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1. In the preceding Lessons the manner in which the incomings of the human body are converted into its outgoings has been explained. It has been seen that new matter, in the form of vital and mineral foods, is constantly appropriated by the body, to make up for the loss of old matter, in the shape, chiefly, of carbonic acid, urea, and water, which is as constantly going on.

The vital foods are derived directly, or indirectly, from the vegetable world : and the products of waste either are such compounds as abound in the mineral world, or immediately decompose into them. Consequently, the human body is the centre of a stream of matter which sets incessantly from the vegetable and mineral worlds into the mineral world again. It may be compared to an eddy in a river, which may retain its shape for an indefinite length of time, though no one particle of the water of the stream remains in it for more than a brief period.

But there is this peculiarity about the human eddy, that a large portion of the particles of matter which flow into it have a much more complex composition than the particles which flow out of it. To speak in what is not altogether a metaphor, the atoms which enter the body are, for the most part, piled up in large heaps, and tumble down into small heaps before they leave it. The force which they set free in thus tumbling down, is the source of the active powers of the organism.

2. These active powers are chiefly manifested in the form of motion-movement, that is, either of part of the body, or of the body as a whole, which last is termed locomotion.

The organs which produce total or partial movements of the human body are of three kinds : cells exhibiting amæboid movements, cilia and muscles.

The amæboid movements of the white corpuscles of the blood have already described, and it is probable that similar movements are performed by many other simple cells of the body in various regions.

The amount of movement which each cell is thus capable of giving rise to may appear perfectly insignificant ; nevertheless, there are reasons for thinking that these ameboid movements are of great importance to the economy, and may under certain circumstances be followed by very notable consequences.

3. Cilia are filaments of extremely small size, attached by their bases to, and indeed growing out from, the free surfaces of epithelial cells (see Lesson XII.); there being in most instances very many (thirty for instance), but, in soine cases, only a few cilia on each cell. In some of the lower animals, cells may be found possessing only a single cilium. They are in incessant waving motion, so long as life persists in them. Their most common form of movement is that each cilium is suddenly bent upon itself, Lecomes sickle-shaped instead of straight, and then more slowly straightens again, both movements, however, being extremely rapid and repeated about ten times every second. These two movements are of course antagonistic; the bending drives the water or fluid in which the cilium is placed in one direction, while the straightening drives it back again. Inasmuch, however, as the bending is much more rapid than the straightening, the force expended on the water in the former movement is greater than in the latter. The total effect of the double movement therefore is to drive the fluid in the direction towards which the ciliun is bent ; that is, of course, if the cell on which the cilia are placed is fixed. If the cell be floating .free, the effect is to drive or row the cell backwards ; for their movements may continue even for some time after the epithelial cell, with which they are connected, is detached from the body. And not only do the movements of the cilia thus go on independently of the rest of the body, but they cannot be controlled by the action of the nervous system. Each cilium seems to be composed of contractile substance, and the cause of its movement would appear to be the alternate contraction and relaxation of its opposite sides along its whole length or at its base only ; but why these alternations take place is unknown.

Although no other part of the body has any control over the cilia, and though, so far as we know, they have no direct communication with one another, yet their action is directed towards a common end-the cilia, which cover extensive surfaces, all working in such a manner as to sweep whatever lies upon that surface in one and the same direction. Thus, the "cilia which are developed upon the epithelial cells, which line the greater part of the nasal cavities and the trachea, with its ramifications, tend to drive the mucus in which they work, outwards.

In addition to the air-passages, cilia are found, in the human body, in the ventricles of the brain, and in one or two other localities ; but the part which they play in man is insignificant in comparison with their function in the lower animals, among many of which they become the chief organs of locomotion.

4. Muscles (Lesson I. § 13) are accumulations of fibres, each fibre having a definite structure which is different in the striated and unstriated kinds (see Lesson XIII.). These fibres are bound up by fibrous (or connective) tissue with blood-vessels, &c. into small bundles; and these bundles are again similarly bound up together in various ways so as to form muscles of various shapes and sizes. Every fibre has the power, under certain conditions, of shortening in length, while it increases its other dimensions, so that the absolute volume of the fibre remains unchanged. This power is called muscular contractility; and whenever, in virtue of this power, a muscular fibre contracts, it tends to bring its two ends, with whatever may be fastened to them, together.

The condition which ordinarily determines the contraction of a muscular fibre is a change of state in a nerve fibre, which is in close anatomical connection with the muscular fibre. The nerve fibre is thence called a motor fibre, because, by its influence on a muscle, it becomes the indirect means of producing motion (Lesson XI. § 6.).

Muscle is a highly elastic substance. It contains a large amount of water (about as much as the blood), and during life has a clear and semi-transparent aspect.

When subjected to pressure in the perfectly fresh state, and after due precautions have been taken to remove all the contained blood, striated muscle (Lesson XII. § 15) yields a Auid which undergoes spontaneous coagulation at ordinary temperatures. At a longer or shorter time after death this coagulation takes place within the muscles themselves. They become more or less opaque, and, losing their previous elasticity, set into hard rigid masses, which retain the form which they possess when the coagulation commences. Hence the limbs become fixed in the position in which death found them, and the body passes into the condition of what is termed the “death-stiffening," or rigor mortis.

After the lapse of a certain time the coagulated matter liquefies, and the muscles pass into a loose and flaccid condition, which marks the commencement of putrefaction.

It has been observed that the sooner rigor mortis sets in, the sooner it is over ; and the later it commences, the longer it lasts. The greater the amount of muscular exertion and consequent exhaustion before death, the sooner rigor mortis sets in.

Rigor mortis evidently presents some analogies with the coagulation of the blood, and the substance which thus coagulates within the fibre (myosin or muscle-clot as it is sometimes called) is in many respects not unlike fibrin. It forms at least the greater part of the substance which may be extracted from muscle by dilute acids, and is called syntonin (see Lesson VI. § 4). Besides myosin, muscle contains other varieties of proteid material about which we at present know little ; a variable quantity of fat; certain inorganic saline matters, phosphates and potash being, as is the case in the red blood-corpuscles, in excess ; and a large number of substances existing in small quantities, and often classed together as extractives.” Some of these extractives contain nitrogen ; the most important of this class is kreatin, a crystalline body which is supposed to be the chief form in which nitrogenous waste matter leaves the muscle on its way to become urea.

The other class of extractives contains bodies free from nitrogen. Perhaps the most important of these is lactic acid, which seems always to be formed when a muscle contracts or when it enters into rigor mortis. For it is a curious fact that a muscle when at rest has a neutral or alkaline reaction as shown by testing it with litnius, but becomes acid when it has been contracting for some time or become rigid by death.

Most muscles are of a deep, red colour ; this is due in part to the blood remaining in their vessels ; but only in part, for each fibre (into which no capillary enters) has a reddish colour of its own, like a blood-corpuscle but fainter. And this colour is probably due to the fibre possessing a small quantity of that same hæmoglobin in which the blood-corpuscles are so rich.

Muscles may be conveniently divided into two groups, according to the manner in which the ends of their fibres are fastened ; into muscles not attached to solid levers, and muscles attached to solid levers.

5. Muscles not attached to solid levers.—Under this head come the muscles which are appropriately called hollow muscles, inasmuch as they enclose a cavity or surround a space; and their contraction lessens the capacity of that cavity, or the extent of that space.

The muscular fibres of the heart, of the blood-vessels, of the lymphatic vessels, of the alimentary canal, of the urinary bladder, of the ducts of the glands, of the iris of the eye, are so arranged as to form hollow muscles.

In the heart the muscular fibres are of the striated kind, and their disposition is exceedingly complex. The cavities which they enclose are those of the auricles and ventricles ; and, as we have seen, the fibres, when they contract, do so suddenly and together.

The iris of the eye is like a curtain, in the middle of which is a circular hole. The muscular fibres are of the smooth or unstriated kind (see Lesson XII.), and they are disposed in two sets : one set radiating from the edges of the hole to the circumference of the curtain ; and the

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