THE SKELETON AND MUSCLES 



335 



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organic compounds. Place a small bone 

 in dilute hydrochloric acid and leave it 

 for several days. The acid will dissolve 

 out the mineral matter, leaving the soft 

 parts, the cartilage at the ends, the 

 periosteum and the organic matter of the 

 bone tissue. The bone so decalcified 

 does not lose its shape but can be twisted 

 and bent without breaking. Burn a bone 

 in the fire. The organic matter disap- 

 pears leaving the mineral matter only, 

 which is now white and brittle. The 

 animal matter, therefore, gives the bone 

 flexibility, while the mineral matter gives 

 it stiffness. 



Bone is nourished by blood that passes 

 in blood-vessels through the periosteum 

 into a series of microscopic canals that 

 perforate the bone tissue. 



Joints. Movements between the limbs 

 are accomplished by joints or articulations. 

 There are four classes of joints, the ball 

 and socket, like that of the femur or leg 

 with the hip bone; the hinge joint, as at 

 the elbow or ankle; the gliding joint, as 

 the wrist and knee joints; and the pivot 

 joint as exhibited by the articulation of 

 axis and atlas in the neck. Figure 171 FIG. 170. Humerus, 

 shows a perfect example of a ball and bisected length- 

 socket joint, the hip joint. On the left, 

 the femur is shown held in place by bands 

 of strong ligaments (H L) attaching it to 

 the pelvic bone, and so arranged as to 

 give perfect freedom of movement. On 

 the right, these bands have been cut away to show the 



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wise, a, marrow- 

 cavity; b, hard 

 bone ; c, spongy 

 bone; d, articular 

 cartilage. (After 

 Martin.) 



