THE VERTEBRAL COLUMN 19 



the elbow. This may be remembered by flexing the knees and 

 elbows, and noting that the medullary arteries run down as 

 though impelled by gravity. 



THE BONES OF THE TRUNK 



The parts of the trunk are the vertebral column, the ster- 

 num and ribs, the hyoid, and bones of the skull. 



The clavicle and scapula do not belong to the trunk; they 

 form the shoulder girdle. 



The os innominatum goes to form the pelvic girdle, com- 

 pleted behind by the sacrum, which belongs to the trunk. 



THE VERTEBRAL COLUMN 



The vertebral column is composed of a series of vertebrae 

 (verto, to turn), originally thirty-three in number. The upper 

 twenty -four remain separate as movable or true vertebrae; 

 these are succeeded by five united into the sacrum; then follow 

 four dwindled segments united into the coccyx. These lower 

 nine are the fixed or false vertebrae. 



Beginning at the skull, there are seven cervical, twelve 

 dorsal or thoracic connected with ribs, five abdominal or 

 lumbar, five sacral, and four coccygeal vertebrae. The number 

 in the cervical region is constant; those between the dorsal 

 and lumbar may vary reciprocally. If there are but eleven 

 pairs of ribs, the twelfth dorsal vertebra will have lumbar 

 characteristics; if thirteen pairs, the first lumbar will have 

 dorsal characteristics. A transitional lumbosacral vertebra 

 is met with, one side connected with the sacrum, the other 

 having a free transverse process. 



Characteristics of Vertebrae. The first two cervical verte- 

 brae are called rotation vertebrae ; all the other true ones, flexion 

 vertebrae. A representative vertebra, like the tenth dorsal, 

 presents a body for the purpose of support, an arch and spinal 

 foramen for protection, and seven processes for leverage. The 

 body or centrum is a short cylinder; the superior and inferior 

 surfaces are flat, with a rim around the circumference. The 

 front and sides are convex horizontally and concave from 



