able material may, under certain conditions, be exposed to arterial blood without oxidation. Yet that this want of action is wholly due to incidental circumstances is shown from the fact that salts of organic acids are much more quickly oxidized in the blood than they are in the open air.

Advantage

decay of one

It is interesting thus to observe how the death of one part of the body ministers to the life of the rest; for the nitrogenized and acttaken of the ive principles of the juices secreted for the accomplishment of digestion are on the descending career, and are truly dying matter. The incipient stage of decay through which they are other. passing reacts on the food, and prepares it in a temporary manner to replace those parts of the body which are ceasing from activity, and about to be removed.

portion to organize an

CHAPTER V.

OF ABSORPTION.

Double Mechanism for Absorption.-The Lacteals and Veins.-Lacteal Absorption.-Description of a Villus.—Analogies in Plants.—Introduction of Fat by the Villi.- The Chyle. Causes of the Flow of Chyle.-Intermediate Changes on its Passage to the Blood.-Action of Peyer's Bodies. — Lymphatic Absorption. — Nature of Lymph. — Structure of the Lymphatic System.― Comparison of Chyle, Lymph, and Serum.-Function of the Lymphatic System.— Production of Fibrin.—Cutaneous Absorption.-Causes of the Flow of Lymph.-Apparent selecting power of the Absorbents.-Connection of the Lacteals and Lymphatics with the Locomotive and Respiratory Mechanism.

Double mech

THE food, after digestion, though in the alimentary tract, is exterior to the animal system. Means have therefore to be resorted to anism for ab- for its introduction into the circulation, and its distribution sorption. to every part. This is accomplished by a double mechanism, one portion of which is adapted to the digestion which has been going on in the stomach, the other to that which is completed in the intestine. The veins which are profusely spread on the walls of the digestive cavity constitute the former apparatus, the lacteal vessels the latter.

a villus.

The lacteal vessels may be described as delicate tubes, conveying maDescription of terials absorbed from the intestine into the blood. Their mode of origin may be understood by considering them as projecting with a fine but blunt end upon the inner coat of the intestine. This projection is covered over with smooth muscle cells and a plexus of blood-vessels, a continuation, as it were, of those of the mucous coat of the intestine itself; they are held together by connective tissue, and over that is cast a covering of cylindric epithelium. This construction con

DISTRIBUTION OF BLOOD-VESSELS TO THE VILLI.

85

stitutes what is called a villus, the shape of which is conical, or perhaps cylindrical. The villus may then be regarded as a process of mucous membrane.

Fig. 23.

wall.

Fig. 23 is a section of the wall of the ileum, a being the villi; b, glands of Lie- Structure of berkuhn; c, muscular layer of mu- the intestinal cous membrane; d, follicles of a Peyer's patch; e, remainder of submucous tissue beneath them; f, circular muscles; g h, longitudinal muscles. (Kolliker.)

Fig. 24 represents the distribution of bloodvessels to the villi of the intestine of the mon

key. The figure was drawn by the camera lucida, a a being the arteries, bb the veins.

The form of the villi differs in different regions of the intestine. the duodenum they are less elevated, laminated, and broader, Fig. 25. In the jejunum, more projecting or cylindroid,

In

Forms of villi.

Fig. 26. In all cases, however, they are abundantly supplied with blood

86

STRUCTURE OF THE VILLI.

vessels. Their epithelial covering of cylindroid cells is shown in the sectional diagram, Fig. 27, a a; at bb is the origin of the lacteal arising obscurely.

Fig. 27.

M

that that end of these cells nearest the cavity of the intestine is in reality open, and in this manner they account for the ready passage of oil globules into them, and also for the appearance of solid foreign bodies, as Osterlein observed.

Though we have described the lacteal as a vessel projecting into the Origin of the interior of the intestine, it is by some viewed rather as a mere lacteal. excavation in the villus. The villi impart to the mucous membrane an aspect sometimes likened to the pile of velvet. On an average, their number upon a square inch is about 10,000. The entire number of these organisms must, therefore, amount to many millions. time it was supposed that the lacteals open directly into the intestine-an opinion which is now universally abandoned. The action of each villus is doubtless more complicated than is generally represented, for the organic fibre cells it contains give to it the power of executing rhythmic motions. When the operation of the lacteal vessels as absorbents was first deThe lacteals tected, it was believed that all nutriment is introduced by not the exclu- their means. But there are many animals wholly destitute absorption. of this system of tubes, for instance, the invertebrates. Even In such cases absorption must necMoreover, though there are no lac

sive organs of

in many fishes the villi are absent. essarily be conducted by the veins. teals on the walls of the stomach, nor, indeed, on that part of the intestinal tube which is higher than the place of introduction of the biliary and pancreatic ducts, there are many substances freely absorbed from the gastric cavity when its pyloric orifice is tied. It has already been mentioned that the stomach absorbs water with remarkable rapidity. The doctrine that the lacteals are the exclusive organs of absorption must, therefore, be abandoned, for it is plain that the venous system participates in this duty.

The function of absorption has therefore to be examined from two points of view. As there are two digestions, one producing a perfect so

T1

ABSORPTION IN PLANTS.

87

venous absorp

lution, and the other an emulsioned, but not dissolved state Conditions of of the food, so there are two absorbent systems, the lacteals lacteal and of and the veins. The lacteals introduce such substances as are tion. not absolutely dissolved, particularly the oils and fats. The veins appear to take up those substances only which are completely dissolved in

water.

Absorption in plants, their ascending sap.

As in many other cases in physiology, so in this, a correct interpretation of the functions of the animal mechanism may be obtained by examining the corresponding structures and functions in plants. In the more perfect of these, the absorption of watery material from the ground, constituting the ascending sap, is brought about by the agency of collections of soft cells, which are placed at the extremity of each rootlet. They are designated spongioles. By their action the fluid is forced up through the tubes of the sap-wood into the leaves, and there exposed to the conjoint agency of the sun and air. A change is thus accomplished, and, from being crude, it turns into elaborated sap, and now descends through the bark, to be distributed to every part of the plant. Its ascent is caused by the cells of the spongioles, its descent by the chemical changes occurring among the cells which are found in the structure of the leaves.

These cells both of the roots and of the leaves-are far from continuing their action for an unlimited period of time. At the most, their existence is transient. Those of the roots are gradually lost by decay, or converted into solid structure, as the elongation of the organs through the ground goes on. Those of the leaves are equally transitory. At periodic intervals, both in deciduous and evergreen plants, the fall of the leaf occurs-a new organism succeeding in another summer to make up for the one which has passed away.

Whatever nutrient material is taken from the soil in the case of plants is introduced by the aid of a cellular structure, and the cells die after accomplishing their duty.

Analogy between the lacroots.

teals and plant

It was once a saying among physiologists that the lacteals are the roots of animals, and in this there is, in reality, a great deal of truth, for between the rootlet of a plant and the lacteal of an animal there is a conspicuous relation, not only in structure, but also in function. As is seen in Fig. 27, upon each villus of the intestinal tube there is a layer of cylindric cells, underneath which the lacteal vessel takes its rise, for it does not open by a free orifice on the interior of the intestine, but its flask-shaped, loop-like, or convoluted origin is obscurely seen in the midst of the cells. The aspect which the villi present, from its doubtful nature, has led to the erroneous conclusion that, as soon as active digestion goes forward, cells rapidly develop within the epithelium, and continue to do so as long as the intestine contains

88

lacteals into

the blood.

INTRODUCTION OF FAT.

digested matter; that they become turgid with chyle, and have a diameter of about the 100 of an inch; that, as they select material, they throw it into the lacteal tube, either by bursting or deliquescing, and at the same time set free broods of germs from which new cells are developed. So far, therefore, as their duration is concerned, if this be their true history, they are even more transitory than the corresponding cells of plants. That the lacteals are connected with respiratory digestion seems to be Fat is intro- plainly indicated by the circumstances of their occurrence. duced by the None of them are found upon the stomach, nor even on that part of the duodenum which is above the entrance of the hepatic and pancreatic ducts, but below that point they are scattered in profusion all over the small intestine. The digestion of fatty bodies not taking place until the food has gained the duodenum, vessels for the absorption of the emulsions to which that digestion gives rise are not required until after that point is passed. Correctly speaking, however, the lacteals are only lymphatics which are taking up oil presented to them. In view of the use which the oils subserve in the animal economy, the lacteals are in reality an appendix to the respiratory system. There can be no doubt that through their channel oils and fats, under the form of emulsions, are transmitted to the blood. The analysis of the chyle shows that it is always rich in fat, and, indeed, it is supposed by some physiologists that the objects just described as cells, surrounding the origin of the lacteals, are nothing more than oil or fat globules accumulated there and waiting to be taken up, or that the disappearance and exuviation of the so-called cells is an optical deception, due to their walls becoming permeated with oil.

The manner in which oil globules collect round the villus I have remarked as being very strikingly displayed

in the case of the gray squirrel after feeding on fatty nuts. As shown in Fig. 28, the whole structure looks as if it were distended with oil globules, a a, in the midst of which the origin of the lacteal, b b b, may be doubtfully and dimly discerned.

cells of the villi.

Fig. 28.

Half-diagram of villi of the gray squirrel after feeding on nuts.

Although it can not be admitted that the Evolution and production and deliquescence function of the of the cells of villi is a demonstrated fact, and that on this the action of the lacteals as absorbent vessels for the most part depends, the rapid evolution and disappearance of these cells is by no means a physiological impossibility. Botanists assert that, in a single night, the Bovista giganteum, a puff-ball, can develop from a mere point to such a size that it must contain fifty thousand millions of cells-a number that seems almost incredible.

The