122 



THE CAT. 



[CHAP. iv. 



The bag, formed of areolar tissue lined with scaly epithelium; is 

 called the SYNOVIAL CAPSULE or membrane. It is more or less 

 connected with the fibrous bands, or ligaments, which bind the 

 bones together in the neighbourhood of the joints and generally 

 help to restrain movement to certain directions. 



In certain joints, as in that of the cat's knee, additional structures 

 are interposed, called INTER-ARTICULAR CARTILAGES. These are 

 formed of fibro-cartilage, and, though placed between the bones, are 

 not within the cavity of the synovial sac, but attached to its outer 

 surface, which is so folded or doubled as to embrace them. 



Movable joints are of different kinds, with corresponding diversities 

 of form. Thus they may be such that the bones are capable of 

 nothing more than a slight gliding movement one on the other, the 

 apposed articular surfaces being nearly flat and even. Such joints 

 are termed PLANIFORM, or ARTHRODIA, and as examples may be 

 selected the bones of the carpus and tarsus. 



A second kind of joint is that called BALL-AND-SOCKET, or ENAR- 

 THRODIA, where one rounded portion of bone is received into a 

 corresponding cup or socket. When the cup is very deep, the cavity 

 is said to be cotyloid as in the hip-joint ; when it is shallow, it is 

 said to be ylenoid, as in the shoulder. The shallower is the cavity, 

 the freer and wider is the power of motion. If the terminal con- 

 vexity is elongated it is termed a condyle as is the articular head 

 of the lower jaw. 



A third and a more complex kind of articulation is called a 

 HINGE or GINGLYFORM JOINT, and is also called pulley-like or 

 trochlear. In such joints the surface of one bone is more or less 

 cylindrical, and fits into a corresponding socket. The former gene- 

 rally exhibits a median groove with a projection on each side of it, 

 while the other bone has a corresponding median prominence bordered 

 by two concavities ; but various arrangements of opposing curves may 

 exist, tending to limit motion more or less completely to one plane. The 

 best example of such a joint is that of the elbow, a more imperfect 

 one is that of the knee. This kind of joint may be so complicated 

 as to form what is called a double hinge-joint (as between the meta- 

 carpal of the pollex and the trapezium). In that articulation each 

 bone is concave in one direction, and convex in the direction at right 

 angles with the former, that is to say, each bone presents a saddle- 

 shaped surface, and the two articulate together, as a rider is placed 

 with respect to the saddle he bestrides. 



There is another kind of joint, rarely found, and termed a RING 

 and COLLAR- JOINT, or lateral ginglymus. It is when the head of a 

 bone is received into a strong ring or collar, formed of ligament, 

 which allows the bone to rotate round its own axis. Such a joint 

 exists between the upper (proximal) parts of the two bones of the 

 fore- arm. 



The last kind of joint to be noticed is that called a PIVOT- JOINT, 

 an example of which is furnished by the two uppermost bones of the 

 neck. Here one bone serves as an axis or pivot on which the other 



