CHAP.V.] BONE. 103 



compact tissue, giving expansion and lightness to those parts of the 

 bone. In the intermediate portion, or shaft, the compact tissue is 

 highly developed, affording great strength in the situation where 

 that quality is the most needed. 



The compact external surface of bone (except on its articular as- 

 pects) is covered by a firm tough membrane, termed the periosteum, 

 which, like the perichoiidrium investing cartilage, consists of white 

 fibrous tissue, densely interwoven in all directions. The cancelli are 

 filled with fat, or medulla, the marrow of bone. They are lined by 

 a delicate membrane, called the medullary membrane, which serves 

 to support the fat. In the shaft of the long bones the medulla 

 is contained, not in ordinary cells, but in one great canal, which 

 occupies the centre of the shaft, the medullary canal. Here the 

 medullary membrane lines the compact tissue that forms the wall 

 of the cavity. 



Both the periosteum and the medullary membrane adhere inti- 

 mately to the bone. Both are abundantly supplied with blood- 

 vessels, which, after ramifying upon them, send numerous branches 

 into the bone. These membranes are of great importance to the 

 nutrition of the bone, inasmuch as they support its nutrient vessels; 

 and, if either of them be destroyed to any great extent, the part in 

 contact with them necessarily perishes : and they not only cover the 

 outer and inner surfaces of the bone, but also send processes, along 

 with the vessels, into minute canals traversing the compact tissue, 

 and are, through the medium of these, rendered continuous with one 

 another. 



The great variety of uses to which the bones are applied in the 

 construction of the skeleton, occasions much difference of shape as 

 well as of size. The following arrangement comprehends all these 

 varieties, and is that commonly adopted. We classify them as, 1 , 

 Long bones ; 2, Short ; 3, Flat ; 4, Irregular. 



The long bones form the principal levers of the body; their 

 length greatly exceeds their breadth and thickness. In descriptive 

 anatomy, a long bone is divided into a shaft, or central part, and 

 two extremities. The shaft is never perfectly straight, it is more or 

 less curved, as in the femur ; and has always an appearance as if, 

 while yet in a soft and flexible condition, it had received a twist, 

 and its extremities had been turned in opposite directions. This is 

 very manifest in the femur and humerus ; more especially in the 

 latter, where the groove, in which the radial nerve runs, is just what 

 one might fancifully suppose to have resulted from such a cause as 

 that above named. 



