general term _tissue_ is applied to the various textures of which the organs are composed. For example, the osseous tissue forms the bones; the fibrous tissue, the skin, tendons, and ligaments.] so strong that the bones are sometimes broken without injuring the fastenings.

II. CLASSIFICATION OF THE BONES.

For convenience, the bones of the skeleton are considered in three divisions: the _head_, the _trunk_, and the _limbs_.

1. THE HEAD.

THE BONES OF THE SKULL AND THE FACE form a cavity for the protection of the brain and the four organs of sense, viz.: sight, smell, taste, and hearing. All these bones are immovable except the lower jaw, which is hinged [Footnote: A ring of cartilage is inserted in its joints, something after the manner of a washer in machinery. This follows the movements of the jaw, and admits of freer motion, while it guards against dislocation.] at the back so as to allow for the opening and shutting of the mouth.

THE SKULL is composed, in general, of two compact plates, with a spongy layer between. These are in several pieces, the outer ones being joined by notched edges, sutures (su'tyurs,), in the way carpenters term dovetailing. (See Fig. 4.)

FIG. 4.

[Illustration: _The Skull._--1. _frontal bone;_ 2, _parietal bone;_ 3, _temporal bone;_ 4, _the sphenoid bone;_ 5, _ethmoid bone;_ 6, _superior maxillary (upper jaw) bone;_ 7, _malar bone;_ 8, _lachrymal bone;_ 9, _nasal bone;_ 10, _inferior maxillary (lower jaw) bone._]

The peculiar structure and form of the skull afford a perfect shelter for the brain--an organ so delicate that, if unprotected, an ordinary blow would destroy it. Its oval or egg shape adapts it to resist pressure. The smaller and stronger end is in front, where the danger is greatest. Projections before and behind shield the less protected parts. The hard plates are not easy to penetrate. [Footnote: Instances have been known where bullets, striking against the skull, have glanced off, been flattened, or even split into halves. In the Peninsular Campaign, the author saw a man who had been struck in the forehead by a bullet which, instead of penetrating the brain, had followed the skull around to the back of the head, and there passed out.] The spongy packing deadens every blow. [Footnote: An experiment resembling the familiar one of the balls in Natural Philosophy ("Steele's Popular Physics," Fig. 6, p. 26), beautifully illustrates this point. Several balls of ivory are suspended by cords, as in Fig. 5. If A be raised and then let fall, it will transmit the force to B, and that to C, and so on until F is reached, which will fly off with the impulse. If now a ball of spongy bone be substituted for an ivory one anywhere in the line, the force will be checked, and the last ball will not stir.] The separate pieces with their curious joinings disperse any jar which one may receive, and also prevent fractures from spreading.

FIG. 5.

[Illustration]

The frequent openings in this strong bone box afford safe avenues for the passage of numerous nerves and vessels which communicate between the brain and the rest of the body.

FIG. 6.

[Illustration: _The Spine; the seven vertebræ of the neck, cervical; the twelve of the back, dorsal; the five of the loins, lumbar;_ a, _the sacrum, and_ b, _the coccyx, coming the nine "false vertebræ."_ (p. 3).]

2 THE TRUNK.

THE TRUNK has two important cavities. The upper part, or _chest_, contains the heart and the lungs, and the lower part, or _abdomen_, holds the stomach, liver, kidneys, and other organs (Fig. 31). The principal bones are those of the _spine_, the _ribs_, and the _hips_.

THE SPINE consists of twenty-four bones, between which are placed pads of cartilage. [Footnote: These pads vary in thickness from one fourth to one half an inch. They become condensed by the weight they bear during the day, so that we are somewhat shorter at evening than in the morning. Their elasticity causes them to resume their usual size during the night, or when we lie down for a time.] A canal is hollowed out of the column for the safe passage of the spinal cord. (See Fig. 50.) Projections (processes) at the back and on either side are abundant for the attachment of the muscles. The packing acts as a cushion to prevent any jar from reaching the brain when we jump or run, while the double curve of the spine also tends to disperse the force of a fall. Thus on every side the utmost caution is taken to guard that precious gem in its casket.

THE PERFECTION OF THE SPINE surpasses all human contrivances. Its various uses seem a bundle of contradictions. A chain of twenty-four bones is made so stiff that it will bear a heavy burden, and so flexible that it will bend like rubber; yet, all the while, it transmits no shock, and even hides a delicate nerve within that would thrill with the slightest touch. Resting upon it, the brain is borne without a tremor; and, clinging to it, the vital organs are carried without fear of harm.

FIG. 7.

[Illustration: B, _the first cervical vertebra, the atlas;_ A, _the atlas, and the second cervical vertebra, the axis;_ e, _the odontoid process;_ c, _the foramen._]

THE SKULL ARTICULATES with (is jointed to) the spine in a peculiar manner. On the top of the upper vertebra (atlas [Footnote: Thus called because, as, in ancient fable, the god Atlas supported the world on his shoulders, so in the body this bone bears the head.]) are two little hollows (_a_, _b_, Fig. 7), nicely packed and lined with the synovial membrane, into which fit the corresponding projections on the lower part of the skull, and thus the head can rock to and fro. The second vertebra (axis) has a peg, _e_, which projects through a hole, _c_, in the first.

FIG. 8.

[Illustration: _The Thorax or Chest._ a, _the sternum;_ b _to_ c, _the true ribs;_ d _to_ h, _the false ribs;_ g, h, _the floating ribs;_ i, k, _the dorsal vertebræ._]

The surfaces of both vertebræ are so smooth that they easily glide on each other, and thus, when we move the head side wise, the atlas turns around the peg, _e_, of the axis.

THE RIBS, also twenty-four in number, are arranged in pairs on each side of the chest. At the back, they are all attached to the spine. In front, the upper seven pairs are tied by cartilages to the breastbone (sternum); three are fastened to each other and to the cartilage above, and two, the floating ribs, are loose.

The natural form of the chest is that of a cone diminishing upward. But, owing to the tightness of the clothing commonly worn, the reverse is often the case. The long, slender ribs give lightness, [Footnote: If the chest wall were in one bone thick enough to resist a blow, it would be unwieldy and heavy As it is, the separate bones bound by cartilages yield gradually, and diffuse the force among them all, and so are rarely broken.] the arched form confers strength, and the cartilages impart elasticity,--properties essential to the protection of the delicate organs within, and to freedom of motion in respiration. (See note, p. 80.)

FIG. 9.

[Illustration: _The Pelvis._ a, _the sacrum;_ b, b, _the right and the left innominatum._]

THE HIP BONES, called by anatomists the innominata, or nameless bones, form an irregular basin styled the _pelvis_ (_pelvis_, a basin). In the upper part, is the foot of the spinal column--a wedge-shaped bone termed the _sacrum_ [Footnote: So called because it was anciently offered in sacrifice.] (sacred), firmly planted here between the widespreading and solid bones of the pelvis, like the keystone to an arch, and giving a steady support to the heavy burden above.

3. THE LIMBS.

TWO SETS OF LIMBS branch from the trunk, viz.: the upper, and the lower. They closely resemble each other. The arm corresponds to the thigh; the forearm, to the leg; the wrist, to the ankle; the fingers, to the toes. The fingers and the toes are so much alike that they receive the same name, _digits_, while the several bones of both have also the common appellation, _phalanges_. The differences which exist grow out of their varying uses. The foot is characterized by strength; the hand, by mobility.

FIG. 10.

[Illustration: _The Shoulder Joint._ a, _the clavicle;_ b, _the scapula._]

1. THE UPPER LIMBS.--THE SHOULDER.--The bones of the shoulder are the collar bone (clavicle), and the shoulder blade (scapula). The _clavicle_ (_clavis_, a key) is a long, slender bone, shaped like the Italic _f_. It is fastened at one end to the breastbone and the first rib, and, at the other, to the shoulder blade. (See Fig. 1.) It thus holds the shoulder joint out from the chest, and gives the arm greater play. If it be removed or broken, the head of the arm bone will fall, and the motions of the arm be greatly restricted. [Footnote: Animals which use the forelegs only for support (as the horse, ox, etc.), do not possess this bone. "It is found in those that dig, fly, climb and seize."]

THE SHOULDER BLADE is a thin, flat, triangular bone, fitted to the top and back of the chest, and designed to give a foundation for the muscles of the shoulder.

THE SHOULDER JOINT.--The arm bone, or _humerus_, articulates with the shoulder blade by a ball-and-socket joint. This consists of a cup-like cavity in the latter bone, and a rounded head in the former, to fit it,-- thus affording a free rotary motion. The shallowness of the socket accounts for the frequent dislocation of this joint, but a deeper one would diminish the easy swing of the arm.

FIG. 11.

[Illustration: _Bones of the right Forearm._ H, _the humerus;_ R, _the radius; and_ U, _the ulna._]

THE ELBOW.--At the elbow, the humerus articulates with the _ulna_--a slender bone on the inner side of the forearm--by a hinge joint which admits of motion in only two directions, _i. e._, backward and forward. The ulna is small at its lower end; the _radius_, or large