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5. Conduction and Radiation.-Just as light is the vibratory motion of the particles of a luminous body, and reveals itself by affecting the nerves of the eye; so it is this vibratory motion of the particles of a hot body, when communicated to the nerves, which causes the sensation of heat. These vibrations pass in all directions, and are hence called radiant heat. (See OPTICS, p. 25.) Again, as sound is transmitted by solids as well as by the air, so heat is transmitted by solids. When the end of a poker is placed in the fire, a vibratory motion is imparted to the atoms of that end; but this motion is communicated from atom to atom, till the other end also becomes heated. Metals shew the greatest facility in the passing of heat in this way; in other words, they are the best conductors of heat. The principle on which heat is transmitted through a fluid, as described at p. 39, and represented in the figure there, is called convection, because the particles of the body change their position, and, as it were, convey the heat; in the conduction of metals, no particle changes its position, the motion is merely passed from one to the other.

Having hitherto treated of the heating of a body, we will now consider the process of cooling. The motion going on among the atoms of a heated body is communicated to the ether, and the heat is said to radiate; thus the hot body expends energy, the motion of its own atoms gradually diminishes, and it is said to cool. Suppose, then, it were desirable to keep it from cooling, what could be done to prevent it? If the hot body were covered with another, the heat must first be conducted through this covering before it can be radiated. Now, different bodies have different powers of conduction, so that if a hot body be covered with a bad conductor, it will be kept hot for a long time. This is the object of wearing clothes-not to warm one's body, but to keep its heat from being radiated. When a piece of red-hot metal is exposed to the air, the heat radiates from the outside, and the outer coating of cooled metal becomes a conductor to the heat in the interior. When the body cooling is a bad conductor, which is quite a different thing from the radiation of heat from the outside, the internal heat is preserved for a long time. The lava that runs as a red-hot liquid from volcanoes, and spreads out in great sheets, after cooling on the surface, so that people may walk over it, retains its heat under this crust for years, because it is a bad conductor.

6. Evaporation and Dew.-One of the most interesting phenomena connected with heat is DEW, the nature of which will be perfectly intelligible after the above explanations. When treating of the heating of liquids, we saw that, with a degree of heat much less than what would raise a liquid to the boiling-point, vapour is formed at the surface of the liquid. The formation of vapour in this way is called evaporation. The heat of the sun is continually causing evaporation from all bodies of

water, or from everything wet; hence it is that anything wet by and by becomes dry, and even water in an open vessel will dry up. There is always more or less of this vapour in the air, even when the sky is clearest. It is only when the vapour, from being cooled in colder air, becomes partially liquefied, that it appears as fog, mist, or cloud. Dew, however, is not fog or mist deposited on the ground. After the sun has set on the evening of a hot summer day, the heat of the ground radiates into the air, or the grass, say, becomes cool, while the heat from the interior is not conducted quickly enough to keep up the temperature. The vapour in the air, coming in contact with this cooled surface, is now condensed into the watery particles of dew. One of the most remarkable things about dew is, that it is not deposited, at least to the same extent, on a cloudy night as under a clear, cloudless sky. This, at first sight, seems a contradiction, but only on the supposition that the moisture falls from the clouds, not when we remember how it is really formed. For the clouds radiate back to the earth the heat which has been radiated from it; so that the surface of the earth does not become colder than the air above it, and therefore the vapour is not condensed. Heat is always transmitted from one body to another which is colder. As was seen above, a certain amount of heat or of motion communicated to ice expands it into water, and a further amount expands the water into vapour. When the surface of the earth is colder than the air containing vapour, the heat of that vapour is transmitted into the ground, and the vapour becomes water or dew; and if the ground is extremely cold, the heat in the water keeping it in the liquid state, is further transmitted into the ground, the watery particles become solid and receive the name of hoar-frost.

PHYSIOLOGY OF THE HUMAN BODY.

THE HUMAN BODY is a most skilfully contrived machine, composed of a great number of different parts, or organs, all admirably adapted for the work they have to do, or, as it is technically expressed, the functions they have to perform. Thus, the limbs, the eyes, the ears, and the nose are organs which respectively perform the functions of motion, seeing, hearing, and smelling.

The functions performed by the organs of the body are of two kinds : (1) those that have to do with the building up and keeping in repair of the body itself; (2) those that bring the individual into connection with surrounding objects. The former have been called the functions of NUTRITION, or of ORGANIC LIFE, and include the digestion of the food, with the absorption of the nutritive materials contained in it, the circulation of the blood, and respiration; the latter are called the functions of RELATION, or of ANIMAL LIFE, and include all the forms of motion and of sensation. In the following lessons, however, we propose to describe first the bony framework, with its covering of muscles and skin; then, the apparatus for keeping the whole fit for use; and lastly, the nervous system, with the different forms of sensation which are the true links of connection between a human being and the outer world.

The Bony Skeleton.

BONE is a hard substance, composed of two kinds of material, an animal matter, called gelatine,2 and a mineral or earthy matter, consisting principally of lime. To the former it owes its elasticity and toughness, and to the latter its hardness. The general appearance of bone is that of a network of minute canals, usually running lengthwise, and connected here and there by cross branches. Towards the outside, the substance of the bone is harder and more compact; and the whole is covered with

1 From Greek organon, an instrument.

2 From Latin gelo, to freeze, because the liquid gelatine takes the consistence of jelly when

cold,

a membrane, called the periosteum.1 The long, roundish bones of the legs and arms have this peculiarity, that they are hollow in the middle, and are filled with a soft, fatty substance, called marrow.

The skeleton is divided into the head, the trunk, and the extremities. 1. The Head-The bony framework of the head, called the skull, is divided into two parts, the cranium and the face. THE CRANIUM 2 is the shell which contains the brain, and is composed of eight bones, of which the following six form its top and sides: the frontal or coronal bone, so called from Latin corona, a crown, because it forms the crown of the head,

Fig. 41.

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marked 1 on fig. 41; the two parietal bones, so called from Latin paries, a wall, because they form the walls or sides of the head, of which only one, marked 2, is seen in the figure; the occipital bone, from Latin occiput, the back part of the head, marked 3; the two temporal bones, from Latin tempus, temporis, the temple, marked 4. The base is formed by other two, the sphenoid, marked 5, and the ethmoid, not visible in the figure. The sphenoid, from Greek sphen, a wedge, is so called because it wedges in

and locks together all the bones of the head and face, being attached to fourteen distinct bones; and the ethmoid, from Greek ethmos, a sieve, from its being perforated with a large number of holes, for the passage of nerves from the brain to the face, and of blood-vessels into the brain. It will be observed that the bony plates forming the upper part of the cranium are joined by ragged edges. These joinings are an elaborate system of dove-tailing, binding together the different pieces as firmly as if the whole were one, while certain purposes are also served by the cranium consisting of separate pieces. At the sides of the skull, some of the bones overlap each other, those above being supported by those beneath. Altogether, the bones of the skull form an arched or vaulted covering of extraordinary strength for the protection of the brain.

THE FACE is made up of fourteen bones, all of which, except the lower jawbone, are immovably fixed to each other, and to the bones of the

1 From Greek peri, about, and os, a bone.

2 Latin cranium, Greek kranion, from kara, the head.

cranium. By their union these bones form five cavities, which contain and protect the organs of sight, smell, and taste. The two principal bones are the upper maxillary or jaw bones, marked 8 on fig. 41. Overlapping, and joined to them at the sides, are the two malar 2 or cheek bones, marked 6. These are joined behind to the two palate3 bones, which form the back part of the roof of the mouth. The inside walls of the cavities for the eyes are partly formed by two small bones, called the lachrymal bones, because there are holes through them for the passage of the ducts or canals which convey the tears from the eyes to the nose. The greater part of the nose is formed of cartilage or gristle, so that the bony part, formed of the two nasal bones, marked 7, is not very prominent. The nasal cavity is divided into two by a partition, which is partly formed by a bone called the vomer, from its resemblance to a ploughshare; and its outer walls are formed of two small turbinated bones, which are thin bony plates, in the form of a scroll or horn, the use of which will be explained when we treat of the organ of smell. Last of all, comes the lower maxillary or lower jawbone, marked 9.

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2. The Trunk.-The most important part of the trunk is the SPINE7 or backbone, so called from its spikes or points. It consists of a large number of small pieces so jointed together as to make it exceedingly flexible. Each of the pieces is called a vertebra [Latin, 'a joint'], and is attached to the two between which it lies by strong elastic ligaments; while between each pair is a cushion of cartilage, which is thickest in the lower part of the spine, and serves a very important purpose. When the body is jolted in any way, for example, in jumping from a height, these cushions act like the buffers of a railway train, and neutralise the shock. From its being composed of these vertebræ, the spine is called the vertebral column. Each vertebra is perforated with a round hole; and from the manner in which they are joined together, these holes form a continuous canal, which contains and protects the spinal cord. This substance, although sometimes erroneously called the spinal marrow, is quite distinct from the real marrow found in the long bones of the legs and arms. It is, in fact, next to the brain, the most important part of the nervous system, in connection with which it will afterwards be described. Behind the backbone are three rows of projections; one in the centre, forming the ridge felt along the back, and seen farthest to the right in the figure, called the spinous processes; and one on each side of these, called the transverse processes. In the whole

1 From Latin maxilla, diminutive of mala, a jaw.

3 From Latin palatum, the roof of the mouth.

5 Latin vomer, a ploughshare.

7 Latin spina, a thorn.

2 From Latin mala, a jaw.

4 From Latin lachryma, a tear.

6 From Latin turbo, turbinis, a whirl.

8 From Latin ligo, to bind.

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