CHORD ATA — VERTEBRATA 329 



while the elbow and knee, at the junction of humerus and radius, 

 femur and tibia respectively, are hinge joints. 



Joints. — The articulating surfaces of bones at immovable 

 joints, as between the bones of the skull, are separated by a 

 fibrous membrane only, hence the contiguous bone surfaces 

 are irregular and they are stronger thus than when smooth. 

 At joints where there is very slight movement, as between con- 

 tiguous vertebrae, each surface is faced with a layer of cartilage 

 and the bone surface is but slightly rough. At joints where 

 movement is very free, as the ball-and-socket and the hinge 

 joints, the bordering surfaces are also covered with cartilage, 

 but the two bones are likewise held apart by a fluid ; the bone 

 surfaces here are very smooth. Hence in fossil bones the place 

 and extent of movement between bone surfaces is easily noted. 



Muscles. — The comphcated muscular apparatus, by which 

 the various parts of the body are moved, is primarily derived 

 from the vertical muscle segments (myomeres) lying on each 

 side of the spinal column (as seen typically in Amphioxus and 

 the fish). But in the cat, as in all forms above the fish, this 

 arrangement is obscured, since the muscles are greatly modified 

 both in form and position in adaptation to terrestrial life. The 

 muscles, commonly called flesh, are made up of fibers (coarsest 

 in fish), which are bound together into bundles. These bundles, 

 or true muscles, are usually fastened by each end to a rough- 

 ened surface of a bone by means of the tough membrane of the 

 periosteum, the larger the muscle the larger being the roughened 

 surface or projection for attachment. Since muscles make up 

 the bulk of the cat's body, the muscular system largely deter- 

 mines the shape of its trunk and limbs (see page 18). 



More than four hundred of the muscles found in the cat's 

 body occur also in man with the same general location, function 

 and nerve supply. 



All muscles are either voluntary, under control of the 

 will, or involuntary, incapable of control by will. The vol- 



