376 HUMAN PHYSIOLOGY 



most spongy area ? Find the marrow which partially fills the central cavity. 

 What is its color ? Blood vessels and nerves pass into the marrow through 

 small openings in the compact bone. Try to find a rather large opening 

 (near one end) through which the artery passes into the bone. Draw in 

 your notebook a bone cut lengthwise, and label all the parts in the drawing. 



A long bone of the human body shows the same general charac- 

 teristics as above described. The surface of the bone is covered 

 with periosteum or connective tissue. Under the periosteum is 

 found a layer of hard, compact bone. This covers a layer of spongy 

 bony tissue under the heads or ends of the bone. The ends of the 

 bone are covered with pads of cartilage. The internal cavity is 

 filled with yellow marrow, the cells of which contain considerable 

 fat. 



Home Experiment. — • Rub some marrow on a piece of paper. Hold 

 the paper to the light. What substance is present? 



Structure of a Flat Bone. Laboratory Exercise. — Compare the long bone 

 with aflat one, for example, a rib. The following differences in structure 

 will be noted. The outer wall is hard bone as in the leg bone. The entire 

 interior, however, is made up of spongy tissue, its spaces being more or less 

 completely filled with red marrow. We thus find that in all long bones, 

 where strength must he combined with lightness, the bone is hollow. For 

 example, in birds, most of the large bones contain air spaces ; the weight of 

 the bird is thus decreased, a peculiar adaptation for flight. In a flat bone, 

 lightness and strength are obtained by having a layer of solid bone outside 

 the soft spongy material. 



Substances present in Bone. Home Experiment. — Burn a bone in a hot 

 fire. This removes the animal material. Now test with hydrochloric 

 acid. What happens? Leave another bone for several days in 10 to 20 

 per cent hydrochloric acid solution. The mineral matter is thus removed. 

 After the bone has been taken from the acid, it may be bent or twisted 

 into knots without breaking. 



The bone-making cells make bony tissue largely from lime. 

 This is easily shown by the above experiments. Some other 

 mineral matters are left behind after testing for lime, common 

 table salt and silica being the most abundant. These materials 

 are, of course, taken into the body with food. 



Growth of the Skeleton. — In some of the lower animals studied, 

 for example the limy sponge, we found certain cells of the animal 

 take up lime from the water or food taken into the body and lay 

 this down as a secretion in the form of spicules. In somewhat 

 the same manner certain cells undertake the work of skeleton 

 formation in man. Water, milk, and other foods furnish the 

 mineral matter necessary. The result is that the skeleton grows 



