Chap. 9 



PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 



143 



/"X.X'^^" 



Fig. 9.7. The flexibility of the vertebral column: in a walking salamander which 

 swings from side to side like a fish; in a fighting cat that arches its back as easily 

 as a bucking bronco. A human "backbone" bends forward, backward, and side- 

 wise. 



capsule (Fig. 9.8). The end of each bone is capped with cartilage and folds 

 of thin synovial membrane project into the capsule of the joint from the sides. 

 This membrane secretes the synovial fluid, a lubricator that is transparent 

 and viscid like the white of egg. When the synovial membrane of the knee 

 becomes inflamed, its excess secretion often accumulates as "water on the 

 knee." 



Long Bones. The humerus of the arm or femur of the leg may be taken as 

 an example of the general structure of long bones (Fig. 9.9). The cellular 

 structure of bone is described in Chapter 7. 



The tubular plan of long bones makes them much stronger than rods of the 

 same size and weight. Two arrangements of their bony tissue, the compact 

 bone mostly surrounding the hollow shaft and the spongy (cancellous) bone 

 at the ends, create strength and lightness at the same time. Spongy bone is a 

 network of plates laid down in lines running in the directions which best meet 

 the stress that falls upon the particular part, such as the weight borne by the 

 head of the femur (Fig. 9.8). It contains spaces filled with red bone marrow 

 in which the red and some of the white blood cells are formed (Chap. 7). An 

 important layer of connective tissue, the periosteum, surrounds all bones. It 



