436 
MOVEMENT 
hardened. In Figure 386 is shown an X-ray photograph of 
a recently broken femur. In Figure 387 the same bone is 
shown. A large “ callus ” of new bone has formed around 
the broken ends, which gradually hardens, making this part 
of the bone stronger than ever. 
344. Structure of Bone. — One of the long bones from the 
arm or leg illustrates best the several parts. There is the long 
shaft with knobs on each end. These enlargements are the 
heads of the bone. The outer surface is covered with a 
tough membrane. If such a bone is sawed lengthwise, the 
following additional parts can be recognized: the long 
central cavity of the shaft filled with marrow; the hard bone 
of the shaft; the spongy bone toward each end; and the 
thin layer of cartilage covering each head. 
LABORATORY STUDY 
Study the skeleton, and examine long, flat, and irregular bones. 
How is the bone modified to do its work ? 
STUDENT REPORT 
Make a report on the skeletal structures of a nim als as follows: 
External 
Absent 
Jointed 
Not 
Jointed 
Horny 
Bony 
Internal 
Paramecium . 
Crayfish . . . 
Clam .... 
Frog .... 
Man, etc. . . 
345. Muscles. — The muscles are the lean parts of the 
flesh of animals. They are covered by the skin and are usu¬ 
ally dark in color. Birds are an exception, for their breast 
meat is generally white. Muscles are of two kinds : volun¬ 
tary (governed by the will), such as those which we use in 
