acidity of the chyme, is resumed as soon as the latter is neutralized, the pancreatic and intestinal juices operating powerfully in this direction.

As the chyle is thrust along the small intestines by the grasping action of the peristaltic contractions, the dissolved matter which it contains is absorbed, in the ordinary way, by the vessels of the villi. The minute particles of fatty matter, on the other hand, are squeezed through the soft substance of the epithelium into that of the villi; and so, in the long run, into the vessels; just as mercury may be squeezed by pressure through the pores of a washleather bag.

As the network of capillaries lies outside the lacteal radicle in each villus, it would appear that the blood-vessels must carry off the greater part of the chyle; but much of it enters the lacteals, fills them, and only enters the blood after a roundabout passage through the mesenteric lymphatics and the thoracic duct. (Lesson II. §§ 5, 6.)

25. The digested matters, as they are driven along the small intestines, gradually become deprived of their peptones, fats, and soluble amyloids, and are forced through the ileo-cæcal valve into the cæcum and large intestine. Here they acquire an acid reaction and the characteristic fæcal odour and colour, which become more and more marked as they approach the rectum. It has been supposed that a sort of second digestion occurs in the upper part of the large intestine.

LESSON VII.

MOTION AND LOCOMOTION.

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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. 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, or of the fluids which it contains, are of two kinds: Cilia and Muscles.

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.). They are in incessant waving motion, so long as life persists in them; and the motion of a cilium continues even for some time after the epithelial cell, with which it is connected, is detached from the body. Not only does the movement of the cilia thus go on independently of the rest of the body, but it cannot be controlled by the action of the nervous system. The cause of the movement of each cilium would appear to be the alternate contraction and relaxation of opposite sides of its base; 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 ramifica

tions, 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 of which 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 connexion 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 fluid which undergoes spontaneous coagulation at ordinary temperaAt a longer or shorter time after death this coagulation takes place within the muscles themselves.

tures.

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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.

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 bloodvessels, of the lympathic vessels, of the alimentary canal, 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