THE SKELETON 



375 



passes. Part of the base of the cranium is formed by the sphenoid and eth- 

 moid bones. Part of the former bone extends upward between the frontal 

 and temporal bones. The skull shows wonderful adaptations for its varied 

 functions. The brain case is compactly built, its arched roof giving 

 strength. The eye and inner 

 ear are protected in sockets 

 of bone. The lower jaw works 

 upon a hinge, and furnishes 

 attachment for strong mus- 

 cles which move the jaw. Try 

 to add other adaptations to 

 this list. 



Long bone; C, internal cavity; 

 H.B., hard bone; S.B., spongy bone. 



The skull; F., frontal bone; P., parietal bone; T., 

 temporal bone ; SP., sphenoid bone; O., occipital 

 bone ; U.J., superior maxillary (upper jaw) bone ; 

 L.J., inferior maxillary (lower jaw) bone. 



Internal Structure of a Long Bone. 

 Laboratory Exercise. — A better under- 

 standing may be gained of the internal 

 structure of the long bone from observa- 

 tions made, for example, on a long leg 

 bone of a sheep (obtained fresh from the 

 butcher). In such a bone, notice the 

 rounded ends; these are covered during 

 life by a layer of gristle (cartilage). 

 Notice also the roughened projections 

 (called processes) which serve for the at- 

 tachment of muscles. What parts of the 

 bone are provided with such projections ? 

 In a bone cut lengthwise, notice that the 

 outer surface is covered with a thin layer 

 of connective tissue; this is called the 

 periosteum. The periosteum permits of 

 the entrance into the bone of blood vessels 

 and nerves. It also forms bone from its 

 inner surface. In cases of injurj^ the 

 bone inside the periosteum ma}' be re- 

 moved by a surgeon. New bony tissue 

 will then be rebuilt by the periosteum. 

 Notice also the difference between the end 

 of the bone and the middle part or shaft. 

 The entire outer layer is compact; the 

 inner, spongy in structure. Where is the 

 most compact mass of the bone? The 



