MUSCULAR TISSUES. a3 
The articulations of bones are of two types—immovable 
articulations or synarthroses, and movable articulations, diar- 
throses, or joints. In the former, motion is either absent or at 
least greatly restricted. In the latter, it is definitely provided for 
through the presence of joint-structures. Thus in a joint (Fig. 13) 
the apposed surfaces of the bones are accurately modelled in 
relation to one another, and are moreover covered by a layer 
of cartilage, the latter forming a joint cushion. Between the two 
surfaces is a space, the cavity of the joint, containing a viscid 
material, the synovia, which serves for lubrication. The space is 
enclosed by a connective tissue capsule. The strength of the joint 
depends largely on the enclosing capsule, but it is 
usually greatly increased by the presence of 
accessory ligaments. In the more complex joints, 
such as that of the knee (Fig. 13 B), interarticular 
cartilages (menisci) are enclosed between the bone 
surfaces, and the latter are connected directly 
by short ligamentous cords. The various liga- 
ments of a joint permit free motion of the bones, 
but only up to a certain point. : 
Several differences are observable in joints 
according to the form of the apposed surface and 
the kind of motion provided for. Thus in the i 
ball-and-socket joint or enarthrosis, exemplified Gari tone 
: : section of the mus- 
by those of the shoulder and hip, a bone is able to cular tunic of the 
move in various directions about its base of 
attachment, although actually, in the limbs, this motion is almost 
restricted to an anteroposterior direction. In the ginglymus or 
hinge-joint, as exemplified by the distal articulations of the limb, 
motion is restricted to a single plane. The gliding joint or arth- 
rodia is one in which a slight degree of motion is made possible by 
one surface slipping over the other; it is exemplified in the accessory 













articulations of the vertebral arches. 
3. Muscular Tissues. 
Muscular tissues are the active portions of the individual 
muscles of the skeleton and of the muscle coats of visceral organs. 
Their chief feature consists in the elongation of the cells to form 
