The Skeletal System 93 



inner parts of these animals are well shielded against mechanical in- 

 jury, and, in the case of the insect, against desiccation. For the verte- 

 brates, much of this function is taken over by the integumentary system 

 and its derivatives. Nevertheless the internal skeleton shields the 

 delicate nervous system by means of the brain case and backbone; the 

 lungs and heart are enclosed within the protective box of the ribs, 

 breastbone, and vertebral column. 



3. Body Form. The general form of the body is determined by 

 the shape of the skeletal elements. These individual parts, of course, 

 have their abrupt angles smoothed by overlying muscles and fat. The 

 general body form of prehistoric animals has been determined from 

 careful study of skeletal remains. Even the features of prehistoric 

 man are familiar due to the many paintings which have been made 

 from reconstructions of this type. 



4. Muscle Attachment. For both the insects and the verte- 

 brates the skeletal system serves as a place to attach muscles which make 

 bodily movements possible. \'arious bony protuberances in the verte- 

 brate skeleton serve as anchorage points for most of the large skeletal 

 muscles. 



5. Movement. The exoskeleton of the insects is divided into 

 various segments and the appendages are jointed. Similarly the endo- 

 skeleton of the vertebrates is composed of many bones of various shapes 

 and sizes. It is to these various parts that the organism owes its ability 

 to move. By muscle contractions, the individual bones or parts are 

 able to change their positions and thus bring about movements. 



6. Blood Cell Formation. It is only among the vertebrates that 

 the skeletal system assumes this important function of producing the 

 red blood corpuscles as well as many of the white blood cells. These 

 cells are produced in the central marrozu cavities of the long bones of the 

 appendages. 



7. Mineral Reserves. In recent years, it has become apparent 

 that the bones may be recognized as being a source through which the 

 calcium and phosphorus level of the blood is kept constant. Recent 

 work has indicated that in a fifty-day period, one-fourth of the mineral 

 content of the bones is changed. This is a constant, dynamic process 

 in which the calcium and phosphorus of the bones are being removed 

 and stored. By this means, the level of these two minerals in the blood 



