CHABACTERISTICS OF THE ANIMAL BODY 31 



at the extremities it increases in amount and forms nearly the en- 

 tire mass. It is less resistant than compact bone tissue, but is 

 useful in giving the bones surface without adding to the weight, 

 e. g., ends of bones where they articulate to form joints. 



When dried and bleached, bones are white in color with a tinge 

 of yellow. When exposed in the living animal the white is tinged 

 with bluish-pink, due to the blood in the innumerable small vessels 

 of the bone. Overlying the compact tissue of a fresh bone is found 

 a thin, tough, unelastic membrane called the periosteum. It is 

 present on all parts of the bone except the articular surfaces, where 

 it is replaced by cartilage. Blood-vessels and nerves for the bone 

 run in the periosteum. 



Bone-marrow is the soft material found in the center of bones. 

 Marrow is of two varieties — red and white. The red marrow pro- 

 duces certain kinds of red blood-cells. It is found in the shafts 

 of long bones and in flat bones. In the young animal it is rela- 

 tively much grater in amount than in the old animal. Yellow 

 marrow is about 96 per cent, fat and appears to have no particu- 

 lar function. It replaces part of the red marrow in the bones of 

 mature animals. 



The bones of young animals may become permanently dis- 

 torted under the weight of the body or the traction of the muscles 

 attached to them. In old animals the organic matter in the bones 

 is replaced to a large extent by inorganic matter, thus rendering 

 them much more brittle and liable to fracture. 



B. The master tissues comprise the muscles and nerves. They 

 will be described in special chapters, as they are of sufficient impor- 

 tance to deserve more detailed consideration than can be given 



here. 



THE ORGANS OF THE ANIMAL BODY 



Organs are well-defined parts, such as the heart, hver, or brain. 

 They are composed of several kinds of tissues, but possess, as a rule, 

 only one or, at most, a few forms of activity. There are several 

 methods of classifjdng the various body organs for study. The 

 systematic method is most suitable for our purposes. It divides 

 the body into systems or groups of organs as follows : 



(1) Skeletal system, or organs of support. 



(2) Muscular system, or organs of motion. 



(3) Digestive system, or organs of digestion. 



(4) Respiratory system, or organs of respiration. 



