140 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



concrete. The combination of the materials makes bone hard and resistant to 

 strain. Bone can support a greater weight than granite without being crushed. 

 Despite its great firmness, it is moderately flexible especially in young animals. 

 The flexibility of the human skull at birth is well known; even in an adult the 

 skull can stand some compression before it cracks. Bone may be deprived of 

 either mineral matter or collagen and yet keep its shape. Soaking in dilute 

 hydrochloric acid will remove the minerals; burning will remove the animal 

 substance (mainly protein) (Fig. 9.10). The proportion of calcium to living 

 matter varies with age, with the amount of vitamin D in the diet, and other 

 factors. The body's calcium supply is regulated by the parathyroid glands 

 that are located on either side of the thyroid gland (Fig. 15.1). Calcium also 

 indirectly controls the coordinated activity of muscles by slowing down the 

 transmission of nerve impulses to them. When there is an excess of impulses, 

 the secretion of the parathyroids circulating in the blood extracts calcium 

 from the supply in the bones. This, in turn, circulated in the blood, slows the 

 activity of nerves and muscles. On the other hand, if the body becomes 

 sluggish, the parathyroid secretion is diminished and less calcium is called 

 forth from the bones. Again, the parathyroids may be too active and may rob 

 the bones of their calcium and produce abnormal formations. Sometimes this 

 is deposited as kidney stones. 



Discoveries by Tracers. The behavior of calcium and phosphorus in the 

 tissue of living bone has been observed by means of their isotopes used as 

 tracer substances. The movements of radioactive calcium and phosphorus are 

 detected by a sensitive instrument (Geiger counter) placed on the outside of 

 the body (Chap. 2). Radioactive calcium has been demonstrated in the bones 

 of mice 24 hours after its injection into the veins. Radioactive phosphorus was 

 immediately deposited in the teeth, in the ends of bones, and in the ring of 

 healing (callus) in a bone which had been fractured. Radioactive phosphorus 

 in the form of a solution of sodium phosphate has also been given to human 

 patients either by mouth or by injection into the veins and its movement in 

 the body and its behavior in the bone followed by the Geiger counter. Such 

 explorations are more and more frequently made in the treatment of broken 

 and diseased bones. 



Skeletons of Vertebrates and Their Ancestors 



Notochord and Vertebral Column. Vertebrates are named from the chain of 

 bones which composes the vertebral column, the oldest part of the skeleton 

 and the support to which their development and dominance are supremely 

 indebted (Fig. 9.6). "Having backbone" has long come to mean having 

 strength and resolution. With a flexible, dorsal, median backbone, and the 

 bilaterally symmetrical appendages which developed later, the vertebrates 

 gained agility first in water and then on land. They moved about more. 



