A. The sternum, or breast-bone.

B B. The ribs, which rise a little from behind, and fall as they come forward, by which they acquire a greater flexibility.

C C. The cartilaginous points of the short ribs, by which their expansive and compressive powers are much increased.

D E. Part of the vertebral column, or back-bone.

Because, by that arrangement, concussions of the skull, which might be fatal to the brain, are deadened, and injuries from accident greatly modified.

Because the functions of those organs require considerable space, while their importance in the system of life, renders it essential that they should be securely protected from the probabilities of accident.

Because, by the inflation and contraction of the lungs, their capacity is constantly changing. When man takes a moderate inspiration, he inhales about thirty cubic inches of air, and the lungs increase in size one-eighteenth of their whole capacity. Consequently, were they enclosed in a frame of fixed dimensions, it must needs be, to that extent at least, larger than is necessary, when the frame is made to dilate and contract with the capacity of the lungs.

So perfect is the Almighty contrivance, that not only are the ribs made to protect the lungs, but, by their elasticity, and the contractions and dilations of the muscles which lie between them, they assist the lungs in their labours, and work with them in perfect harmony.

Because lightness is thereby combined with strength. There is a provision by which, in the extremities of bones, where an enlarged surface is required, lightness is still combined with the necessary degree of strength.

The bones are made up of a cellular formation; and this generally occurs in parts which are much called into action, in the various movements of the body.

A. Lower part of the bone of the thigh.

B. Head of the bone of the leg.

Because a considerable leverage is gained, by which the advantages of quickness of motion, and increase of mechanical power, are secured.

Because the motions of the hands and feet are very varied and complicated. There are no less than twenty-eight bones in one hand and wrist; and about as many in a foot and ankle. To these are fastened a great number of ligaments and muscles, by which their varied compound movements are controlled. But for the complexity of the mechanism of our hands and feet, our motions would be extremely awkward, and many of the valuable mechanical inventions which now benefit mankind, could never have been introduced. The bones of the hands and feet are in number equal to one-half of the whole of the bones of the body.

Ligaments consist of bands and cords of a tough, fibrous, and smooth substance, by which the bones are bound together and held in their places, allowing them freedom to move, and supplying smooth surfaces over which they glide.

Because the bones would otherwise be constantly liable to slip from their places.

Tendons are long cords, of a substance similar in its nature to cartilage, by which the muscles are attached to the bones.

A. The ball, or head of the thigh bone.

B. The socket, showing the ligament in the socket, which holds the head of the bone in its place, but allows it free motion.

C. Ligaments tied from bone to bone, giving firmness to the parts.

Because, by this arrangement, the large muscles by which the extremities are moved, may be placed at some distance from the bones upon which they act, and thus the extremities, instead of being large and clumsy, are small and neat.

There are about four hundred and forty-six muscles that have been dissected and described, and the actions of which are perfectly understood. But there is probably a much larger number of muscles, and of compound actions of muscles, than the skill of man has been able to recognise.

Every muscle is made up of a number of parallel fleshy fibres, or threads, which are bound together by a smooth and soft tissue, forming a sheath or case to the muscle, and enabling it to glide freely over the surfaces upon which it moves.

A. Lower extremity of the muscle which draws the fore-arm towards the upper-arm, bends the elbow, raises the hand to the head, and is powerfully exerted in pulling, lifting, &c.

C. A muscle which gives off four long tendons, which pass under the ligaments of the wrist, one to each finger, and by which the fingers are bent upon the palm of the hand, as in grasping, &c.

F. Tendon of a muscle which draws the little finger and the thumb towards each other.

The ligaments may be seen enfolding the finger-joints, and also crossing the wrist, underneath the tendons.

The muscles are compressed into tendinous cords at their ends, by which they are united to the bones.

They are arranged in pairs, having reciprocal actions—each muscle having a companion muscle by which the part which it moves is restored to its original position, when the influence of the first muscle is withdrawn, and the stimulus given to bring back the part.

Because the muscles at the back of the arm, whose tendons are attached to the back of the fingers, contract and restore them to their former position.

The degree of strength of a muscle depends upon the healthy condition of the muscle, the amount of stimulus which it receives at the time of exertion, and the manner in which its powers are applied.

The great muscle of the calf of the leg has been found, when removed from a dead body, to be capable of sustaining a weight equal to seven times the weight of the entire body.

The muscles are excited to action by the nerves, which they receive from the spinal cord.

Because the muscles of the heart are involuntary muscles—that is, they are independent of the will, and receive a continuous nervous stimulus which is not under the controul of the mind.

A. A large ligament, which covers the knee pan, or moveable bone of the knee, by which the ends of the bones of the thigh and leg are kept from slipping over each other.

B. A muscle which passes underneath the cartilages of the ankle, and gives off four tendons, which are distributed to the toes, and by which they are extended in elongating the foot, walking, &c.

C. Part of the muscle which forms the fleshy bulb of the calf of the leg, and which terminates in the large tendon attached to the heel, called the tendon of Achilles.

D. One of the ligaments which bind the tendons and the bones of the ankle.

E. Arteries proceeding from the large vessel descending the leg, by which the toes are supplied.

Because, as they supply the mechanism through which we adapt ourselves to our varying wants and circumstances, it was necessary that they should be placed under the controul of the mental power, and be moved only in accordance with man's necessities.

Because, as the necessity for the heart's motion is fixed and unalterable, the constant motion of the heart could be best secured by giving it a fixed nervous influence, by which it might be unfailingly prompted to fulfil its functions.

If the movements of man's heart were subject to his will, he would be constantly required to regard the operations of that organ; and so large an amount of mental care and physical exertion would have to be employed in that direction, that man's sole work would be to keep himself alive. Hence we see the goodness of the Creator in giving life to man, and in keeping the vital impulses under his divine care.

The nerves are branches of the brain and the spinal cord; they are distributed in great numbers to all the active and sensitive parts of the body.

The spinal cord is a long and large cord of nervous matter, which extends from the brain through a continuous tube formed by corresponding hollows in the bones of the back. It serves as a nervous trunk for the distribution of nerves, just as the aorta distributes branches of blood-vessels.

Being a very vital part of the system, and from the delicacy of its structure liable to injuries, it is set in the back-bone for protection; and so great is its security that it is only by force of an unusual kind that it can be injured.

Because in the bones, on each side of the spinal cord, there are smaller grooves for the transmission of the nervous branches.

Of the brain, the spinal cord, and the branches which are called nerves.

A. B. Veins of the fore-arm.

B. Canal formed in the muscle, through which a trunk-vein emerges.

C. Canal formed in the muscle, through which a large nerve emerges.

D. Canal through which a vein enters to communicate with the deep muscles of the arm.

It consists of a thin membrane, or sheath, surrounding a greyish oily matter, which forms the nervous marrow. In the centre of this marrow is usually found a small fibre, which is supposed to be the essential part of the nerve; and most nerves consist of a number of these sheaths enclosing fibres running in parallel directions.

The term nervous fluid is used to express our ideas of the mode by which the brain and spinal cord influence the remote parts: just as we say the electric fluid, without knowing that such a fluid exists. It is the most convenient form of expression.

There are:—

The nerves of motion are those which, in obedience to the will, stimulate the muscles to act, and apportion the amount of stimulation they convey to the degree of exertion required.

A A A. The facial nerve emerging from underneath the ear, and distributing branches to the cheeks, temple, forehead, &c. This nerve excites the muscles of the face, and is chiefly instrumental in producing the expressions of the countenance under the changing emotions of the mind.

B B B. Muscles by which various motions are imparted to the head, face, mouth, &c., under the stimulus of the nerves.

The nerves of sensation are those which impart a consciousness to the brain that its commands to the nerves of motion have been obeyed, and how far they have been fulfilled.

The nerves of special sense are those through which we hear, see, feel, smell, and taste.

The nerves of sympathy, or the system of sympathetic nerves, are those which are distributed to the internal organs, and which are independent of the will. They regulate the motions of the heart, the lungs, the stomach, &c., and stimulate the organs of secretion, so that those organs work in harmony with each other.

As the internal organs are all more or less dependent upon each other, and unite their functions for similar ends, it is obvious that there should prevail among them a mutual consciousness of their state. Otherwise, when the stomach had formed chyme, the liver might have no bile ready to fulfil its office; the absorbents might be in a state of rest at the moment when nutrition was set before them; and the heart might beat slowly, while the lungs were in active exertion to obtain additional blood to support an active exercise. The sympathetic system of nerves therefore regulates and harmonises these internal functions.

Because the light which is reflected from them enters our eyes and produces images of their forms upon a membrane of nerves called the retina, just as images are produced upon a mirror.

Because the membrane which receives the images of objects is connected with the optic nerve which transmits to the brain impressions made by the reflections of light, just as other nerves convey the effects of feeling, hearing, tasting, &c.

Because the lenses and humours of the eye collect the rays of light coming from every direction, and, bringing them into a focus, transmit them to the retina, where each ray impresses upon the nervous surface the qualities it received from the object which reflected it.

A. Portion of bone through which the optic nerve passes in its communication between the brain and the eye.

B. The optic nerve, from before which an external muscle has been cut away, leaving its two attachments.

C. The globe of the eye.

D. The muscle which turns the eye outward, and which is counteracted by a muscle on the other side.

E. The muscle which passes through a loop, or staple of cartilage I, and turns the eye obliquely. It is counteracted by a muscle situated underneath.

F. The muscle situated underneath, which turns the eyeball upwards, and is counteracted by

G. The muscle which turns the eyeball downwards.

H. The muscle attached to a bone which turns the eyeball upwards.

I. The cartilaginous loop through which a muscle passes.

J. The front chamber of the eye filled with a clear fluid.

K. Fragment of the bone by which one of the muscles is fastened.

Because it sometimes happens that a muscle of the eye acts too powerfully for its companion muscle, and draws the eye too much on one side.

Because the pupil is an opening through which the rays of light pass into the chamber of the eye. There is, therefore, nothing in the pupil, of the eye to reflect light.

Because the iris, a ring of extremely fine muscles which surround the pupil, contracts when too much light falls upon the retina, and dilates when the light is feeble. It therefore enlarges or diminishes the size of the pupil to regulate the admission of light.

A. The pupil of the eye through which the light enters.

B. The iris, which dilates or contracts, and thereby increases or lessens the size of the pupil.

C. The three coats of the eye, called the sclerotic, choroid, and retina.

D. The ciliary processes, or hair-like muscles, which have a slight vibratory motion which they impart to the fluids of the eye.

E. The dark coat of the choroid, the coat forming the retina removed.

Because, besides those optical laws which bring upon the two retinas the exactly corresponding images of the same objects, the optic nerves meet before they reach the brain, and blend the impulses which they convey.

Because the eyes require to be defended from floating particles in the air, and to be kept moist and clean. The eyelids form the shutters of the eye, defending it when waking, by closing upon its surface whenever danger is apprehended, moistening its surface when it becomes dry, and covering it securely during the hours of sleep.

Because the eyelashes assist to modify the light, and to protect the eye, without actually closing the eye-ids. When the eyelids are partially closed, as in very sunny or dusty weather, the eyelashes cross each other, forming a kind of shady lattice-work, from the interspaces of which the eye looks out with advantage, and sees sufficiently for the guidance of the body.

Because the crystalline lens of the eye is a moveable body, and is pushed forward, or drawn back by fine muscular fibres, according to the distances of the objects upon which we look. By these means its focus becomes adjusted.

From the blood, which flows abundantly to the eyes, and is circulated in capillary vessels that are spread out upon the membranous coats of the eye-balls.

A and B. The sclerotic, choroid, and retina, the three layers or coats which form the walls of the globe of the eye, and enclose its humours.

C C. The iris.

D. The front chamber of the eye, filled with watery humour.

E. The pupil, through which the rays of light pass to

F. The crystalline lens.

G G. The vitreous humour enclosed in cells formed by the hyaloid membrane.

H. An artery which supplies blood to the crystalline lens, and which passes through the centre of the optic nerve.

G. The optic nerve, showing the sheath in which the nerve is enclosed.

Because, under the emotions of the mind, the circulation of blood in the brain, and in its nearest branches, becomes considerably quickened. The eyes receive a larger amount of blood, and the secretion of the lachrymal glands being increased, the fluid overflows, and tears are formed. The use of tears is probably to keep the eyes cool during the excitement of the brain. They are formed also during laughing, but less frequently.

Because an excess of light enters the eye before the iris has had time to adjust the pupil to the amount of light to be received.

Because the iris has so reduced the pupil while we were looking at the bright light, that immediately upon turning to a darker object, the pupil is too small to admit sufficient rays to enable us to see.

A A. Capillary veins distributed over the sclerotic coat.

B. One of the trunks of the optic nerve.

C. A nerve communicating with the ciliary processes.

D. A vein running parallel with the nerve to the ciliary processes.

E. Side view of the iris.

Because the iris has relaxed and enlarged the pupil, therefore we receive more rays of light from the comparatively dark object, and are enabled to see it more clearly.

Because the cat's eye is so sensitive to light that the iris closes the pupil almost entirely to shut out the too powerful light.

Because the tympanum of the ear receives impressions from sounds, and transmits those impressions to the brain in a similar manner to that in which the retina of the eye transmits the impressions made upon it by light.

The external ear is a natural ear-trumpet, and serves to collect the vibrations of sound, and to conduct them towards the internal ear.

Because the external ear, being formed of tough cartilaginous substance, and being very simple in its organisation, is but little liable to injury.

Because they prevent the intrusion of insects, and of particles of dust, by which otherwise the faculty of hearing would be impaired.

The insect called the earwig is popularly supposed to be so named from its tendency to get into the human ear, and cause pain and madness by penetrating to the brain. An earwig, however, is no more likely to get into the ear than any other insect whose habit it is to penetrate the corollas of flowers; and should an insect enter the ear, it could get no further than the membrane of the tympanum, which spreads all over the auditory passage, just as the parchment of a drum spreads over the entire circumference of that instrument. The fact is, that the wing of the insect, when spread, resembles the external ear in shape. It is similar to the wing of the stag beetle (see illustration), and this fancied resemblance of the wing of the insect to the ear of man may have given rise to the name of ear-wing, which became corrupted to earwig.

Because, by the peculiar resinous property which it possesses, it improves the sound-conducting power of the auditory canal through which it prevails.