muscular fibres around the termination of the rectum into a ring-like sphincter muscle, which keeps the aperture firmly closed, except when defecation takes place.

23. The only secretions, besides those of the proper intestinal glands, which enter the intestine, are those of the liver and the pancreas the bile and the pancreatic juice. The ducts of these organs have a common opening in the middle of the bend of the duodenum; and, since the common duct passes obliquely through the coats of the intestine, its walls serve as a kind of valve, obstructing the flow of the contents of the duodenum into the duct, but readily permitting the passage of bile and pancreatic juice into the duodenum (Figs. 36, 38, 42). Pancreatic juice is an alkaline fluid not unlike saliva in many respects; it differs, however, in containing a considerable quantity of proteid material. Bile we have already studied.

After gastric digestion has been going on some time, and the semi-digested food begins to pass on into the duodenum, the pancreas comes into activity, its bloodvessels dilate, it becomes red and full of blood, its cells secrete rapidly, and a copious flow of pancreatic juice takes place along its duct into the intestine.

The secretion of bile by the liver is much more continuous than that of the pancreas, and is not so markedly increased by the presence of food in the stomach. There is, however, a store of bile laid up in the gall-bladder; and as the acid chyme passes into the duodenum, and flows over the common aperture of the gall and pancreatic ducts, a quantity of bile from this reservoir in the gall-bladder is ejected into the intestine. The bile and pancreatic juice together here mix with the chyme and convert it into what is called chyle.

24. Chyle differs from chyme in two respects. In the first place, the alkali of the bile neutralizes the acid of the chyme; in the second place, both the bile and the pancreatic juice appear to exercise an influence over the fatty matters contained in the chyme, which facilitates the subdivision of these fats into very minute separate particles. The chyme, in fact, which results from the digestion of fatty food, is a mere mixture of watery fluid with oily matters, which are ready to separate from it

and unite with one another. In the chyle, on the other hand, the fatty matters are suspended in the fluid, just as oil may be evenly diffused through water by gradually rubbing it up with white of egg into what is termed an emulsion; or as the fat (that is, the butter) of milk is naturally held suspended in the watery basis of milk.

The chyle, with these suspended particles, looks white and milky, for the same reason that milk has the same aspect-the multitude of minute suspended fatty particles reflecting a great amount of light.

The conversion of starch into sugar, which seems to be suspended wholly, or partially, so long as the food remains in the stomach, on account of the acidity of the chyme, is resumed as soon as the latter is neutralized, the pancreatic and intestinal juices operating powerfully in this direction.

Recent observations moreover have shown that pancreatic juice has a powerful effect on proteid matters, converting them into peptones differing little, if at all, from the peptones resulting from gastric digestion. It would appear too that fats are not only minutely divided or emulsionized by the bile and pancreatic juice, ¿.e. acted upon mechanically, but also to a small extent converted by a chemical change into soaps, and thus rendered more soluble.

Hence it appears that, while in the mouth amyloids only, and in the stomach proteids only, are digested, in the intestine all three kinds of food-stuffs, proteids, fats and amyloids are either completely dissolved or minutely subdivided, and so prepared for their passage into the vessels.

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 osmosis into the vessels of the villi. The minute particles of fatty matter, on the other hand, which, not being dissolved, are incapable of osmosis, pass bodily through the soft substance of the epithelium into that of the villi, and so into the beginning of the lacteal.

The exact manner in which this is effected is at present a matter of dispute. The contents of the intestine are undoubtedly subject to pressure from the peristaltic con

tractions of the muscular walls; and this may help to squeeze the fat into the villi, just as mercury may be squeezed through the pores of a piece of wash leather. The process, however, is probably not one of mere pressure only.

As the network of capillaries lies outside the lacteal radicle in each villus, it would appear probable that the blood-vessels must carry off the greater part of the more soluble matters of the chyle. It is possible, however, that some of these pass by simple diffusion into the lacteals as well as into the blood-vessels. We are not, in fact, in possession of exact knowledge as to which constituents of the chyle pass into the lacteals, and which into the blood-vessels (or which into both), except on one point; and that is, that the minutely divided fat passes not into the blood-vessels, but into 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. has been supposed that a sort of second digestion occurs in the upper part of the large intestine.

It

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 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 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 which each cell is thus capable of giving rise to 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 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 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