Читать книгу A Practical Physiology: A Text-Book for Higher Schools - Albert F. Blaisdell - Страница 9

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36. The Trunk. The trunk is that central part of the body which supports the head and the upper pair of limbs. It divides itself into an upper cavity, the thorax, or chest; and a lower cavity, the abdomen. These two cavities are separated by a movable, muscular partition called the diaphragm, or midriff (Figs. 9 and 49).

The bones of the trunk are variously related to each other, and some of them become united during adult life into bony masses which at earlier periods are quite distinct. For example, the sacrum is in early life made up of five distinct bones which later unite into one.

The upper cavity, or chest, is a bony enclosure formed by the breastbone, the ribs, and the spine. It contains the heart and the lungs (Fig. 86).

The lower cavity, or abdomen, holds the stomach, liver, intestines, spleen, kidneys, and some other organs (Fig. 59).

The bones of the trunk may be subdivided into those of the spine, the ribs, and the hips.

The trunk includes 54 bones usually thus arranged:

1 Spinal Column, 26 bones:7 Cervical Vertebræ.12 Dorsal Vertebræ.5 Lumbar Vertebræ.1 Sacrum.1 Coccyx.

2 Ribs, 24 bones:14 True Ribs.6 False Ribs.4 Floating Ribs.

3 Sternum.

4 IV. Two Hip Bones.

5 V. Hyoid Bone.

37. The Spinal Column. The spinal column, or backbone, is a marvelous piece of mechanism, combining offices which nothing short of perfection in adaptation and arrangement could enable it to perform. It is the central structure to which all the other parts of the skeleton are adapted. It consists of numerous separate bones, called vertebræ. The seven upper ones belong to the neck, and are called cervical vertebræ. The next twelve are the dorsal vertebræ; these belong to the back and support the ribs. The remaining five belong to the loins, and are called lumbar vertebræ. On looking at the diagram of the backbone (Fig. 9) it will be seen that the vertebræ increase in size and strength downward, because of the greater burden they have to bear, thus clearly indicating that an erect position is the one natural to man.

Fig. 16.--The Spinal Column.

This column supports the head, encloses and protects the spinal cord, and forms the basis for the attachment of many muscles, especially those which maintain the body in an erect position. Each vertebra has an opening through its center, and the separate bones so rest, one upon another, that these openings form a continuous canal from the head to the lower part of the spine. The great nerve, known as the spinal cord, extends from the cranium through the entire length of this canal. All along the spinal column, and between each two adjoining bones, are openings on each side, through which nerves pass out to be distributed to various parts of the body.

Between the vertebræ are pads or cushions of cartilage. These act as "buffers," and serve to give the spine strength and elasticity and to prevent friction of one bone on another. Each vertebra consists of a body, the solid central portion, and a number of projections called processes. Those which spring from the posterior of each arch are the spinous processes. In the dorsal region they are plainly seen and felt in thin persons.

The bones of the spinal column are arranged in three slight and graceful curves. These curves not only give beauty and strength to the bony framework of the body, but also assist in the formation of cavities for important internal organs. This arrangement of elastic pads between the vertebræ supplies the spine with so many elastic springs, which serve to break the effect of shock to the brain and the spinal cord from any sudden jar or injury.

The spinal column rests on a strong three-sided bone called the sacrum, or sacred-bone, which is wedged in between the hip bones and forms the keystone of the pelvis. Joined to the lower end of the sacrum is the coccyx, or cuckoo-bone, a tapering series of little bones.

Experiment 7. Run the tips of the fingers briskly down the backbone, and the spines of the vertebræ 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 vertebræ more prominent.

Experiment 8. To illustrate 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 vertebræ come into play, and the head can be turned 30° more.

Experiment 9. To show how the spinal vertebræ make a firm but flexible column. Take 24 hard rubber overcoat buttons, 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 vertebræ. The most striking features of the spinal column may be illustrated 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 vertebræ, 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 vertebræ 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.

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 (Υ). 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.