THE BONES. 19 



open into the marrow, and receive their blood-vessels from it, 

 while others open on the surface beneath the periosteum, and 

 admit blood-vessels. 



Each canal is the centre of a complete system of blood 

 supply to the district. Little tubes run outwards, like spokes 

 from a wheel, communicating with each other and with numer- 

 ous other cavities (seen in Fig. 4, as black specks) in such a 

 way as to carry the blood to every part of the bone. 



6. Growth and Repair of Bone. The system of blood 

 supply in the bone, so admirable in its arrangement, furnishes 

 the osseous structure with nutriment for growth and develop- 

 ment ; also, for the repair of tissue, as the bone is subject to 

 the same continual decay and death of the minute cells as 

 are the other tissues, and ordinary repair is constantly being 

 made. 



7. Healing of Broken Bone. When the bono is broken, 

 these vessels -supply the material necessary for repair or union 

 of the fragments. The plasma of the blood is poured out 

 upon and around the fractured ends, at first gluing them, 

 together, arid holding them firm until new bone cells are 

 thrown out. Jn this way the two ends are knit together, an(x 

 soon the bone becomes as strong as before it was broken. 

 But in order that this work of nature may properly proceed, 

 the fractured bone must be kept at rest and the adjoining 

 muscles relaxed. This is best done by placing the limb in 

 an easy position and applying suitable splints. When, from 

 carelessness of the patient or uneasiness 'on his part, the 

 broken bone is not kept in a state of repose, nature attempts 

 to fix the part by pouring out a more copious quantity of 

 plasma. This extra material, callus, as it is called, makes the 

 bone at the seat of fracture much larger. In time, however, 

 this will be absorbed. 



8. Effects of Alcohol on Growth of Bone. First 



amongst the signs of the evil effects of alcohol, wha taken to 



