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The function of Respiration is performed in the lungs with common air. Air is composed of about 1 part of a gas called oxygen,1 and 4 parts
of another, nitrogen, 2 with an exceedingly small quantity of a third, carbonic acid. The change effected on air by respiration is the giving off one of these gases, oxygen, and receiving another, carbonic acid, in its place.
The chief difference between arterial and venous blood is, that the former contains oxygen, while the latter contains carbonic acid; and also that when the venous blood is sent from the heart to the lungs, its errand is simply to discharge this carbonic acid, and receive in exchange a supply of oxygen, with which it returns to the heart, ready to be sent off again throughout the body. It is clear then that in the lungs there must be (1) a provision for the blood coming in contact with the air; (2) an arrangement by which the blood in contact with the air shall be continually changing; and (3) an arrangement by which the air in contact with the blood shall be continually changing.
Let us consider, first, the provision for bringing the blood into contact with the air. It consists generally in having an immense extent of internal surface, covered by a complete network of minute vessels, through which the blood to be acted upon flows, separated from the air which has been inhaled only by an extremely thin membrane, so that the effect is the same as if the two were in actual contact. But how is this immense extent of surface obtained in a small compass; for the lungs are of moderate size? It is accomplished by the peculiar structure of the lungs. When the main air-tube or windpipe descends to the level of the lungs, it divides into two branches, called the bronchi [plural of Latin bronchus, the windpipe], one going to each lung (B, fig. 57). These again divide and subdivide into smaller tubes, called bronchial tubes, as 6, b. They subdivide in this way, until at length an immense number of minute tubes is formed, each of which opens into a small sac, a little wider than itself. Each of these, again, is partitioned off into a number of cells. Into the smallest of these cells the air has access, and it is on their walls that the network of capillaries is spread, in which the blood comes in contact with it. When the venous blood has returned to the heart, it is propelled from the right ventricle into the pulmonary artery. It must be remembered that this artery contains true venous blood. This artery soon divides into two, and sends a branch to each lung (g, i, fig. 54); and these ramify so as to send a minute artery along with each of the minute air-tubes, till they reach the lung-sacs already mentioned. The blood now distributes itself into capillaries on the walls of the cells and of the adjoining air-passages. The blood having in this manner come in contact with the air, and thus been rendered fit for being again sent out through the body, now passes into the first roots of the veins (so called, although they contain true arterial blood), which unite and reunite to form the four pulmonary veins that pour the blood, thus arterialised, into the left auricle of the heart, e, e', fig. 54.
1 From Greek oxys, acid, and gennao, to produce.
2 From Greek nitroji, soda, and gennao, to produce.
The action of the lungs is very much like what takes place on squeezing a hollow india-rubber ball in the hand, and allowing it to expand alternately. By a peculiar arrangement of the ribs, and by the contraction of the diaphragm (the floor of the chest, as it were, which, when uncontracted, rises up into the chest with a convex surface), the capacity of the chest may be very much increased. Suppose, then, the chest to he expanded; the lungs, being of a spongy nature, are also free to expand, and immediately the air rushes in through the windpipe, and fills them, just as happens when the ball is held in the open hand. But now the ribs fall, the muscles of the diaphragm relax, and the chest is contracted so as to squeeze the air out of the lungs, as happens when the hand is closed on the ball. The muscles that contract to cause these various movements act independently of the will, so that the action is kept up during sleep, which it could not be if it were voluntary.
Secretion and Excretion.
We have seen that the two functions of the Hood are to deposit materials for building up and keeping in repair the different parts of the body, and to carry off the waste matter arising from the decay continually going on. The various processes by which the products of this waste are drawn off from the blood receive the general name of secretion;1 arid the organs for the purpose, all of which are constructed on the same principle, are called glands.** These secretions are of two kinds. One kind is merely a purification of the blood, in carrying off something which would be injurious to the system, while the other kind extracts from the blood a fluid which is intended to serve a purpose in the economy of the system. The products of the former are called excretions,3 as the exhalations from the skin, urine, &c.; the products of the latter are secretions proper, as the saliva, gastric juice, &c. The general structure of a gland is a collection of follicles, or minute membranous bags, surrounded with a network of capillaries, from which the materials of the secretion, or the injurious substance to be excreted, are drawn. They are divided into two kinds, according to the manner in which they discharge their contents. In some cases, the secreting follicles discharge their contents separately, as the sweat glands and sebaceous glands of the skin. The work of secretion is really performed by cells or exceedingly minute bags, lining the follicles; and another form of discharge is when these cells, without being contained in follicles into which they discharge the fluid secreted, lie on the surface of a membrane, and discharge their contents upon it directly, as in the case of the mucous membranes.
In the more complicated secreting organs, such as the liver or kidneys, one or more tubes which carry off the secretion ramify in the organ into very minute branches; and at the point of each of the smallest twigs is a follicle, lined with cells, which discharge their contents into the follicles, to be carried off by the tubes.
The Liver is the organ which secretes the bile, and is a body of a darkbrown colour, divided into two lobes, and lying opposite the stomach, on the right side. Part of the bile is sent direct from the liver to the intestine, but there is a reserve kept in the gall-bladder, a little bag attached to it. The bile is drawn from the venous blood returning from the abdomen, which is made to pass through the liver in its passage to the heart.
The Kidneys lie in the lumbar region, in front of the backbone, one on each side, and are imbedded in fat. The function of this organ is to excrete the urine, in which are carried off, dissolved in a large quantity of water, several substances which would act as poisons if left in the system.
I From Latin secerno, to put aside or separate. 2 Latin rjlans, an acorn, a gland.
3 From Latin exccrno, to sift.
Other secreting organs are the pancreas, a large oblong gland behind the stomach, which secretes the pancreatic juice, described under Digestion; the spleen, situated near the upper left-hand corner of the stomach, the function of which seems to be to act as a storehouse of nutritive material, which can be taken advantage of by the circulation, in certain emergencies of the system; and the mammaryl glands or breasts (in women), which secrete milk for the nourishment of infants.
The Nervous System.
The functions of organic life are merely secondary and subservient to the higher or animal life. All the arrangements of the organism of the body, considered as a complete structure, are adapted to connect the Spirit of which it is the tenement with other beings and things around it—to receive sensations, to feel, to think, to will, and to act under the guidance of the mind or of instinct. For the purpose of carrying on the functions of this higher life of the human being, we find what might be called a complete telegraphic system. There is a head-o&ce, to which telegraphic messages are sent from all parts of the body, telling what is going on, and asking for instructions as to what ought to be done in the circumstances; and forthwith messages are sent back with the required instructions. When one places the hand on something burning, a message immediately goes off to the head-office from the hand, that it is being burned, and back comes a message to the muscles of the arm to withdraw the hand from the dangerous spot; and it is done. Or, again, suppose a man to be crossing a street, and a vehicle to come up behind him. First, there goes a message from the ears to the head-office that there is a sound of wheels behind. At once a message is sent off to the muscles of the neck to turn the head, that the eyes may be able to report how matters stand. Probably the eyes send in a message to the effect that a cab is close behind. When this is taken into consideration in the head-office, the conclusion is come to, that it would be advisable to have the body removed out of the way as quickly as possible; and accordingly, a message is at once sent to the muscles of the legs, telling them to be active; and the visible effect is that the man jumps out of the way. This is no fanciful description; but a process similar to that here described is what actually takes place in most actions, although these may be done in an instant. Now, this telegraphic system is the nervous system; the head-office is the brain; the telegraph wires are an immense number of nerves, or nervous fibres, spread over the surface of the body, -being especially abundant in the organs of sense.
1 From Latin mamma, the breast.
What is the nature of the connection that exists between the mind and the action of the brain, no one has yet been able to tell; we only know that there is a connection. Nor can the nature of nervous influence be -explained. The nearest approach to an explanation that can be given, is to say that it acts like electricity; and, indeed, the similarity between the action of the nervous system and that of the electric telegraph is greater than at first might be supposed. When treating of the muscles, we saw that they have the power of contracting. The principal agent in producing contraction of the muscles is the stimulus conveyed to them through the nerves.
The substance of which the nervous system is composed is of two -distinct kinds, called, from their appearance, white and gray matter. The white matter consists of straight fibres, lying side by side, and bound into bundles; these bundles are again bound together into larger bundles; and in this way are formed the nervous trunks which are spread throughout the body, and which are entirely formed of this white or fibrous matter. The gray matter is found, to a great extent, in the ganglia,1 or nervecentres, which are masses of nerve-matter occurring at intervals on the trunks. This gray matter is not of a fibrous texture, like the white, but consists of cells; and it seems, that in some way or other, the nervefibres of the trunks communicate with the cells of the gray matter in the ganglia. The office of the nerve-fibres in the trunks is to convey the influence of something going on in one part of the system to another part. These influences, as has been indicated, are sent in two directions —the impressions made on the sensitive parts of the body are conveyed to the nerve-centres, and give rise to what are called sensations; and when these sensations cause certain emotions, or an exercise of the will, an influence is sent outward to the muscles, which are thereby excited to contraction. And it has been proved that these different messages are conveyed by two different sets of fibres: those which carry an influence to the nerve-centres being called sensitive or afferent;2 and those which carry an influence from these to the muscles, motor (because for the most part giving rise to motion) or efferent?
The nervous system consists of two parts—a central part, called the cerebrospinal axis, which consists of the brain and the spinal cord, lodged within the skull and the vertebral column; and an outside part, consisting of nervous trunks proceeding from the central part, and distributed to all parts of the body. The brain is composed of a number of ganglionic masses, each of which is at the head of a special department;
1 Plural of ganglion, Greek, a knot. 2 From Latin ad, to, and/ero, to bring.
3 From Latin ex, out from, and fero, to bring.