188 OSSEOUS SYSTEM. 



curate experiment, made by Mr. Hunter, proves this; he 

 bored holes in the middle of a long bone, and although 

 the bones had grown considerably longer, the holes were 

 no farther apart. 



Q. In what way does a bone grow after the twentieth 

 year? 



#. It grows in thickness. Observe the difference in a 

 thigh bone eighteen years old and one of forty. 



Q. What evidence is there that exhalation and absorp- 

 tion go on in the bones ? 



t/2. The experiment of feeding with madder makes this 

 manifest; this colouring substance is alternately deposited 

 and removed. It is never coloured when gelatine alone is 

 in the bone; now this proves that the calcareous exhala- 

 tion is the vehicle of it. 



Q. Can you, by giving gelatine or phosphate of lime, 

 supply a defect of these constituents in the bony system? 

 *#. No; unless you can bring the organic sensibility of 

 the exhalents of the bones in due relation to the blood 

 containing these principles, the vascular system of the 

 bones would reject it. 



Q. What changes take place in the bones of old persons? 

 J2. They become of a grayish colour, and they are hea- 

 vier in old people, owing to the preponderance of calca- 

 reous deposition. 



Q. Beclard mentions a cause of the brittleness of bones 

 in advanced life, what is it? 



*ft. Usually, as the medullary canal enlarges, so does . 

 the exterior of the bone; but sometimes, in advanced life, 

 the canal continues to enlarge, at the expense of the thick- 

 ness of the walls, and hence the bone is more brittle. 



