butcher and were offered for sale, a wise cook would refuse to buy it. He would complain that even boiling would not make it tender.

These muscles which we have been studying belong to the skeleton. They are always attached to bones and are therefore called skeletal or voluntary muscles. There are indeed two classes of muscles:

1. Voluntary muscles, of which there are five hundred. They are called voluntary because each is under the power of our will. Through them we walk and run and climb and swim; through them we talk and sing and play the piano and cover ourselves with glory on the athletic field. They serve us when we give our commands. Not so, however, with the second class.

2. Involuntary muscles. These form the heart and are also in the walls of the arteries and of the alimentary canal, -the food tube. They are deaf when we command, but they continue to be busy whether we are asleep or awake, whether we stand or sit, whether we laugh or sigh or cry, run or climb or swim. Whatever we do, they are unceasingly occupied with the internal work of the body, pumping the blood round through heart and blood vessels, caring for the food we eat, and carrying on those central, vital processes of life over which we have no conscious control.

Taken as a whole, the muscular machinery of any human being weighs as much as all the rest of his body put together and weighed in a lump. A few separate muscles are given in the picture on page 27, but there is no special reason why we should learn their names

heart. Nevertheless the biceps is not easily forgotten. It is the muscle best known to every boy, for it may give him pride or disgrace him, according to the pulling power which has been developed in it by its master.

CHAPTER V

STIFF SUPPORT FOR GROUPS OF MUSCLES

A certain teacher who owned a skeleton was in the habit of throwing it over his shoulder when he carried it from the storeroom where he kept it to the lecture room where he showed it to his students. And as he walked it hung from his back as a clattering set of dangling bones. It is true that the separate bones were held together at the joints by artificial contrivances; but that was all. By no chance could the skeleton have stood on its own unaided legs. Those who saw this group of bones for the first time understood as never before that bones are as dependent on ligaments and muscle to keep them together as are tendon and muscle dependent on bone to hold them in place.

Examine a bone fresh from the butcher's. Notice the outside, — firm and closely woven, as it has to be, to supply a surface for muscles to hold to. Look at the inside. There we find looser texture. We know now how it happens that the bone is not only large and strong but light and firm. It is indeed by no means a solid thing. A magnifying glass shows the spaces even better yet. You may now see smooth channels on the outside,

THE CENTRAL SUPPORT OF THE HUMAN BODY

A, skull (composed of 22 bones); B, spinal column (composed of 33 vertebræ); C, sternum or breastbone; D, ribs (12 on each side); E, clavicle; F, humerus; G, radius; H, ulna; I, carpal (7 bones in each wrist and ankle); /, metacarpal (5 bones in the palm of each hand and instep); K, phalanges (14 bones that form the fingers and the toes of each hand and foot); L, pelvis (composed of 4 bones); M, femur; N, tibia; O, fibula

along which the blood vessels ran, and tiny openings from the surface to the interior, into which the smallest blood vessels went, to keep up the life of the bone.

A chemist will take the same bone and study it in another and a different way. He will keep it in acid for a while and will then tell us that he has taken

all the lime out; that he

has left nothing but gela

tin. He may tie it into a knot and let us see how pliable it is. Taking another bone, he will hold it in fire for a while, and when we touch it the whole structure will go to pieces like a heap of ashes. "That is mostly lime," he will tell us; "I have burned out the organic

matter which becomes gelatin when boiled." A cook will take a bone with no meat on it, will boil it for several hours, set the liquid away to cool, and when it is cold she will have a thick jelly to add to her soups. It is gelatin made from the bone. From these and other experiments we learn that bones are made up of lime and of an animal substance, which becomes gelatin after being cooked. We also learn that in old age bones