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power from life.
but some vegetables contain it, as the extensive classes of fungi and cruciformia, and in cafein, a principle extracted from coffee, there is actually a greater quantity of it, than in most animal substances. Circulation is not found to exist in the lowest class of animals. As for respiration, the leaves of plants so exactly resemble, in their action, the lungs of animals, that they are now familiarly spoken of by vegetable physiologists as respiratory organs.
What life is, we know not; what life does, we know well. Life counteracts the laws of gravity. If the fluids of our bodies followed the natural tendency of fluids, they would descend to our feet, when we stood, or to our backs, when we lay. The cause, why they do not, may be referred immediately to the action of the heart and vessels ; but it is evident, that they derive that
Life resists the effects of mechanical powers.-Friction, which will thin and wear away a dead body, actually is the cause of thickening a living one. The skin on a labourer's hand is thickened and hardened, to save it from the effects of constant contact with rough and hard substances. The feet of the African, who, without any defence, walks over the burning sands, exhibit always a thickened covering; and a layer of fat, a bad conductor of heat, is found deposited between it, and the sentient extremities of the nerves.-Pressure, which thins inorganic matter, thickens living matter. A tight shoe produces a corn, which is nothing more than a thickened cuticle. The same muscle, that with ease raised a hundred pounds when alive, is torn through by ten when dead.
Life prevents chemical agency. The body, when left to itself, soon begins to putrefy; the several parts of which it is composed, no longer under the influence of a higher controlling power, yield to their chemical affinities; new combinations are formed ; ammoniacal, sulphuretic, carburetted, and other_gases are given off, and nothing remains but dust. This never happens during life.
Life modifies the power of heat. Beneath a tropical
sun, or within the arctic circle, the temperature of the human body is found unaltered, when examined by the thermometer. Some have exposed themselves to air, heated above the point at which water boils ; yet a thermometer, placed under the tongue, stood at the usual height of about 98°; and the sailors, who, under Captain Parry, wintered so near the north-pole, when examined in the same way, constantly afforded the same results.
Finally, life is the cause of the constant changes that are going forward in our bodies. From the moment that our being commences, none of the materials, of which we are composed, continue stationary. Foreign matter is taken in, and, by the action of what are termed the assimilating functions, becomes part of our composition ; while, on the other hand, the materials, of which our frame had been built up, being now unfit any longer for the performance of the necessary duties, are dissolved, as it were, into a liquid or gaseous form, conveyed by the absorbents from the place which the new matter comes to occupy, and finally expelled from the system.
PERCEVAL B. LORD.
The integuments form that substance, which covers every part of the surface of the body. They constitute what is termed the hide, in various animals, and consist of three parts ; the scarf-skin, a mucous net work below, and the true skin.
The scarf-skin, or cuticle, which is intended to protect the parts below, and to preserve their sensibility, is itself insensible. A blister will raise the cuticle, and render it apparent. Strong work will harden it, as in the hands of labouring people ; and, after many severe complaints, the scarf-skin peels off, just as it does in some animals, as serpents, which cast their skin at certain periods.
The scarf-skin has in it numerous minute holes or pores, by means of which perspiration is effected, and through which the hair issues.
The colour of the scarf-skin varies very little in the different races of mankind : even in the negro it is very little darker than in the European. The seat of colour is, in fact, a very thin layer of soft substance, which is interposed between the scarf-skin and the cutis, or trueskin, and is termed the mucous net-work. In the negro it is of a very dark colour : and the colouring matter is capable of being communicated to water. The true skin, and the parts below, are of the same colour, both in whites and blacks.
There are five principal varieties of colour in the human species, and all of them dependent on the different shades of the mucous coat: the first is the European, or white; the second is the Mangolian yellow, or olive; the third is the American red, or copper colour ; the fourth is the Ethiopian, or black ; the fifth is the Malay brown, or tawny.
The true skin constitutes-the organ of touch. This power exists in the greatest degree at the ends of the fingers, in slight elevations of the skin, called papillæ. The immediate organs of sensation are, however, small white threads, called nerves, which are more or less immediately derived from the brain, and these are diffused very plentifully over the ends of the fingers, and particularly over the papillæ, which, by this means are calculated to communicate minute impressions with great accuracy.
Most animals have, independently of the general diffusion of sensibility over the surface, some particular part which possesses the sense of touch in a pre-eminent degree. The nose or snout is a very common organ for this purpose, in many animals; and in the elephant, large and unwieldy as it appears, the extremity of the trunk is provided with an organ, as small and delicate as the human finger, and capable of taking hold of very small objects, as needles or pins, with great facility.
Some animals have an exceedingly thick epidermis or scarf-skin, as the elephant and hippopotamus. Those
that live in the air, have their cuticle dry and horny ; fish, on the contrary, have it mucous, or oily, so as to prevent injury by the action of the water upon it. Some animals, as has already been observed with regard to serpents, cast their cuticle once a year, and this in so perfect a way, that even the rotundity of the eye is discoverable in the exuviæ. The greater part of silkworms, and of the caterpillars of butterflies, cast off their cuticles seven times, and some insects even ten times, before they pass into the state of chrysalis.
There is a peculiarity in the attachment of the skin of the frog and toad to the body, which is not found in other animals. It is only adherent at a few points ; being in other respects a loose bag inclosing the body; whereas, in most animals, it is closely adherent to the muscular surface beneath.
The bones form, as it were, the foundation of the body; and, besides being a basis or ground-work for the soft parts, are intended to enclose and support some organs, which are of the first importance in the animal frame.
The skull or cranium, which contains the brain, is fixed at the top of the vertebral column, or bones of the back : in the centre of these bones, is a hollow space, destined for the reception of the spinal marrow, a substance which is a prolongation of the brain, and resembles it a good deal in nature and function.
At a little distance from the skull commence the ribs, which are all fixed behind to the bones of the back, and the greater number to the breast-bone before. Their curvature forms a cavity, which is called the chest, and contains the heart and lungs.
At the lower part of the vertebral column is placed a firm, thick, strong, and irregular, bony structure,
called the hips, which encircle a sort of hollow space termed the pelvis or bason.
At the upper part of the ribs, are the shoulder blades, into which the upper extremities are articulated or jointed ; and at the lower part of the pelvis are articulated the lower extremities.
The form, magnitude, and mode of junction of bones, vary, according to the design which they are intended to serve. Where length is required, with flexibility at particular parts, we have bones, like those of the arm and leg, of firm texture, with joints at certain intervals. In the
hand and foot, there is, by means of the numerous joints of the fingers and toes, and the mechanism of the wrist and ancle, a facility given to the various important actions of the hand, and to the more limited motions of the foot.
In the back, great solidity is required, and the motion in any one part of it is very small. In some of the joints, the power of motion is in all directions, as in the shoulder and hip ; while in the elbow and knee, there is only the power of bending or extending them.
The joints which compose the shoulder and hip are of the description which is called, in mechanics, the ball and socket. The bone of the arm is attached to the shoulder blade, which is connected with the breast bone, by the intervention of the collar bone, or clavicle.
The ends of bones are covered with a gristly substance, called cartilage, which, together with the oil
, or synovia, as it is called, which is secreted in every joint, prevents them being injured by the constant friction to which they are exposed.
The bones, hard and substantial as they appear, were originally nothing more than soft pulp, contained within a membraneous covering, which gradually became harder, and, at the proper period, acquired solidity sufficient for all the purposes of life. The younger a person is, the greater is the quantity of jelly; and in old people there is a much larger proportion of ossified matter. Some fish have their bones composed entirely of cartilage, as the shark, skate, sturgeon.