Muscular Tissues. 



33 



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 

 ball-and-socket joint or enarthrosis, exemplified 

 by those of the shoulder and hip, a bone is able to 

 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 



Fig. 14. Involun- 

 tary muscle, from a 

 section of the mus- 

 cular tunic of the 

 intestine. 



