the canal about equals that which is removed by absorption, so that the contents at the ileo-cæcal valve are about as fluid as they are in the duodenum; in the large intestine on the contrary, especially in its later portions, the contents become less and fluid. At the same time a characteristic odour and colour are developed, and the remains of the food, now consisting either of undigestible material, or of material which has escaped the action of the several digestive juices, or withstood their influence, gradually assume the characters of fæces.

LESSON VII.

MOTION AND LOCOMOTION.

I. 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 food, is constantly appropriated by the body, to make up for the loss of old matter, which is as constantly going on in the shape, chiefly, of carbonic acid, urea, and water.

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 enter the body for the most part, piled up into large heaps, and tumble down into small heaps before they leave it. The energy 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 amaboid movements of the white corpuscles of the blood have been 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 to which each cell is thus capable of giving rise may appear perfectly insignificant; nevertheless, there are reasons for thinking that these amoeboid 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 certain epithelial cells (see Lesson XII.); there being in most instances very many (thirty for instance), but, in some 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, becomes sickle-shaped instead of straight, and then more slowly straightens again, both movements, however, being extremely rapid and repeated about ten times or more 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 cilium 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

the cilia may continue their movements 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 appear not to be controlled by the action of the nervous system. Each cilium is comparable to one of the mobile processes of a white corpuscle. A ciliated cell differs from an amoeboid cell in that its contractile processes are permanent, have a definite shape, and are localised in a particular part of the cell, and that the movements of the processes are performed rhythmically and always in the same way. But the exact manner in which the movement of a cilium is brought about is not as yet thoroughly understood.

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 a few 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 XII.). These fibres are bound up into small bundles by fibrous (or connective) tissue, which carries the vessels and nerves; 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 property, under certain conditions, of shortening in length, while it increases its other dimensions, so that the absolute volume of the fibre remains unchanged. This property is called muscular contractility; and whenever, in virtue of this property, 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, as we have seen (Lesson V. §31), the passage along the nerve fibre, which is in close anatomical connection with the muscular fibre, of a nervous impulse, i.e. of a particular change in the substance of the nerve which is propagated from particle to particle along the 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 fluid 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. This stiffening is accompanied by a change in the chemical reaction of the muscle, for while living muscle, when tested with litmus is faintly alkaline or neutral, at least when at rest, it becomes distinctly acid as rigor mortis sets in. And it is a curious fact that a similar acidity is developed even in a living muscle, when it contracts.

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,