Regeneration

of the organism itself. Whether the size is due to more cells being present, as seems probable, or to the cells being larger, or to both, has not, so far as I know, been determined for man. In a mollusk, Crepidula fornicata, in which large and small adult individuals occur, it has been shown by Conklin ('98) that the difference is due entirely to the larger number of cells in the larger individual. In this case external conditions, in so far as they retard the maximum possible growth of the individual, are responsible for the differences in size. The distinction is, in this case, rather between large normal individuals and dwarfs, than between giants and normal or average individuals.

The voluntary muscles of the body of man grow larger, and may be said to hypertrophy, as a result of doing certain kinds of work. The muscles of the hand and arm grow large through use, and become smaller again if not used ; but the muscles of the fingers of a musician do not hypertrophy, although the total amount of work done may be very large. It is only when muscular work is done against great resistance that enlargement of the muscles takes place. The factors that may bring about the enlargement will be discussed later.

The kidneys seem to give the most satisfactory evidence of compensating hypertrophy. Nothnagel1 states that it has been shown in man, in the rabbit, and in the dog, that when one kidney has been removed the other enlarges; and that this takes place both for young animals, in which the kidneys have not reached their full size, and in adult animals, in which the remaining kidney becomes larger than normal. In the adult the enlargement is due to hypertrophy, in Virchow's sense, in the tubules and in the epithelium of the canals. In the young animal there is, in addition, a hyperplastic growth that leads to an increase in the number of glomeruli, etc.

Experiments have shown that the same amount of urea is excreted by the animal after the removal of one kidney as before ; in fact, this is true immediately after the operation, before any increase in the size of the organ has taken place. This means that, under normal conditions, the kidneys do not perform their maximum of work. It is important to observe in this connection that the remaining kidney gets more blood than it would get if the other were present. Nothnagel sums up the changes that take place in this way : First, the removal of one kidney ; second, an increase in the flow of blood in the remaining kidney ; third, an increase in the functional activity and excretion of this kidney ; fourth, along with the increase in the flow of blood, there is a necessary increase in the amount of food that is

1 Nothnagel gives a review of the subject down to 1886 in an article entitled "Uber Anpassung und Ausgleichting bei palhologischcn Zustanden. Zeilsc/i. f. klinische Medicin" 1886. Vols. X and XI. brought to the kidney in the blood ; fifth, this food is taken up in larger amount than before by the cells, which leads to an increase in the growth of the cells, which produces hypertrophy. The increase in size, looked at from this point of view, Nothnagel says, has nothing mysterious about it. The enlargement seems to be an adaptation ; but the enlargement does not take place because it is an adaptive process, but because it cannot be helped under the conditions that arise. We shall return again to Nothnagel's interpretation, when we come to consider other views.

' Experiments of the sort just described are most easily carried out on the paired organs of the body, such as the salivary glands, the tear glands, the mammae of the female, and the testes of the male. In regard to the latter two organs the evidence, especially in the case of the testes, is conflicting, but the recent experiments of Ribbert seem to give definite results. Nothnagel had found that after the removal of one testis there is no hypertrophy of the other. He pointed out that this result does not stand in contradiction to his hypothesis in regard to the kidneys, for the loss of one testis does not lead to a greater functional activity in the other. Each acts for itself alone. The result shows further, he adds, that the process of hypertrophy is not an adaptive one, but a physical or a physiological process. Ribbert on the contrary thinks that even Nothnagel's statistics give evidence of hypertrophy, and Ribbert's own experiments give unmistakable evidence of a considerable enlargement of the remaining testis. In his experiments, young rabbits were used that were born of the same mother and in the same litter. One of the testes was removed from some of the individuals, and after some months the remaining testis was taken out and its weight compared with that of the control animal. In sixteen out of seventeen experiments there was found to be a noticeable increase in the single testis as compared with either testis of the control animal. The results show that in some cases the single testis weighs almost as much as the two together of the control animal. It is important also to notice that in this case the enlargement has taken place in an organ that has not been active, as was the case with the kidney.

Ribbert has also shown that hypertrophy takes place in the mammae of the rabbit after the removal of some of them. Five out of the eight mammas were removed in three cases, and seven out of the eight in two other cases from young rabbits about two months old. Ribbert found that if the operator is not careful to remove completely all the tissue of a mamma an active regenerative process takes place from the part that remains. After five and a half months the single remaining mamma of one animal measured six and one-half by three and four-fifths centimetres, and the corresponding one in the control

animal five and three-fourths by three and one-half centimetres. The glandular tissue was also found less developed in the control animal. In another experiment the rabbit experimented upon bore young when it was six and a half months old. Soon after the birth of the young and before the mamma had been used the animal was killed and the single mamma that had been left was measured. It was much enlarged and projected more than the normal mammae. It measured nine by five centimetres. In a normal control animal1 the corresponding mamma measured seven by five centimetres. The number of acini was in the proportion of sixteen in the animal operated upon to ten in the normal. The results show a distinct compensating hypertrophy, due to a hyperplastic increase in the number of elements of the gland.

A further example of compensating hypertrophy has been found after the removal of the spleen, when the lymphatic glands of other parts of the body become enlarged. There are also observations which go to show that after the removal of some of the lymphatic glands others undergo an enlargement. Ziegler2 has given a critical review of the various opinions and hypotheses that have been advanced to account for the process of hypertrophy. According to Cohnheim3 hypertrophy in bones, muscles, spleen, and glands is due to hyperaemia, i.e. increased blood supply. He thinks that neither mechanical nor chemical stimuli can cause directly new processes of growth. Recklinghausen 4 thinks that hypertrophy is not due to any extent to an increase in the food supply. Samuel5 explains hypertrophy as due to a removal of, or to a decrease in, the resistance to growth and also to the influence of the nerves. Klebs 6 thinks that three factors enter into the problem, (a) inherited peculiarities, (b} overfeeding, (c) a removal of the controlling influences. Weigert believes that reparative processes are due to the removal of influences that prevent growth, and not to a direct stimulus. He thinks that a stimulus may start a functional act, but can never start a nutritive or a formative one. Good nourishment, for instance, may bring a tissue to a maximum development that is predetermined by innate peculiarities, but " idioplastic forces " are not thereby increased. Pekelharing 7 thinks that hypertrophy is due to a disappearance of a resistance to growth, and also to a stimulus causing proliferation.

We see from these various opinions how little is really known ; 2 Internal. Beitrage zu wissensch. Medicin. Festschrift fur R. Virchow, Vol. II, 1891. 3 Vorlesungen itber allegemeine Pathologic, Vol. I, 1882. 4 Pfandbuch. 7 Uber Endothelwucherungen in Arterien. Beitr. z. pathoL Anat., Vol. VIII, 1890. how little has been determined as yet by experiment as to the causes that bring abouthypertrophy. Many of the views are more or less plausible in the absence of direct, experimental evidence, but it remains for the future to decide as to the correctness of all of them. They are valuable as suggestions, in so far as they show the different possibilities that must be taken into account.

Ziegler first advocated the view, in the first edition of his Le/irbuc/i, that hypertrophy is due to a lessening of the resistance to growth. He thinks that while hyperaemia and transudation may support the new growth, they are never the only cause of the formation of new tissue. While Virchow's view that any injury to the body or to an organ excites proliferation finds support in the work of Strieker and Grawitz, yet the view has been combated by Cohnheim and by Weigert, and is no longer held by many pathologists. Ziegler points out that as a result of his own work, and that of his students, traumatic and chemical lesions are not followed at once by new growth of the tissue, but by degeneration of the tissue, and by changes in the circulation that lead to exudations. The new growth begins, at the earliest, eight hours after the operation, and generally only after twenty-four hours. Also after mechanical, chemical, or thermal injuries, a long interval elapses before phenomena of growth begin. The injury itself does not appear to produce the growth, but brings about those conditions that lead to cell-multiplication. Ziegler discusses what is meant by the idea of a lessening of the resistance to growth. He himself does not mean by this that hypertrophy depends on changes in the physical conditions, because it is known that living phenomena are the outcome of chemical processes and it is, therefore, a priori probable that the effect is brought about by chemical substances in the fluids of the tissues. These substances affect functional actions, and may even bring about regenerative changes.

This action of chemical substances on the formative activity of the cell is theoretically possible in either of two ways ; first, chemical substances of definite concentration are set free, or, second, chemical substances are present in the normal condition that prevent proliferation, but if their influence should be counteracted by other substances the conditions become favorable to growth. It is known in the case of certain unicellular organisms, that derive their nourishment from the surrounding medium, that their increase in number may be retarded by the presence of certain chemical substances. It is also known that certain organisms may themselves produce chemical substances that prevent their own multiplication. It is, therefore, at least conceivable that after a part has been injured a new substance may be produced that acts upon and destroys in the organ itself the substances there present that have prevented its further growth. The other interpretation is that

in the breaking down of the tissue of the organ a substance is produced that excites the cells to proliferation. Klebs suggested that the accumulation of the leucocytes at the wounded surface may act as a stimulus to growth, and that the chromatin of their nuclei might be absorbed by the cells of the tissue, and combining with the nuclei of these cells bring about the new growth. But Ziegler points out that we now know that although the leucocytes are dissolved and absorbed over the wounded surface, no process of absorption, of the sort postulated by Klebs, takes place. Ziegler thinks that Nothnagel is wrong in supposing that an increase in the blood supply, bringing with it an increase in the nourishment, can account for the hypertrophy of the kidney. On the contrary he believes that the growth is the result of an increase in the function of the organ due to the increase of the chemical substance, urea, that is brought to the secreting cells. The muscles of the body also hypertrophy as a result of their activity and not as a result of the additional blood supply.

In connection with these problems of hypertrophy it may be pointed out that, under certain conditions, blood vessels may enlarge and their walls become thickened. To cite a single example, Nothnagel found that if the femoral artery of the rabbit is tied, the blood vessels, that come off immediately above the ligature, and which have already, through their subdivisions, connections in the muscles with other branches of the same femoral artery (that come off below the ligature), grow larger after a time. This he believes to be due, in the first instance, to the increased speed of the blood in the vessels, and thereby the bringing to these arteries of an increased food supply. Other writers have given different interpretations. Ziegler himself believes that several factors may be capable of bringing about the result. He thinks it improbable that the increase in the food supply can alone be the cause, and thinks it much more probable that the increased work that the vessels must perform while carrying more blood will account for the enlargement.

In connection with this discussion it may not be unprofitable to recall that in the regeneration of the lower animals we find simpler conditions in which proliferation of the cells takes place under circumstances where many of the factors suggested in the above discussion are absent. In the first place we find that new growth may occur without any increase in the nourishment that is brought to the organ. Regeneration takes place in the entire absence of food, except so far as it may be stored up in the tissues. Even in a planarian that is starving and decreasing in size, proliferation of new cells will take place if a part is removed. In many of the lower forms there may be proportionately even a much greater proliferation than in the regeneration and hypertrophy in the mammalian organs. It is true

that proliferation may be more active if the tissues are well fed, but this does not show that the presence of food is a factor in the proliferation except so far as it keeps the proliferating cells in their best condition for growth. It is possible in many animals, more especially in some of the lower forms, to force them to grow rapidly by supplying them with a large amount of food, and conversely by decreasing the food to delay the growth. While this shows that the rate of growth is, within certain limits, a function of the amount of food, there may be also other factors that enter into the result, and in all cases there is an upper limit beyond which it is not possible to make the animal grow any larger.

That the presence of certain substances may bring about the enlargement of a part must be admitted as probable. It has been shown, for instance, that after the removal of certain lymphatic glands other glands may become larger. This appears to be due to the greater activity of the gland, brought about probably by the presence of an increased amount of some specific substance. In this instance the result can scarcely be due to a decrease in the physical resistance to growth or to an increase in the blood flow, except so far as this is brought about by the increased activity. It is, of course, possible, even if it cannot be positively shown in the case of the lymphatic glands, that a substance in the blood causes the hypertrophy in certain organs, while in others, as in the kidney, an increase in the blood flow may be also a factor in its hypertrophy.

The view held by several pathologists, that hypertrophy and regeneration may be caused by the removal of a physical resistance to growth, cannot be looked upon as a very probable hypothesis. The experiments in grafting of hydra and lumbriculus show that regeneration may still take place when the physical resistance has been reestablished by grafting two pieces together. These results, which are more fully described in a later chapter, demonstrate that the growth is due to other influences.

A comparison with the lower animals shows that proliferation takes place when all but three of the factors considered in connection with hypertrophy and regeneration in the higher forms have been eliminated. These are, first, the action of substances that act either directly or as counteracting some substance already present, as Ziegler suggests ; second, an innate tendency in the organism to complete itself; and, third, the use of the organ. It is impossible that the second factor enters into the problem of hypertrophy. In those cases in which regeneration takes place when a part of an organ is removed, as in the case of the liver, for example, the result may possibly also involve the second of the two factors, for the process is much like that of morphallaxis in the lower animals.

If it be granted that the growth in a hypertrophied organ is brought about by some substance that increases the function of that organ, can we suppose the phenomenon of regeneration to be due to similar factors ? In other words, can we reduce both phenomena to the same principle ? The case is complicated by two facts that may be illustrated by concrete examples. If a piece is cut from the middle of the body of lumbriculus new cells are produced at both ends of the piece. If we suppose the proliferation is brought about by the accumulation of certain substances in the piece, we must still invoke other factors to account for the differentiation of the proliferated material, since a head forms at one end and a tail at the other. All the hypothesis can do in itself is to account for a proliferation, not for the differentiation, and, both in the case of hypertrophy and in that of regeneration, it is the formation of new structures that we are chiefly concerned with, rather than the simple act of growth or of proliferation. If a piece of a hydra is cut off, the whole piece changes into the typical hydra form. Here there is no extensive process of proliferation, and the change is in the old part. It seems highly improbable that the production of substances in the piece could account for its change of form. These examples will suffice to show that in the process of regeneration it is very improbable that the change is brought about by special substances that may develop or be present in the part. We must suppose that during regeneration the formation of the typical form is not the result of a stimulus originating in a chemical substance acting upon the living material, but due to changes brought about directly in the living part itself.

We must conclude, therefore, that despite the apparently close connection between the phenomena of hypertrophy of uninjured organs and of regeneration, they may often involve different factors. If specific substances can bring about the hypertrophy of an organ, it is still not clear at present whether they do so by directly causing new growth, or whether their presence only stimulates the organ to greater activity and the activity of the organ is the cause of its growth. Since it must be supposed that in each organ a different specific substance brings about its activity and the consequent hypertrophy, it seems more probable that the result is due to the activity itself rather than to a stimulus from the substance. This view is further supported by the fact that in the case of the muscles and of the blood vessels the hypertrophy is directly connected with their use. The greater use brings about a larger supply of blood, but the blood is only different in amount and not in its quality. It must be confessed that it is difficult to see how the use of a part could make its growth increase, for by use the tissues break down ; and we are not familiar with any other processes within the body that make for the

building up of an organ in more than an inverse ratio to its breaking down. We are, however, familiar with phenomena of building up due to an increase in the food supply. It might appear from this to be more in accordance with what we find, to assume that the hypertrophy is solely due to an increase in the food supply ; yet there are other facts known that show that an organ does not increase in size simply because it gets more blood, and that this occurs only when the organs have a greater functional activity. It is a safer conclusion, I think, at present to assume that both the activity of the organ and the increase in its supply of food acting together are factors in the result. On the other hand we are so much in the dark concerning the functioning and growth of organs that we can do little more, as the preceding pages show only too clearly, than speculate in the vaguest sort of way as to what changes take place ; but since the processes seem to be within reach of experimental methods we can hope in the near future to learn more of how the processes of hypertrophy are brought about.

It would not be profitable to enter into a general discussion of the many cases of absorption, or of atrophy of parts of the organism, but a few examples may be given that have a general bearing on the topics discussed in this chapter. The more noticeable cases arise through disuse of an organ, as shown, for example, in the decrease in size of the muscles of man when they are not used. Since this may take place in a single group of disused muscles, when no such change occurs in other muscles of the same individual that are in use, the most obvious explanation is that the decrease is due directly to disuse. Since the blood that goes to all the parts is the same, the diminution cannot be ascribed to any special substance in the blood. The flow of blood into the disused muscle is less than when the muscle is used, and it might be supposed that atrophy is directly caused by the lessened nourishment that the muscle receives. There is also the possibility that the decrease is brought about by the accumulation of certain substances in the disused muscle itself, but since, in general, the breaking down of the muscle is most active when it is usecl, it seems improbable that the result can be due directly to this cause, unless indeed it could be shown that the substances produced by a disused muscle are different from those in an active muscle.

Lack of food, as is known, may cause organs to decrease, the fat first disappearing, and then in succession in vertebrates, the blood, the muscles, the glands, the bones, and the brain. Certain poisons may also affect definite organs and bring about a decrease in size, as when the thymus and mammae decrease from iodine poisoning, and certain extensor muscles after lead poisoning. Atrophy may also be brought about by pressure on a part, as when the feet or waist are compressed. In old age there may be a decrease in some of the organs, as in the bones, the testes and ovary, and even in the heart.

Degenerative changes appear even in the young stages of some animals, as when the tail of the tadpole is absorbed and the arms of the pluteus of the sea-urchin are absorbed by the rest of the embryo. Especially interesting are the cases of absorption that take place when organs are transplanted to unusual situations in the body. Zahn transplanted a foetal femur to the kidney, where it continued to grow but was later absorbed. Fischer transplanted the leg of a bird's embryo to the comb of a cock, where it continued at first to grow, but after some months degenerated. The spleen, the kidney, and the testis have been transplanted, but they degenerate, and, in general, the larger the transplanted piece the more probable its degeneration. Small pieces of the skin have been transplanted from one individual to another, and it has been found that small pieces maintain themselves better than large pieces. Ribbert's recent experiments in transplanting small pieces of different organs have been more successful than earlier experiments in which larger pieces were used. The first difficulty seems to be in establishing a blood supply to the new part, in order to nourish it. If the piece is quite small, it can absorb the substances, necessary to keep it alive, from the surrounding tissues, until the new blood supply has developed.

In the lower animals grafting experiments have been more successful, because the parts can remain alive for a longer time. It is important to find, however, that even in these cases, a part grafted upon an abnormal region of the body is usually absorbed. Rand shows that if the tentacles of hydra become displaced, as sometimes happens when a piece containing the old tentacles regenerates (Fig. 48, A-A3), the misplaced tentacles are absorbed ; and I can confirm this result. In hydra, the hollow tentacles are in direct communication with the central digestive tract, and a displaced tentacle seems to be in as good a position as a normal one, as far as its nourishment is concerned, yet it becomes absorbed.

Rand also found, in other experiments, that when the anterior end of a hydra is grafted upon the wall of another hydra, the piece may maintain itself if it is large ; but it is slowly shifted toward the base of the hydra to which it is grafted, and then the two separate in this region. If the graft is small, it may be entirely absorbed into the wall of the animal to which it is attached. Marshall found that if the head of a hydra is partially split in two,

each half-head completes itself (as Trembley had already shown). The body then begins slowly to separate into two parts, beginning at the angle between the two heads, until finally the two parts completely separate. King (1900) has repeated the experiment in a large number of cases with the same result. It seemed that the division might be brought about by the weight of the halves causing the gradual separation of the body, but King has shown that this is not the case, for, when a double form remained hanging with its head down, it still divided into two parts (Fig. 47, A). In this case, the weight of the two heads would cause the parts to come together rather than to separate, if gravity had any influence of the sort suggested. Marshall and King have also shown that if the posterior end of a hydra is split in two, the two parts do not continue to separate, but one of the two, if the pieces have been split some distance forward, may become constricted from the other, and, producing new tentacles at its apical end, become a new individual.

I have carried out a series of experiments on planarians of a somewhat similar nature. If the posterior end is split in two, the separation extending into the anterior part of the worm (Fig. 44, C\ each half completes itself, but the halves do not separate unless they happen to tear themselves apart. If one of the pieces is cut off, not too near the region of union with the other half, a new posterior end, replacing that cut off, regenerates. If, however, the piece is cut off quite near the region or union of the halves, the piece that is left may be absorbed.

The absorption of misplaced parts in the lower animals cannot be explained, I think, by any lack of nutrition, especially in the case of the tentacles of hydra. The result may be due either to the displaced part not receiving exactly those substances, perhaps food substances, that it gets in its normal position, or it may be due to some formative influence. At present we are not in a position to decide between these alternatives, and, while the former view seems more tangible, and the latter quite obscure, the latter may nevertheless be found to contain the true explanation. If the view that I have adopted in regard to the organization — namely, that it can be thought of as acting through a system of tensions peculiar to each kind of protoplasm — is correct, it may be possible to account for the absorption of misplaced parts by some such principle as this.