroses or ball and socket joints; the condyloid, in 

 which, owing to the head being oval, the movements are not as easy in 

 all directions as when it is spherical ; and the ginglymoid or ginglymus, 

 in which the motion can occur in only one direction, as in a hinge. 

 The farther subdivision of the joints belongs more to anatomy than 

 to physiology. 



The articular surfaces of bones never come into immediate contact. 

 They are tipped with a firm, highly elastic substance, called cartilage ; 

 which, by its smoothness, enables the bones to move easily upon each 

 other ; and may have some influence in deadening shocks, and defending 

 the bones, which it covers. The arrangement of cartilage varies ac- 

 cording to the shape of the extremity of the bone. If it be spherical, 

 the cartilage is thick at the centre, and gradually diminishes towards 

 the circumference. In the cavity, the reverse is the case ; the cartilage 

 is thin at the centre, and becomes thicker towards the circumference ; 

 whilst on a trochlea or pulley its thickness is nearly every where alike. 



An admirable provision against displacement of bones at the articu- 

 lations is seen in the ligaments. These, by the French anatomists, are 

 distinguished into two kinds fibrous capsules, and ligaments properly 

 so called. The former are a kind of cylindrical sac, formed of a firm, 

 fibrous membrane ; open at each extremity, by which they closely 

 embrace the articular ends of bones ; and loose, when the joint admits 

 of much motion. In this way, the articulation is completely enclosed : 

 they generally bear the name of capsular ligaments. The ligaments, 

 properly so called, are bands of the same kind of tissue, which extend 



