282 



BIOLOGY AND HUMAN LIFE 



Cranium 



Sieruiim 



is a tendency toward the development of a fixed number. In 

 mammals, for example, there are always seven vertebrae in the 



cervical, or neck, region; the 

 giraffe has no more than a 

 mouse. The thoracic region 

 has twelve vertebrae. The 

 ribs are of varying lengths 

 and make up a sort of cage 

 for the delicate organs in 

 the thoracic cavity (see Fig. 

 84). They are capable of 

 considerable motion and so 

 permit the breathing move- 

 ments. In birds the bones 

 tend to fuse together and 

 the ribs are not movable. 

 In man the first seven ribs 

 are attached to the breast- 

 bone, or sternum, by straps , 

 of cartilage. The eighth, 

 ninth, and tenth are con- 

 nected by cartilage that is 

 attached to the cartilage of 

 the seventh rib. The elev- 

 enth and twelfth ribs are 

 not attached in front. 



In most mammals, rep- 

 tiles, and fishes the spinal 

 column is nearly straight, or 

 somewhat arched, curving 

 outward on the dorsal side 

 (see Fig. 2). In early stages 

 of human development the 

 column also has the form of 

 a continuous arch from the head to the coccyx; but in adults 

 and in birds it has a more complex curvature (see Fig. 136). 



Pelvis 



Coccyx 



Fig. 135. The human backbone 



Each region has a rather definite number of 

 vertebrae. In some of the lower vertebrates 

 (fishes, reptiles) there is considerable varia- 

 tion. The five vertebrae in the sacral region 

 are fused together, and the pelvic girdle is 

 fused to the sacrum. Four or five small verte- 

 brae make up the coccyx. In early stages of 

 development there may be as many as eight 

 or nine bones in this "tail" region 



