CLASSIFICATION OF THE BONES. 203 



change and renovation is tested by experiment, and the 

 rapidity with which it takes place, is almost incredible. 

 It has been found by examination, that when animals are 

 fed on madder, it penetrates into the bones and tinges 

 them with its colour. In twenty-four hours, the bones are 

 tinged, and in two or three days, the colour is much deep- 

 ened ; but if it be discontinued for a few days, the red 

 colour will be found to have entirely disappeared. A 

 striking proof of the rapidity with which deposition and 

 absorption is carried on in these parts. 



Emily. Are the bones enclosed in any sheath, or 

 investing membrane ? I should presume they would 

 not lie in direct contact with the other parts. 



Dr. B. They are all invested by a thin fibrous 

 membrane, called the periosteum, which by means of 

 its numerous vessels, gives nourishment to the external 

 portions of the bone. An injury therefore, which de- 

 prives any part of the bone of its periosteum, causes 

 the death of those parts ; they become detached for want 

 of nourishment, and their place is supplied by new ones 

 formed from the parts immediately beneath them. The 

 internal cavity of the bone is also provided with its proper 

 membranes which in the same way transmit the vessels 

 that nourish the adjacent parts of the bone. If now you 

 fully understand this account of the formation and nutri- 

 tion _of bone, we will proceed to the next division of our 

 subject. 



Emily. Thus far, I believe, I have a clear idea of 

 the processes you have described, and they are surely, 

 not the least wonderful in the system of the animal econ- 

 omy. 



Dr. B. The bones in the human body amount in num- 

 ber to about two hundred and sixty, and exhibit a great 

 variety of size and figure. They are nevertheless divi- 

 ded into three classes ; the long, short, and flat bones. 

 The short bones are found in those parts whose motions 

 are confined, but complicated, as in the hand and foot. 

 They present considerable extent of surface, with but 



