18 



Anatomy oi? the; Rabbit. 



Cartilage union or synchondrosis occurs in certain situations, as in the 

 basal region of the skull. Union by fibrocartilage or symphysis is 

 characteristic of the articulation of the two sides of the pelvis (symphysis 

 pubis). 



The articulations of bones are of two types — immovable articulations 

 or synarthroses, and movable articulations, diarthroses, 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. 12) 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 



Ftg 12. Section of the elbow-joint of a four-day-old 

 rabbit: c, capsule; e.b., endochondral bone in the 

 distal epiphysis of the humerus; e.a., extensor muscles 

 of the forearm; e.m., extensors of the hand; f.a., flexors 

 of the forearm; f.m., flexors of the hand; h., humerus; 

 ol., olecranon; r.. radius; s.c, synovial cavity; u., ulna. 



(Fig. 13), interarticular cartilages (men- 

 isci) are enclosed between the bone 

 surfaces, and the latter are connected 



Fig. 1.1. Section of the knee-joint .of a 

 four-day-old rabbit: c., capsule; f., distal 

 epiphysis of femur; l.p., patellar ligament 

 (insertion of quadriceps femoris); p.v., 

 popliteal vessels and nerves; t., proximal 

 epiphysis of tibia; +., ligament of the 

 lateral meniscus: x., posterior cruciate 

 ligament; xx , anterior cruciate ligament. 



directly by short ligamentous cords. The 



various ligaments 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 



