THE FRAMEWORK OF THE BODY 239 



bones forming the skull or framework of the head in which the 

 organs of special sense are located. In man the bones of 

 the spinal column are arranged in a slight double curve and 

 the column itself is erect. In the lower animals the spinal 

 column is usually parallel with the earth's surface. The part 

 which includes the head and, in all vertebrates, goes first, is 

 called the anterior portion and the opposite part is the pos- 

 terior. The upper side is the dorsal surface and the under side 

 the ventral surface. Attached to the axis are usually two 

 girdles of bones each bearing a pair of appendages or limbs. 

 In man these appendages are called arms and legs. Between 

 the two girdles are also attached in pairs a number of curved 

 bones or ribs. Man has twelve pairs of ribs, all of which, 

 excepting the last two pairs, are also attached in front to the 

 breast-bone, which is really three bones in one. These bones 

 thus form a sort of bony cage in which are the heart and lungs. 

 The longest bones of the body are found in the appendages. 

 These long bones are nearly cylindrical, hollow and filled with 

 a fatty marrow in which the red blood corpuscles are formed. 

 The flat bones, such as the ribs and the bones of the skull, 

 have no central cavity. The ends of the bones are pitted and 

 ridged for the attachment of the muscles, and the vertebrae 

 have various bony projections for this purpose. The bones 

 are derived from cells so small that one wonders how they 

 can form a substance so strong as bone: the same may be 

 said of the teeth. The bones of all young animals are very 

 soft and flexible at birth but grow harder with age. This ex- 

 plains why a child may escape without broken bones from a 

 blow that would seriously injure an adult. 



203. Joints. The bones of the skeleton are joined together 

 in a variety of ways. Those of the cranium (that part of the 

 skull that encloses the brain) are closely and immovably 

 joined. Such joints are usually called sutures. Other joints 

 are constructed on the familiar hinge pattern, allowing con- 



