« PreviousContinue »
“ Leisure is pain; takes off our chariot wheels ;
How heavily we drag the load of life;
through it, draw one string in each hand, B, B, not towards you in the line A, C, but sideways, till both strings are stretched in a straight line : you will see how much swifter the stone moves than it did before when pulled straightforward. Now this is proved by mathematical reasoning to be the necessary consequence of forces applied obliquely; there is a loss of power but a great increase of velocity. The velocity is the quality required to be gained.
63. By what mechanism are the motions of the arm performed ?
The arm is joined to the body, and moved by numerous powerful muscles;
and is fixed to the breast by the ligaments of the collar-bone. The muscles that move the shoulder-blade lie upon the trunk ; those that move the arm lie upon the shoulder-blade ; those that move the fore-arm lie upon the arm ; and those that move the hand and
the fore-arm. But as the arm requires easy, circular motions, it has a multiplicity of parts to perform them. It has the wrist, for turning it round ; the elbow, for its hinge-like motions ; and the shoulder-joint, on which it rolls; and to assist all those, the moveable shoulder-blade becomes the centre of their motions ; for, after a certain point of elevation, the motion of raising the arm is performed by the action of the shoulder-blade upon the trunk ; when our shoulder-bone is raised to a horizontal position, it is checked by the upper part of the shoulder-joint which hangs over it; and if we elevate our arm still higher, the shoulder-blade rolls, turning upon the point of the collar-bone ; and, as it turns, it glides easily upon those muscles, which lie like a fleshy cushion between it and the trunk over which it is placed.
64. Why are the muscles often removed by means of slender strings from the parts they are designed to rest upon ?
Because, in many cases, the situation of the muscles where they are immediately required would be inconvenient. If the muscles which move the fingers had been plaoed in the palm or the back of the hand, they would have swelled that part to an awkward and clumsy thickness ; the beauty and the proportion of the part would have been destroyed. They are, therefore, disposed in the arm, and even up to the elbow, and act by long tendons strapped down at the wrist, and passing under the ligaments to the fingers and to the joints of the fingers which they are severally to
“ Since God is ever present, ever felt,
In the void waste as in the city full ;
move. In like manner, the muscles which move the toes, and many of the joints of the foot, are disposed in the calf of the leg, instead of forming an unwieldy tumefaction in the foot itself. Thus, also, is it with the nictating membrane over the eye. Its office is in the front of the eye ; but its body is lodged in the back part of the globe, where it lies safe, and where it encumbers nothing.
65. What are tendons ?
Tendons are not only necessary as pulleys to the bones, but to give the limbs a proper form, and preserve their beautiful symmetry, Tendons are seldom required, except where muscles are inserted into bones. There is no tendon in the heart, the stomach, the bowels, or the gullet ; these do not require them, for the motions are wholly contractile, and need no lever power. But where tendons pass over bones and traverse joints, the force is concentrated into narrow bounds, and their long cords being fixed to the extremities of the muscles, pull the bones, and raise them in obedience to our will. Tendons have no visible nerves, and little or no motion.
66. What is cartilage ?
Cartilage is intermediate in hardness with bone, and what are called the soft parts — it is firm and resisting, and yet it has a great deal of elasticity. In some parts of the body there are cartilages serving for continuations of bones, such as those which continue the ribs and connect them to the breast-bone, and they are exactly similar to bones from which the earthy parts have been dissolved by an acid.
67. The cartilaginous crusts which cover the auricular ends of bones are of a very beautiful and peculiar structure. If a piece of bone be sawn towards its articular end, till all be cut through, and then the remaining part, and the cartilage covering it be torn asunder, the cartilage will be found to present an infinity of fibres set perpendicularly on the surface of the bone. When a portion of the bone with its articular cartilage has been soaked in water for some weeks, the cartilage is found to have lost its smooth surface and cohesion, and looks exactly as if the bone had been covered with white reiset.
“ We censure Nature for a span too short;
That span too short we tax as tedious too;
68. What are ligaments ?
Ligaments are composed of numerous straight fibres collected together, and arranged into short bands of various breadth, parallel or radiating, and interwoven with others which cross them. Sometimes the ligament is so formed as to surmount the articular ends of two bones which move upon one another, and here it is called a capsule. Ligaments are not extensible nor elastic; hence, when any attempt is made to stretch them too far, great pain is the result, and inflammation follows, and they are said to be sprained.
CHAPTER III. THE INTERNAL ORGANS OF THE HUMAN FRAME : THE HEART,
LUNGS, LIVER, STOMACH, BRAIN, ETC. 69. How is blood formed ?
The food which we eat is masticated and afterwards swallowed ; it is then received into the stomach, where it is prepared by digestion for the nourishment of the body. Here there is a ffuid formed which changes the food into a substance called chyme, which, passing into the several intestines, is there converted into a milky liquour denominated chyle. This is the fluid matter from which the blood is formed.
70. What is the composition of blood ?
The blood may be described as consisting of innumerable cells suspended in a gelatinous fluid. These cells or globules are formed of thin transparent sacs, evolving a reddish fluid, the quantity and quality of which undergo constant changes, in consequence of its passing to and fro vessels containing a denser fluid. general rule, the deeper or lighter colour of the blood may be said to depend on the greater or smaller number of the globules. The colour may also become darker although the globules are diminished ; and this is caused by the blood cells assuming a more globular form, through its action of passing in the vessels con
“ Blest son of foresight, lord of fate!
taining the denser fluids, and reflects the rays of light in a different manner.
ul. How is the blood in the human body circulated and purified ?
First, the heart sends the blood to all the capillaries through one set of blood vessels called arteries. Then the blood is brought back to the heart by another set of blood-vessels called veins. Next by the blood which returns by the veins is impure, and is sent by another set of arteries from the heart into the lungs, to be purified by the air we breathe. Then it is brought back to the heart by another set of veins. Thus there are two sets of arteries and veins through which the blood is all made to pass ; first, through the body and back to the heart for the purpose of nourishment, and then through the lungs and back for the purpose of purification.
72. The right side of the heart receives and sends out the impure blood. The upper
divisien receives it from the body through two large veins. Then the 'blood passes to the lower division of the heart, where it is sent to the lungs through one large artery with two branches, one for each iung. This is the pulmonary artery. The left side of the heart receives and sends out the pure blood. The upper portion receives it from the lungs through three large reins. Then the blood passes to the lower division of the heart when it is senu to the capillariet all over the body through one large artery called the aorta.
73. Why is the blood regarded as so important an agent in animal economy?
Because the blood is the vehicle of life to every atom of our organization. By properties peculiar to itself, all the various fluids of our body are produced from it, and every particle of bone, muscle, membrane, nerve, and vessel must have existed as an ingredient of the blood, and have been conveyed to its appropriate
“ All men think all men mortal but themselves ;
Themselves, when some alarming shock of fate
place by this circulating spring of energy and nourishment. No vital action is maintained without blood, and should it cease to flow through the brain, all the sense would be speedily shut up, and every function speedily superseded.
74. In the accompanying engraving the circulation of the blood as performed by a single heart is depicted : represents the ventricle, or strong muscular bag of the heart, which when filled with blood contracts upon it, just as any other muscle does, and so forces out the contents through the pipe which arises from it, called
the aorta, just as the contents of an india-rubber bag aro squeezed out through a pipe fixed in its neck. The only difference is, that whereas an external force squeezes the bag, the heart, being muscular, has a power of contraction of its own, and, as it were, squeezes itself; and then, just like the india-rubber bag which regains its shape when the pressure is removed, so the heart, wheh it has squeezed out all the blood, dilates itself again, and is ready to contract anew. The blood having been poured into the great artery, goes through branches up to the head, and down to the lower part of the body, where its minute or capillary terminations are seen to end in veins. Those from the lower part of the body form an inferior great vein; those from
a superior : and the two veins terminate separately into a bag A, called the auricle. The auricle is not nearly so strong as the ventricle, because it has nothing to do with forcing the blood over the body; it is intended merely as a receptacle for the venous blood, till the ventricle be ready to receive it. The auricle is constantly full of blood, which flows to it through the veins in an equable stream, so that whenever the emptied ventricle dilates, the blood from the
auricle rushes in, and distends it for a renewed contraction. The arteries are a set of tubes both dilatable and elastic. Hence at the moment when the ventricle contracts, the blood which is forced into them distends them, increasing their diameter, and producing the feeling communicated to the fingers placed over them, which is called the pulse. The number of the pulse is therefore the number of contractions which the heart is making in a minute; and at the moment when the ventricle dilates, the artery, having the distending force taken off, contracts on its contents. It would now drive part of the blood back again into the ventricle were it not for a valve placed in the artery at its origin, which shuts down the moment the pressure comes on it backwards, so that the force of the elasticity of the artery is expended in propelling the blood forward, not in an equable stream but in successive waves. Again; when the ventricle contracts to throw its blood into the aorta, it would throw back an equal portion into the auricle, were not a valve placed there also, which shuts the moment the ventricle contracts.