MRS. B.
In quadrupeds the phosphat of lime is deposited likewise in their horns, and in the hair or wool with which they are generally clothed.
In birds it serves also to harden the beaks and the quills of their feathers.
When animals are arrived at a state of maturity, and their bones have acquired a sufficient degree of solidity, the phosphat of lime which is taken with the food is seldom a.s.similated, excepting when the female nourishes her young; it is then all secreted into the milk, as a provision for the tender bones of the nursling.
EMILY.
So that whatever becomes superfluous to one being, is immediately wanted by another; and the child acquires strength precisely by the species of nourishment which is no longer necessary to the mother. Nature is, indeed, an admirable economist!
CAROLINE.
Pray, Mrs. B., does not the disease in the bones of children, called the rickets, proceed from a deficiency of phosphat of lime?
MRS. B.
I have heard that this disease may arise from two causes; it is sometimes occasioned by the growth of the muscles being too rapid in proportion to that of the bones. In this case the weight of the flesh is greater than the bones can support, and presses upon them so as to produce a swelling of the joints, which is the great indication of the rickets. The other cause of this disorder is supposed to be an imperfect digestion and a.s.similation of the food, attended with an excess of acid, which counteracts the formation of phosphat of lime. In both instances, therefore, care should be taken to alter the child"s diet, not merely by increasing the quant.i.ty of aliment containing phosphat of lime, but also by avoiding all food that is apt to turn acid on the stomach, and to produce indigestion. But the best preservative against complaints of this kind is, no doubt, good nursing: when a child has plenty of air and exercise, the digestion and a.s.similation will be properly performed, no acid will be produced to interrupt these functions, and the muscles and bones will grow together in just proportions.
CAROLINE.
I have often heard the rickets attributed to bad nursing, but I never could have guessed what connection there was between exercise and the formation of the bones.
MRS. B.
Exercise is generally beneficial to all the animal functions. If man is destined to labour for his subsistence, the bread which he earns is scarcely more essential to his health and preservation than the exertions by which he obtains it. Those whom the gifts of fortune have placed above the necessity of bodily labour are compelled to take exercise in some mode or other, and when they cannot convert it into an amus.e.m.e.nt, they must submit to it as a task, or their health will soon experience the effects of their indolence.
EMILY.
That will never be my case: for exercise, unless it becomes fatigue, always gives me pleasure; and, so far from being a task, is to me a source of daily enjoyment. I often think what a blessing it is, that exercise, which is so conducive to health, should be so delightful; whilst fatigue, which is rather hurtful, instead of pleasure, occasions painful sensations. So that fatigue, no doubt, was intended to moderate our bodily exertions, as satiety puts a limit to our appet.i.tes.
MRS. B.
Certainly. --But let us not deviate too far from our subject. --The bones are connected together by ligaments, which consist of a white thick flexible substance, adhering to their extremities, so far as to secure the joints firmly, though without impeding their motion. And the joints are moreover covered by a solid, smooth, elastic, white substance, called _cartilage_, the use of which is to allow, by its smoothness and elasticity, the bones to slide easily over one another, so that the joints may perform their office without difficulty or detriment.
Over the bones the _muscles_ are placed; they consist of bundles of fibres which terminate in a kind of string, or ligament, by which they are fastened to the bones. The muscles are the organs of motion; by their power of dilatation and contraction they put into action the bones, which act as levers, in all the motions of the body, and form the solid support of its various parts. The muscles are of various degrees of strength or consistence in different species of animals. The mammiferous tribe, or those that suckle their young, seem in this respect to occupy an intermediate place between birds and cold-blooded animals, such as reptiles and fishes.
EMILY.
The different degrees of firmness and solidity in the muscles of these several species of animals proceed, I imagine, from the different nature of the food on which they subsist?
MRS. B.
No; that is not supposed to be the case: for the human species, who are of the mammiferous tribe, live on more substantial food than birds, and yet the latter exceed them in muscular strength. We shall hereafter attempt to account for this difference; but let us now proceed in the examination of the animal functions.
The next cla.s.s of organs is that of the _vessels_ of the body, the office of which is to convey the various fluids throughout the frame.
These vessels are innumerable. The most considerable of them are those through which the blood circulates, which are of two kinds: the _arteries_, which convey it from the heart to the extremities of the body, and the _veins_, which bring it back into the heart.
Besides these, there are a numerous set of small transparent vessels, destined to absorb and convey different fluids into the blood; they are generally called the _absorbent_ or _lymphatic_ vessels: but it is to a portion of them only that the function of conveying into the blood the fluid called _lymph_ is a.s.signed.
EMILY.
Pray what is the nature of that fluid?
MRS. B.
The nature and use of the lymph have, I believe, never been perfectly ascertained; but it is supposed to consist of matter that has been previously animalised, and which, after answering the purpose for which it was intended, must, in regular rotation, make way for the fresh supplies produced by nourishment. The lymphatic vessels pump up this fluid from every part of the system, and convey it into the veins to be mixed with the blood which runs through them, and which is commonly called venous blood.
CAROLINE.
But does it not again enter into the animal system through that channel?
MRS. B.
Not entirely; for the venous blood does not return into the circulation until it has undergone a peculiar change, in which it throws off whatever is become useless.
Another set of absorbent vessels pump up the _chyle_ from the stomach and intestines, and convey it, after many circ.u.mvolutions, into the great vein near the heart.
EMILY.
Pray what is chyle?
MRS. B.
It is the substance into which food is converted by digestion.
CAROLINE.
One set of the absorbent vessels, then, is employed in bringing away the old materials that are no longer fit for use; whilst the other set is busy in conveying into the blood the new materials that are to replace them.
EMILY.
What a great variety of ingredients must enter into the composition of the blood?
MRS. B.
You must observe that there is also a great variety of substances to be secreted from it. We may compare the blood to a general receptacle or storehouse for all kinds of commodities, which are afterwards fashioned, arranged, and disposed of as circ.u.mstances require.
There is another set of absorbent vessels in females which is destined to secrete milk for the nourishment of the young.
EMILY.
Pray is not milk very a.n.a.logous in its composition to blood; for, since the nursling derives its nourishment from that source only, it must contain every principle which the animal system requires?
MRS. B.
Very true. Milk is found, by its a.n.a.lysis, to contain the princ.i.p.al materials of animal matter, alb.u.men, oil, and phosphat of lime; so that the suckling has but little trouble to digest and a.s.similate this nourishment. But we shall examine the composition of milk more fully afterwards.
In many parts of the body numbers of small vessels are collected together in little bundles called _glands_, from a Latin word meaning acorn, on account of the resemblance which some of them bear in shape to that fruit. The function of the glands is to _secrete_, or separate certain matters from the blood.