Obrázky na stránke
PDF
ePub

to tell, from the direction in which a needle swings, the direction of the current which acts upon it. I have no doubt of being able, by means of an image, to make this point clear to you. Imagine a little man to swim on the electric current in the direction in which it flows. Let his face be turned towards the needle, then the north pole of the needle will always be deflected in the direction of the little man's left hand. It was this very image that guided me when I stated the direction of the deflection in the last paragraph.

This rule holds good not only when the wire is above the needle, but when it is below it, or beside it. Following out the rule, you would find that if the current runs from south to north when the wire is beside the needle, the north pole would be pressed downwards, for the swimmers left hand would, in this case, point downwards. If the wire be under the needle, the swimmer, to turn his face to the latter, must swim upon his back ; and hence, when the current flows from south to north, under the needle, the deHection of the north pole is the reverse of that produced when the current flows in the same direction above the needle.

This gives you, I trust, a general notion of the power we possess over the movements of a magnetic needle acted upon by an electric current. And you will easily conceive that we may make use of the deflections as signs, to which we may attach a particular meaning. A deflection towards the right may represent one letter or word, and a deflection towards the left another. By interrupting and re-establishing the current, we can make as many deflections as we please, and in any direction we please ; and when we agree beforehand that certain deflections shall mean certain things, the possibility of conveying messages in this way becomes quite intelligible. The electric current travels through hundreds of miles of wire in an instant ; an operator in Manchester or Edinburgh can deflect a needle in London, and then messages can be conveyed from both of these towns to London. The deflection of a needle has been hitherto made use of in this country for telegraphic purposes, but there are several other kinds of signs which may be made use of. To describe these would, however, require a whole book, and I shall be perfectly content if I have conveyed to you a clear notion of the possibility of an Electric Telegraph.

DESCRIPTION OF THE HUMAN BODY.

INTRODUCTION.

The human frame is composed of a series of systems interwoven together, each discharging its own function, and working together for the common good. 1. The Bones forin a hard framework. 2. The Joints allow of motion. 3. The Muscles contract and perform the motions. 4. The Nervous System is the seat of sensation, regulates motion, and furnishes the instrument of intelligence. The remaining systems are for nourishment. 5. The Digestive system prepares the food.

6. The Absorbent system conveys it into the blood. And 7. The heart and blood-vessels circulate the blood to all parts of the body for its nourishment and renewal.

Man belongs to the higher or mammalian class of the great vertebrate division of the Animal Kingdom, which includes fishes, reptiles, birds, and mammalians, and all are formed on the same general plan or type. The body, composed of the systems mentioned above, is divided into, first, the essential part, the trunk or body proper, including the head ; and secondly, the limbs, which are merely locomotive appendages to the trunk, like oars to a boat. The vertebrate trunk is very simply planned, in the form of two canals, or cavities, and a central stem. It is divided into, or composed of, a series of segments or slices, termed vetebrate segments; each of which presents two rings and a solid centre ; so that, when arranged longitudinally, the result is two canals and a central stem. The central stem is composed of the bodies of the vertebræ. The space behind is called the “neurala canal,” from its lodging the great nervous centres, the brain and spinal cord. The space before is the “ hæmal 3 canal,” so called from its lodging the various organs connected with the formation of the blood from the food. This canal includes not only the stomach and chest, but also the face from the eyes downwards, the nose and mouth forming its double inlet. This is the great and simple idea of a vertebrate body?

1 See p. 37.

2 Neuron, a nerve.

8 Ilaima, blood.

Fig. 1.

[ocr errors]

Fig. 1. Longitudinal section of the body, showing N N neural canal. H H hæmal canal.

Fig. 2. Transverse section of body, showing neural and hæmal rings, by the repetition of

which the canals are made up. Fig. 2. The pupil is recommended to

colour with red ink the narrow

space between the dotted lines (1 N in Figs. 1 and 2. Fig. 1 is after

Professor Henle of Göttingen.

The other illustrations for this treatise, except Fig. 3 of the skeleton, are designed and sketched by the author.

[ocr errors][merged small]

PART I.

THE OSSEOUS, MUSCULAR, AND NERVOUS SYSTEMS.

THE OSSEOUS SYSTEM. BONES, by their hardness and strength, are useful in four ways. 1. They shut in and protect important and delicate parts, such as the brain, heart, and lungs. 2. They present levers for motion, serving as handles for the muscles to pull by. 3. They form columns of support. And 4. They serve generally, as a framework or scaffolding, to give form to the body.

The SPINE, or back-bone, is composed of thirty-three pieces called vertebræ :3 seven in the neck (cervical) ; twelve in the back (dorsal); and five in the loins (lumbar); of the remaining nine

1 In reading this description of the bones, the pupil should make frequent reference to the figure of the skeleton.

2 08, a bone. Osteology, from osteon, a bone, and logos, a description. 3 Verto, to turn

From their free morion on each other.

[merged small][ocr errors][merged small][merged small][merged small][merged small][merged small]

(pelvic), the next five unite together, in the adult, to form the bone termeil sacrum ; and the last four are small, serving only to attach ligaments and muscles.

A vertebra presents (see the dark portion of Fig. 2.)---1. The body or centre, which, united to its fellows by slices of flexible and elastic fibro-cartilage, forms a column of support, combining the properties of the rod and chain,—allowing the body to bend, and less liable to break than a rigid column would have been. 2. A ring or arch behind the neural arch, for the lodgment and protection of the spinal marrow; notched on each side, so as to leave a series of loopholes for the transmission of the spinal nerves. 3. Outstanding processes or projections for the muscles to pull by ; one on each side (transverse process) also assisting to support a rib, and the spinous process, the projection of which behind under the skin suggested the name of “ spine’l for the entire vertebral column.

The hæmal canal is completed below, at the PELVIS,2 by the innominate or haunch bone ; which meets its fellow in front, and behind, rests on the sacrum. The pelvic arch or girdle, thus formed, serves to support the weight of the body in sitting, or, in standing, to transmit it to the lower limbs. At the Chest, the hämal canal is expanded, for the lodgment of the heart and lungs ; and acts besides as a bellows-like apparatus, for the mechanical at of breathing. It is formed by the ribs, twelve in number on each side, and by the sternum or breast-bone, and the costal: cartilages in front. The upper seven ribs join the edge of the sternum by their cartilages, which are short prolongations of the ribs, allowing of greater motion and elasticity than bone would have done. But the cartilages of the lower ribs do not reach the sternum, thus allowing of greater expansion of this part of the canal, where, as well as below, it is completed by soft parts only.

The SHOULDER resting on the upper part of the chest is composed, behind, of the broad triangular scapula, or shoulder-blade, and, in front, of the narrow elongated clavicle, or collar-bone, resting by its inner end on the sternum. It is a hæmal arch modified for the attachinent of the upper limb, the socket for which is presented by the scapula. But for the clavicle, the arms would fall inwards in front of the chest, as in quadrupeds, in most of which the clavicle is absent. It is fully developed only when the limb is used as a wing or hand, not as a supporting foot. The scapula

Spina, a thorn.

2 Poleis, a basin.

3 Costa, a rib.

« PredošláPokračovať »