46 THE FROG 
lies in an undifferentiated portion of the protoplasm. Draw a 
portion of a fiber, showing details of structure, representing the 
fiber about 25 mm. in diameter. 
B. General Anatomy of the Skeletal Muscles.—A skeletal 
muscle consists of a bundle of striated muscle fibers bound 
together by connective tissue and fastened at each end to different 
bones, by means of strong cords called tendons. Of the two 
attachements of a muscle, one is usually to a more fixed and cen- 
tral part, the other to a more movable and peripheral part; the 
former attachment is called the origin of the muscle, the latter its 
insertion. , 
Since muscles can exert force only while shortening and not 
while elongating (in other words, since they can pull but cannot 
push), many of the muscles or sets of muscles are arranged in 
pairs, the members of a pair working in apposition to each other. 
The many actions of which the frog is capable require a large 
number of separate muscles, only a few of which can be con- 
sidered here. To illustrate the mode of attachment, functions 
and mutual relationships of the skeletal muscles a study will be 
made of the principal muscles of the hind limbs. 
The motions of which a limb is capable are conditioned, not 
only by the number and arrangement of its muscles, but by the 
form of its joints, some of which allow only certain movements. 
In the hind limb the ball-and-socket joint at the head of the 
femur allows great freedom of motion, but other joints permit 
of movement approximately in only one plane. 
With respect to the kind of motion produced the muscles of 
the limbs of the frog may be classified as abductors, adductors, 
rotators, flexors, and extensors. When the thigh of the frog is 
pulled forward (7. e., away from the median line), it is said to 
be abducted; when it is pulled backward (1. e., toward the median 
line), it is said to be adducted. When a muscle of the thigh is 
placed obliquely its effect is generally to rotate the thigh on its 
long axis. When the leg (crus) of the frog is bent so as to 
form an angle with the thigh, the leg is said to be flexed; when 
it is brought in line with the thigh, the leg is said to be extended. 
The leg is incapable of rotation. The foot may be flexed, ex- 
tended, or slightly rotated. 
