Experiment 7. Run the tips of the fingers briskly down the backbone, and the spines of the vertebrae will be tipped with red so that they can be readily counted. Have the model lean forward with the arms folded across the chest; this will make the spines of the vertebrae more prominent.

Experiment 8. _To ill.u.s.trate the movement of torsion in the spine, or its rotation round its own axis_. Sit upright, with the back and shoulders well applied against the back of a chair. Note that the head and neck can be turned as far as 60 or 70. Now bend forwards, so as to let the dorsal and lumbar vertebrae come into play, and the head can be turned 30 more.

Experiment 9. _To show how the spinal vertebrae make a firm but flexible column._ Take 24 hard rubber overcoat b.u.t.tons, or the same number of two-cent pieces, and pile them on top of each other. A thin layer of soft putty may be put between the coins to represent the pads of cartilage between the vertebrae. The most striking features of the spinal column may be ill.u.s.trated by this simple apparatus.

38. How the Head and Spine are Joined together. The head rests upon the spinal column in a manner worthy of special notice. This consists in the peculiar structure of the first two cervical vertebrae, known as the axis and atlas. The atlas is named after the fabled giant who supported the earth on his shoulders. This vertebra consists of a ring of bone, having two cup-like sockets into which fit two bony projections arising on either side of the great opening (_foramen magnum_) in the occipital bone. The hinge joint thus formed allows the head to nod forward, while ligaments prevent it from moving too far.

On the upper surface of the axis, the second vertebra, is a peg or process, called the _odontoid process_ from its resemblance to a tooth.

This peg forms a pivot upon which the head with the atlas turns. It is held in its place against the front inner surface of the atlas by a band of strong ligaments, which also prevents it from pressing on the delicate spinal cord. Thus, when we turn the head to the right or left, the skull and the atlas move together, both rotating on the odontoid process of the axis.

39. The Ribs and Sternum. The barrel-shaped framework of the chest is in part composed of long, slender, curved bones called ribs. There are twelve ribs on each side, which enclose and strengthen the chest; they somewhat resemble the hoops of a barrel. They are connected in pairs with the dorsal vertebrae behind.

The first seven pairs, counting from the neck, are called the _true_ ribs, and are joined by their own special cartilages directly to the breastbone.

The five lower pairs, called the _false_ ribs, are not directly joined to the breastbone, but are connected, with the exception of the last two, with each other and with the last true ribs by cartilages. These elastic cartilages enable the chest to bear great blows with impunity. A blow on the sternum is distributed over fourteen elastic arches. The lowest two pairs of false ribs, are not joined even by cartilages, but are quite free in front, and for this reason are called _floating_ ribs.

The ribs are not horizontal, but slope downwards from the backbone, so that when raised or depressed by the strong intercostal muscles, the size of the chest is alternately increased or diminished. This movement of the ribs is of the utmost importance in breathing (Fig. 91).

The sternum, or breastbone, is a long, flat, narrow bone forming the middle front wall of the chest. It is connected with the ribs and with the collar bones. In shape it somewhat resembles an ancient dagger.

40. The Hip Bones. Four immovable bones are joined together so as to form at the lower extremity of the trunk a basin-like cavity called the pelvis. These four bones are the sacrum and the coccyx, which have been described, and the two hip bones.

[Ill.u.s.tration: Fig. 17.--Thorax. (Anterior view.)]

The hip bones are large, irregularly shaped bones, very firm and strong, and are sometimes called the haunch bones or _ossa innominata_ (nameless bones). They are united to the sacrum behind and joined to each other in front. On the outer side of each hip bone is a deep cup, or socket, called the _acetabulum_, resembling an ancient vinegar cup, into which fits the rounded head of the thigh bone. The bones of the pelvis are supported like a bridge on the legs as pillars, and they in turn contain the internal organs in the lower part of the trunk.

41. The Hyoid Bone. Under the lower jaw is a little horseshoe shaped bone called the hyoid bone, because it is shaped like the Greek letter upsilon (?). The root of the tongue is fastened to its bend, and the larynx is hung from it as from a hook. When the neck is in its natural position this bone can be plainly felt on a level with the lower jaw and about one inch and a half behind it. It serves to keep open the top of the larynx and for the attachment of the muscles, which move the tongue. (See Fig. 46.) The hyoid bone, like the knee-pan, is not connected with any other bone.

The Bones of the Upper Limbs.

42. The Upper Limbs. Each of the upper limbs consist of the upper arm, the forearm, and the hand. These bones are cla.s.sified as follows:

Upper Arm: Scapula, or shoulder-blade, Clavicle, or collar bone, Humerus, or arm bone,

Forearm: Ulna, Radius,

Hand: 8 Carpal or wrist bones, 5 Metacarpal bones, 14 Phalanges, or finger bones,

making 32 bones in all.

43. The Upper Arm. The two bones of the shoulder, the scapula and the clavicle, serve in man to attach the arm to the trunk. The scapula, or shoulder-blade, is a flat, triangular bone, placed point downwards, and lying on the upper and back part of the chest, over the ribs. It consists of a broad, flat portion and a prominent ridge or _spine_. At its outer angle it has a shallow cup known as the _glenoid cavity_. Into this socket fits the rounded head of the humerus. The shoulder-blade is attached to the trunk chiefly by muscles, and is capable of extensive motion.

The clavicle, or collar bone, is a slender bone with a double curve like an italic _f_, and extends from the outer angle of the shoulder-blade to the top of the breastbone. It thus serves like the keystone of an arch to hold the shoulder-blade firmly in its place, but its chief use is to keep the shoulders wide apart, that the arm may enjoy a freer range of motion. This bone is often broken by falls upon the shoulder or arm.

The humerus is the strongest bone of the upper extremity. As already mentioned, its rounded head fits into the socket of the shoulder-blade, forming a ball-and-socket joint, which permits great freedom of motion.

The shoulder joint resembles what mechanics call a universal joint, for there is no part of the body which cannot be touched by the hand.

[Ill.u.s.tration: Fig. 18.--Left Scapula, or Shoulder-Blade.]

When the shoulder is dislocated the head of the humerus has been forced out of its socket. The lower end of the bone is grooved to help form a hinge joint at the elbow with the bones of the forearm (Fig. 27).

44. The Forearm. The forearm contains two long bones, the ulna and the radius. The ulna, so called because it forms the elbow, is the longer and larger bone of the forearm, and is on the same side as the little finger. It is connected with the humerus by a hinge joint at the elbow. It is prevented from moving too far back by a hook-like projection called the _olecranon process_, which makes the sharp point of the elbow.

The radius is the shorter of the two bones of the forearm, and is on the same side as the thumb. Its slender, upper end articulates with the ulna and humerus; its lower end is enlarged and gives attachment in part to the bones of the wrist. This bone radiates or turns on the ulna, carrying the hand with it.

Experiment 10. Rest the forearm on a table, with the palm up (an att.i.tude called supination). The radius is on the outer side and parallel with the ulna If now, without moving the elbow, we turn the hand (p.r.o.nation), as if to pick up something from the table, the radius may be seen and felt crossing over the ulna, while the latter has not moved.

[Ill.u.s.tration: Fig. 19.--Left Clavicle, or Collar Bone. (Anterior surface.)]

45. The Hand. The hand is the executive or essential part of the upper limb. Without it the arm would be almost useless. It consists of 27 separate bones, and is divided into three parts, the wrist, the palm, and the fingers.

[Ill.u.s.tration: Fig. 20.--Left Humerus.]

[Ill.u.s.tration: Fig. 21.--Left Radius and Ulna.]

The carpus, or wrist, includes 8 short bones, arranged in two rows of four each, so as to form a broad support for the hand. These bones are closely packed, and tightly bound with ligaments which admit of ample flexibility. Thus the wrist is much less liable to be broken than if it were to consist of a single bone, while the elasticity from having the eight bones movable on each other, neutralizes, to a great extent, a shock caused by falling on the hands. Although each of the wrist bones has a very limited mobility in relation to its neighbors, their combination gives the hand that freedom of action upon the wrist, which is manifest in countless examples of the most accurate and delicate manipulation.

The metacarpal bones are the five long bones of the back of the hand.

They are attached to the wrist and to the finger bones, and may be easily felt by pressing the fingers of one hand over the back of the other. The metacarpal bones of the fingers have little freedom of movement, while the thumb, unlike the others, is freely movable. We are thus enabled to bring the thumb in opposition to each of the fingers, a matter of the highest importance in manipulation. For this reason the loss of the thumb disables the hand far more than the loss of either of the fingers. This very significant opposition of the thumb to the fingers, furnishing the complete grasp by the hand, is characteristic of the human race, and is wanting in the hand of the ape, chimpanzee, and ourang-outang.

The phalanges, or finger bones, are the fourteen small bones arranged in three rows to form the fingers. Each finger has three bones; each thumb, two.

The large number of bones in the hand not only affords every variety of movement, but offers great resistance to blows or shocks. These bones are united by strong but flexible ligaments. The hand is thus given strength and flexibility, and enabled to accomplish the countless movements so necessary to our well-being.

In brief, the hand is a marvel of precise and adapted mechanism, capable not only of performing every variety of work and of expressing many emotions of the mind, but of executing its orders with inconceivable rapidity.

The Bones of the Lower Limbs.

46. The Lower Limbs. The general structure and number of the bones of the lower limbs bear a striking similarity to those of the upper limbs.

Thus the leg, like the arm, is arranged in three parts, the thigh, the lower leg, and the foot. The thigh bone corresponds to the humerus; the tibia and fibula to the ulna and radius; the ankle to the wrist; and the metatarsus and the phalanges of the foot, to the metacarpus and the phalanges of the hand.

The bones of the lower limbs may be thus arranged:

Thigh: Femur, or thigh bone,

Lower Leg: Patella, or knee cap, Tibia, or shin bone, Fibula, or splint bone,

Foot: 7 Tarsal or ankle bones, 5 Metatarsal or instep bones, 14 Phalanges, or toes bones,

making 30 bones in all.