CHAKACTERISTICS OF ANIMALS 33 



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

 joints. 



The skeleton of the primitive embryo consists of fibrous and 

 cartilaginous tissues. These are changed to bone by a gradual 

 deposition of earthy salts. The process is termed ossification, 

 and is effected by bone-producing cells, called osteoblasts. One 

 or more centers of ossification that are quite definite in most 

 bones indicate where true bone formation begins. The division 

 or epiphyseal line between the different areas may be clearly 

 seen in the bones of the limbs until the animal is several years old 

 (see Fig. 75). 



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 

 cartilage is found. Blood-vessels and nerves for the bone run in 

 the periosteum. When it is destroyed, that part of the bone 

 beneath it is likely to die as the bone-building cells are contained 

 in the periosteum. 



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

 In color it is either red or white. The red marrow produces 

 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 relatively 

 much greater in amount than in the old animal. Yellow marrow 

 is about 96 per cent fat and appears to have no particular function. 

 It replaces part of the red marrow in the bones of mature animals. 



B. The master tissues comprise (I) the muscles and (II) the 

 nerves. They will be described in special chapters, as they are 

 of sufficient importance to deserve more detailed consideration 

 than can be given here. 



THE ORGANS AND SYSTEMS OF THE ANIMAL BODY 



Organs are well-defined parts, such as the heart, liver, and 

 brain, that perform a particular function. They are always 

 composed of several kinds of tissues. Groups of organs that 

 have work of a common nature to perform are known as systems. 

 There are seven clearly defined systems in all mammals : 



(1) Skeletal system, or organs of support. 



(2) Muscular system, or organs of motion. 



(3) Digestive system, or organs of digestion. 



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