Motor System — Muscles and Skeleton 



Osteoblasts Calcifying connective-tissue bundles Bone matrix 



103 



Fig. 99. Development of dermal (secondary) bone from mesenchyme. From a 

 section of the mandible of a human embryo of four months. ( X 240.) (Modi- 

 fied from Bremer: "A Text-Book of Histology," Philadelphia, The Blakiston 

 Company.) 



Depending on method of development, two kinds of bone are dis- 

 tinguished. In one the skeletal structure is first formed in cartilage in 

 the early embryo. Later the cartilage is invaded by bone-forming tissue, 

 gradually destroyed, and eventually entirely replaced by bone (Fig. 98) . 

 Bones developing in this way are called "cartilage bones" or 

 "replacing bones." Membrane-bones are produced directly by 

 embryonic tissue, not preceded by cartilage (Fig. 99). Most membrane- 

 bones develop in deep extensions of the embryonic tissue which other- 

 wise produces the dermal layer of the skin, and are accordingly called 

 "dermal bones." Membrane-bones are often called "investing" 

 bones because they commonly develop closely adjacent to cartilage or 

 cartilage-bones, as when the embryonic cartilaginous lower jaw be- 

 comes "invested" by thin sheets of dermal bone. Whether of cartilage 

 or bone, the growth of vertebrate skeletal parts requires no periodic 

 "molting." The growth of the skeleton keeps pace consistently with 

 that of the whole animal. 



In general, skeletal structures serve for mechanical protection 

 from impact of external agencies, or for support or internal parts, or 

 for attachment of muscles. The protective function of the verte- 

 brate skeleton is most important in relation to the central nervous 

 organs (the brain and spinal cord) and to such delicately constructed 

 sense-organs as the olfactory (organ of smell), eye, and ear. Some 

 bones of the skull are protective only. The long bones of a limb provide 

 internal support and serve for attachment of muscles. The vertebral 

 column provides protection for the spinal cord dorsally and, in the tail, 

 to the caudal blood-vessels ventral! y. It is important also as a longi- 



