SELECTING POWER OF THE ABSORBENTS.

99

all such cases every thing points out that the lymphatic vessels are the avenues through which the liquid is introduced.

In what manner does the lymph move? In reptiles there are found what are termed lymphatic hearts, which are merely dilated Cause of the portions of a tube exhibiting pulsation. Of these, in the flow of lymph. frog, two pairs may be discovered, one behind the hip-joint, and situated so superficially that the motions can be plainly seen; the other is at the anterior part of the chest. The pulsating movements of these organs, of course, impel the liquid acted on in the direction determined by the valves with which the vessels are so profusely supplied, that is, to the general circulation, and the lymph finally enters the blood-vessels.

But in the higher tribes these organs of impulsion are absent, and the circulation through the vessels is determined by the agencies mentioned in the case of the lacteals. 1st. By the constant accumulation of liquid at the origin of the tube; 2d. By every muscular movement, either voluntary or involuntary, which produces a compression of the tube, the valves all opening one way, and therefore causing the included liquid to pass in one direction only; 3d. By the exhaustive action at the mouth of the lymphatic, arising from the passage of the blood. It ought, perhaps, to be prominently pointed out, as belonging to the second of these causes, that the pulsation of the arterial trunks adjacent to any lymphatic brings the power of the heart itself into operation in an indirect

way.

Though the absorbents will receive many different bodies and transmit them to the veins, the action does not take place in an in- Apparent sediscriminate manner. Certain substances, such as the fats lecting power and albumen, find a ready entrance, but admission to others of absorbents. is wholly denied. Thus it has long been known that if coloring matter be introduced into the intestine, it by no means follows that the chyle will be tinged. If an animal be compelled to take litmus-water, the chyle will still be found colorless or white. On such facts was founded the old doctrine that these organs possess a low species of intelligence, distinguishing among different substances, permitting some to enter them, and refusing a passage to others. Many years ago I showed that these fanciful cases are capable of a simple physical explanation. Thus I found that if blue litmus water was tied up in a bladder, or a piece of peritoneum, and sunk in a vessel of alcohol, though the water would rapidly infiltrate into the alcohol, the coloring matter would be stopped just as it is in the intestine. But, in reality, there is no need of such experiments to satisfy us of the fictitious nature of this selecting power. If we fill a lamp half full of oil and half of water, and immerse in it a wick long enough to dip into both, if the wick be previously soaked in oil, it will withdraw from the lamp oil alone, and continue to do so until the

100

FORMATION OF FIBRIN.

lamp ceases to burn; but if it be first soaked in water, it will wholly refuse to take the oil, and remove the water alone, until all is escaped by evaporation. But did ever any one impute to the wick of a lamp a power of intellectuality, no matter how obscure, or suppose that there is any thing mysterious in such a selecting operation? A perpetual reference of the most common facts to mysterious agencies has been the great barrier to the advance of medical science. This system was introduced by the alchemists and quacks of the Middle Ages, and even now it will take many books and many years before physiology can be rescued from such visionary theories.

Connection of lacteals and lymphatics

with motion

From the point to which our descriptions have brought us, we have to regard this part of the absorbent mechanism as connected with two great animal functions, motion and respiration. Both its divisions, the lymphatics and the lacteals, in preparand respira- ing fibrin from albumen, make provision for the repair of the muscular tissues, and are therefore to be regarded as a portion of the motive apparatus. But the lacteals are charged with a farther duty, and in a double manner are connected with the respiratory mechanism, for they not only introduce fats into the system, but give origin to the cells of the blood, which are the carriers of oxygen.

tion.

We may therefore close this chapter with a few remarks, 1st. On the connection of the absorbent system with the provisions for motion; 2d. On its connection with the respiratory function, as more particularly displayed by the preparation of blood-cells.

1st. The connection of the absorbent system with the provisions for motion is through its function of preparing fibrin from albumen.

From the membrane which lines the plexus of tubes of which the mesFabrication enteric and lymphatic glands are composed, crowds of nucleated of fibrin. cells are continually arising. As to the function of these cells, there can be little doubt that it is in part to effect the translation of a portion of albumen, which has been introduced along with the oil globules, into fibrin, and accordingly we find that the chyle, analyzed at different parts of its course, yields different products. As has been stated already, intercepted before its passage through these glands, very little fibrin is found, but collected from points beyond, the quantity of fibrin steadily increases and that of albumen declines. The plexus of tubes has therefore for its object to expose its contents to the influence of the cells.

Now what are the chemical conditions under which the transmutation of albumen into fibrin takes place? The problem is most clearly presented in the case of the incubation of a bird's egg. The white of the egg, consisting chiefly of albumen, gradually loses that form, and passes into the state of fibrin as the development of the muscular tissues of the

FORMATION OF BLOOD-CELLS.

101

is furnished for

young chicken is effected; but the change can not take place except oxygen be received through the shell; and, indeed, in all cases in which albumen passes into fibrin, it does so only in the presence of oxygen. But in the case of the absorbent glands, from what source does the requisite oxygen come? These glands have just been de- Manner in scribed as plexuses of the absorbent tubes, among the rami- which oxygen fications of which arteries and veins are abundantly distrib- the making of uted, the blood not getting access to the interior of the ab- fibrin. sorbent, but running in its own vessels, as it were, side by side, and branching on the naked walls of the plexus; and, just as in the placental circulation the arterial blood of the mother vivifies or furnishes oxygen to the foetal blood, so in this instance the arterial blood enables the cells to discharge their duty of converting the albumen into fibrin, which passes onward to the general circulation for the renovation of the muscular tissues.

Since the hourly consumption of fibrin may be taken at 62 grains, the quantity produced by the action of these cells must be the same. We may therefore affirm that the fibrin-producing mechanism yields about one grain in each minute of time.

2d. Contemporaneously with the elaboration of fibrin is the development of the proper chyle corpuscles. Through the evolution Formation of of these and the absorption of fat, the chyle vessels present a blood-cells. connection with the respiratory apparatus.

If any weight is to be given to the views of Ascherson, the occurrence of fat globules in the chyle is essential to these cellular productions. He found that when globules of oil are placed in a solution of albumen, they become coated over with a film of that substance in a coagulated state, and hence was led to infer that this is the starting-point of cell production generally.

The chyle corpuscles are the embryos of the true red blood-cells, the latter being derived from them by gradual development. As will appear more in detail when we come to the description of the blood, in vertebrated animals there are two distinct classes of red blood- Two successive cells, which appertain to distinct periods of life. The first, forms of bloodwhich are found in man previously to the time of formation cells in man. of the chyle and lymph, are nucleated, and have the power of reproduction by fissuring of the nucleus.

But a distinct set gradually replaces the preceding. These cells have no nucleus; they are flattened, bi-concave, and in man circular. They possess no power of reproduction either by fissuring or otherwise. Their origin is from the chyle corpuscle, the granular interior of which clears up, and is succeeded by a deep red tint. The transition from the first to the second of these forms takes place at an early period, and may be

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ABSORPTION BY THE BLOOD-VESSELS.

regarded as complete in the human embryo of two months old. After that time blood-cells are generated upon the second plan, from the chyle corpuscles alone.

It is a significant circumstance that this transition from the reproductive to the non-reproductive blood-cell is coincident usually with the disappearance of the external branchiæ, or the closing of the branchial fissures. There can be no question that the destined function of the perfect blood-cell is the introduction of oxygen to the system. In their origin and in their object they are therefore in relation with the respiratory mechanism.

CHAPTER VI.

ABSORPTION BY THE BLOOD-VESSELS.

Proof of Absorption by the Blood Capillaries.—Occurs as a physical Necessity.-Nature of Capillary Attraction.-Its Phenomena in the Rise and Depression of Liquids.—Conditions for producing a Flow in a Capillary Tube.-Passage of Liquids through minute Pores.-General Propositions respecting Capillary Attraction.—Endosmosis and Exosmosis.-They depend on Capillary Attraction.—Force against which these Movements may take place.—Illustrations of selecting Power.-General View of the entire Function of Absorption, lacteal and venous. THAT the blood-vessels of the stomach and intestinal tube participate Substances are in the function of absorption is demonstrated by many different facts. Medicaments placed in the stomach after its illaries. pyloric orifice has been tied will produce their specific effect almost as rapidly as under natural circumstances; and, since there are no proper lacteals upon that organ, and its lymphatics seem to be inadequate, the absorption of these agents can have taken place through the blood-vessels only.

absorbed by

the blood cap

This conclusion is substantiated by an examination of the blood of the gastric and mesenteric veins. It varies with the stage of digestion and the nature of the food. At first there is a general lowering of the percentage amount of the solid ingredients, this being evidently the result of the absorption of water. At a more advanced period, the relative proportion of albumen, or rather of albuminose, rises, and along with it the extractive, gelatine, and sugar increase. As with the chyle in the lacteals, so with the blood in the mesenteric veins, coagulation takes place imperfectly, or perhaps not at all. It is stated that the mesenteric blood of a fasting animal does not differ from the ordinary venous blood.

The position of the blood-vessels, both on the mucous surface of the stomach and particularly on the villi of the intestine, is favorable to the

PHYSICAL NECESSITY OF VASCULAR ABSORPTION.

103

discharge of this function. The term venous absorption, employed to express it, is perhaps somewhat incorrect, since there is no reason that a venous capillary should have any advantage over an arterial one in this respect. The rapidity with which substances in a state of solution are taken up from these cavities has been well demonstrated by such instances as those of the detection of the ferrocyanide of potassium in the urine within 2 minutes of its having been deposited in the stomach, or by the death of dogs in a similar short period after strong alcohol had been administered to them, their blood being found to be charged with that combustible substance.

Among substances thus finding their way into the circulation by direct vascular absorption may be enumerated such soluble salts as have little affinity for the tissues, mineral and organic acids, alcohol, ether, volatile oils, vegetable alkaloids, and coloring matters, as those of rhubarb, madder, gamboge.

sels occurs as a

In fact, if there were not these physiological considerations, we should have to admit absorption by the blood-vessels as a mat- Absorption by ter of physical necessity; for, under the circumstances of the blood-vestheir situation, they must take up soluble matters presented physical necesto them. Through the pores of their delicate structure sub- sity. stances in the liquid state will pass to mingle with the blood.

Though we have treated of respiratory or lacteal absorption as specifically distinct from absorption by the blood-vessels, the circumstances here alluded to evidently point out that the resulting action of the villi of the intestines is of a mixed kind; for, though the epithelial cells and the commencing pouch of the lacteal may exert a definite influence, the network of blood-vessels which lies immediately beneath the epithelium must be engaged in precisely the same manner as the network of bloodvessels between the gastric follicles. The permeation of the walls of these tubes by substances in a state of solution is dependent, as we are now to see, upon a purely physical principle, which is just as applicable in the one case as it is in the other. The leading solid ingredients of the chyle being fat and albumen, the former is perhaps introduced by the proper lacteal structure, and the latter, taken up by the vascular network, exudes in part again from it into the lacteal arrangement.

In the case of absorption, as in that of respiration, hereafter to be described, there is a physical principle in operation which it is necessary to understand. I shall proceed to explain it on this occasion as far as is needful for the present purpose, and complete the description in the chapter on the function of respiration. The peculiar views here set forth, so far as they differ from those ordinarily expressed, I believe to be warranted by my own experiments elsewhere published.

The absorbent action of the blood-vessels depends on the force known