DEATH AND DISSOLUTION OF THE ORGANISM
1. It is an old saying that we cannot understand life unless we understand death. The dead body, if its temperature is not too low and if it contains enough water, undergoes rapid disintegration. It was natural to argue that life is that which resists this tendency to disintegration. The older observers thought that the forces of nature determined the decay, while the vital force resisted it. This idea found its tersest expression in the definition of Bichat, that “life is the sum total of the forces which resist death.” Science is not the field of definitions, but of prediction and control. The problem is: first, how does it happen that as soon as respiration has ceased only for a few minutes the human body is dead, that is to say, will commence to undergo disintegration, and second, what protects the body against this decay while the respiration goes on, although temperature and moisture are such as to favour decay?
The earlier biologists had already raised the question why it was that the stomach and intestine did not digest themselves. The hydrochloric acid and the pepsin in the stomach and the trypsin in the intestine digest proteins taken in in the form of food; why do they not digest the proteins of the cells of the stomach and the intestine? They will promptly digest the stomach as soon as the individual is dead, but not during life. A self-digestion may also be caused if the arteries of the stomach are ligatured. Claude Bernard and others suggested that the layer of mucus protected the cells of the stomach and of the intestine from the digestive enzymes; or that the epithelial layer had a protective effect. Pavy suggested that the alkali of the blood had a protective action. All these theories became untenable when Fermi showed that all kinds of living organisms, protozoans, worms, arthropods, are not digested in solutions of trypsin as long as they are alive, while they are promptly digested in the same solution when dead.[294] This is in harmony with the fact that many parasites live in the intestine without being digested as long as they are alive. Fermi concluded that the living cell cannot be attacked by the digestive ferments, while with death a change occurs by which they can be attacked. But what is this change? Fermi seems to be inclined to think that the “living molecule” of protein is not hydrolysable (perhaps because the enzyme cannot attach itself to it?), while a change in the constitution or configuration of the proteins takes place after respiration has ceased. The fact that the living cell resists the digestive action of trypsin and pepsin has found two other modes of explanation, first, that the cells are surrounded by a membrane or envelope through which the enzyme cannot diffuse, and second, that the living cells possess antiferments. But the so-called antiferments are also said to exist after the death of the cell, whereas after death the cell is promptly digested. Frédéricq, as well as Klug, has shown that worms which are not attacked by trypsin are digested by this enzyme when they are cut into small pieces; although the pieces of course contain the antienzyme. The other suggestion that a membrane impermeable for trypsin protects the cells would explain why living protozoa are not digested by trypsin, but it leaves another fact unexplained, namely, the autodigestion of all the cells after death by enzymes contained in the cells themselves.
2. The disintegration of the body after death is not caused exclusively or even chiefly by the digestive enzymes of the intestinal tract or the micro-organisms entering the dead body from the outside, but by the enzymes contained in the cells themselves. This phenomenon of autolysis[295] was first characterized by Hoppe-Seyler.[296]
All organs suffering death within the organism, in the absence of oxygen, undergo softening and dissolution in a manner resembling that of putrefaction. In the course of that process, albuminous matter gives rise to leucin and tyrosin, fat to free acids and soaps. This maceration, identical with the pathological conception of softening, is accomplished without giving rise to ill odour and is a process similar to the one resulting from the action of water, acids, and digestive enzymes.
In work of this kind, rigid asepsis is required to exclude the possibility of bacterial infection and this was first done by Salkowski, who showed that in aseptically kept tissues like liver and muscle the amount of substances that can be extracted with hot water increases considerably. By the work of others, especially Martin Jacoby and Levene, it was established that the power of self-digestion is shared by all organs. Analysis of the products of the autodigestion of tissues shows that, e. g., the amino acids, which constitute the proteins, are produced. Dakin, Jones, and Levene demonstrated the hydrolytic products of the nucleins, in the case of the self-digestion of tissues.[297]
Again the question arises: Why do the tissues not undergo autolysis during lifetime and what protects them, and the answer is that self-digestion is a consequence of the lack of oxidations. The presence of antiferments must continue after death and cannot be the cause which prevents the self-digestion during life, since nothing indicates the destruction of the hypothetical antidigestive enzymes through lack of oxygen. The recent work of Bradley and Morse[298] and of Bradley[299] has thrown some light on the problem. These authors found that proteins of the liver which are indigestible can be made digestible by the liver enzymes if an acid salt or a trace of acid is added to the mixture. A m/200 HCl solution gives marked acceleration of the autodigestion of the liver. This would explain why autodigestion takes place after oxidations cease. In many if not all the cells, acids are constantly formed during lifetime, e. g., lactic acid, which through oxidation are turned to CO2, and this diffuses into the blood so that the H ion concentration in the cells does not rise materially. If, however, the oxidations cease, as is the case after death, the formation of lactic acid continues, but the acid is not oxidized to CO2 and thus removed, and as a consequence the H ion concentration increases in the cells and the self-digestion of proteins, which the digestive enzymes contained in the cells themselves could not attack formerly, becomes possible. Acid increases the digestibility of a protein, probably by salt formation. Theoretically we should not be surprised that while in the liver an increase in the CH favours autolysis in other tissues the same result is produced by the reverse effect. We might say that the preservation of a certain CH probably at or near the point of neutrality during life prevents self-digestion, while the gross alteration of the CH in either direction after death (or after the cessation of oxidations in the tissues) induces autolysis. Bradley indeed suggests that many of the phenomena of autolysis during lifetime, such as atrophy, necrosis, involution, might be due to an increase in the CH in the tissues.
These facts agree with the suggestion of Fermi that in the living cell the proteins cannot be attacked by the digestive enzymes but relieves us of the necessity of making the monstrous assumption of a “living molecule” of proteins as distinct from a “dead” molecule. The difference between life and death is not one between living and dead molecules, but more likely between the excess of synthetic over hydrolytic processes.
In the second chapter we mentioned the interesting idea of Armstrong that when a synthesis is brought about by a digestive enzyme (e. g., maltase) not the original substrate is formed (e. g., maltose) but an isomer, in this case isomaltose; and this isomer is not attacked by the enzyme maltase. We thus get a rational understanding of the statement which Claude Bernard used to make but which remained at his time mysterious: la vie, c’est la création. During life, when nutritive material is abundant, through the reversible action of certain enzymes, synthetic compounds are formed from the building stones furnished by the blood. These synthetic isomers cannot be hydrolyzed by the enzymes by which they are formed and hence on account of the isomeric structure are immune against destruction. It is not impossible that the increase of the concentration of acid in the cells after death transforms the isomers into that form in which they can be digested by the enzymes contained in the cell. Another possibility is that the increase in digestibility brought about by an increase in CH in the cell is due to the hydrating effect of acids on proteins with a subsequent increase in digestibility. Whatever the answer may be, the work done since Claude Bernard has removed that cloud of obscurity which in his days surrounded the prevalence of synthetic action in the living and of disintegration in the dead tissues.
3. We have already referred to the connection between the lack of oxygen and the onset of autolysis and disintegration of tissues in the body. It is of interest that there are cells in which the disintegration under the influence of lack of oxygen is so rapid that it can be followed under the microscope. The writer has observed that certain cells undergo complete irreversible dissolution in a very short time under the influence of lack of oxygen, e. g., the first segmentation cells of the egg of a teleost fish Ctenolabrus.[300]
| Fig. 48 | Fig. 49 | |
| Fig. 50 | Fig. 51 |
When these eggs are deprived of oxygen at the time they reach the eight- or sixteen-cell stage, it can be noticed that the membranes of the blastomeres are transformed into small droplets within half an hour or more, according to the temperature. These droplets begin to flow together, forming larger drops. [Figures 48 to 51 show the successive stages of this process.] When the eggs are exposed to the air in time, segmentation can begin again; but if a slightly longer time is allowed to elapse, the process becomes irreversible and life becomes extinct. Such clear structural changes cannot often be observed in the eggs of other animals under the same conditions. Are these changes of structure (apparently liquefaction of solid elements) responsible for death under such conditions? In order to obtain an answer to this question, the writer investigated the effect of the lack of oxygen upon the heart-beat of the embryo of the same fish Ctenolabrus. This egg is perfectly transparent and the heart-beat can easily be watched. When these eggs are put into an Engelmann gas chamber and a current of pure hydrogen is sent through, the heart may cease to beat in fifteen or twenty minutes; it stops beating suddenly, before the number of heart-beats has diminished noticeably, and ceases beating before all the free oxygen can have had time to diffuse from the egg. In one case the heart beat ninety times per minute before the hydrogen was sent through; four minutes after the current of hydrogen had passed through the gas chamber, the rate of the heart-beat was eighty-seven per minute, three minutes later it was seventy-seven, and then the beats stopped suddenly. It is hard to believe that this cessation could have been caused by lack of energy. Hydrolytic processes alone could furnish sufficient energy to maintain the heart-beat for some time, even if all the oxygen had been used up. The suddenness of the standstill at a time when the rate had hardly diminished seems to be more easily explained by a sudden collapse of the machine; it might be that liquefaction or some other change of structure occurs in the heart or its ganglion cells, comparable to that which we mentioned before. In another fish Fundulus, where the cleavage cells undergo no visible changes in the case of lack of oxygen, the heart of the embryo can continue to beat for about twelve hours in a current of hydrogen. In this case the rate of the heart-beat sinks during the first hour in the hydrogen current from about one hundred to twenty or ten per minute; then it continues to beat at this rate for ten hours or more. In this case one might believe that during the period of steady diminution of the tension of oxygen in the heart (during the first hour), the heart-beat sinks steadily while it keeps up at a low but steady rate as long as the energy for the beat is supplied solely by hydrolytic processes; but there is certainly no change in the physical structure of the cells noticeable in Fundulus, and consequently there is no sudden standstill of the heart.
Budgett has observed that in many infusorians visible changes of structure occur in the case of lack of oxygen[301]; as a rule the membrane of the infusorian bursts or breaks at one point, whereby the liquid contents flow out. Hardesty and the writer found that Paramœcium becomes more strongly vacuolized when deprived of oxygen, and at last bursts. Amœbæ likewise become vacuolized and burst under these conditions. Budgett found that a number of poisons, such as potassium cyanide, morphine, quinine, antipyrine, nicotine, and atropine, produce structural changes of the same character as those described for lack of oxygen. As far as KCN is concerned, Schoenbein had already observed that it retards the oxidation in the tissues, and Claude Bernard and Geppert confirmed this observation. For the alkaloids, W. S. Young has shown that they are capable of retarding certain processes of autoxidation. This accounts for the fact that the above-mentioned poisons produce changes similar to those observed in the case of lack of oxygen.[302]
The phenomenon of rapid disintegration when deprived of oxygen (or in the presence of KCN) seems to be general as Child[303] has shown in extensive experiments. Child has used it to show that younger animals disintegrate more rapidly than older or larger ones, and he uses this fact for a theory of senescence. He connects the more rapid disintegration of the young animal with a greater metabolism.[304] Without wishing to doubt Child’s interesting observations the writer is not quite certain whether the more rapid disintegration of the younger forms is not a result of the fact that the walls of membranes in the young are softer than those of the older animals, and hence are more readily liquefied. Such a difference could be due to mere chemical constitution, e. g., the increase in Ca in the membrane with the increase in age. In old age in man the deposit of Ca in the blood-vessels is a frequent occurrence.
These facts may help us to understand the nature of death and dissolution of the body in higher animals. Death in these animals is due to cessation of oxidations, but the surprising fact is that if the oxidations have been interrupted but a few minutes life cannot be restored even by artificial respiration. This suggests that the respiratory ganglia in the medulla oblongata suffer an irreparable injury or an irreversible change (comparable to that just described in the cells of Ctenolabrus) even when deprived of oxygen for only a short time. As a consequence of the irreversible injury to the medulla the respirations cease permanently, the heart-beat must also cease, and gradually the different tissues must undergo the dissolution characteristic of death. While all the cells may be immortal they are only so in the presence of oxygen and the nutritive solution which the circulating blood furnishes. With the proper supply of oxygen cut off they can no longer live.
4. It is an unquestionable fact that each form has a quite definite duration of life. Unicellular organisms are immortal; but for the higher organisms with sexual reproduction the duration of life is almost as characteristic as any morphological peculiarity of a species. No species can exist unless the natural life of its individuals outlasts the period of sexual maturity; and unless the average duration of life is long enough to allow as many offspring to be brought into the world as will compensate for loss by death. The male bee dies before it is a year old, while the queen may live several years. In a certain species of butterflies, the Psychidæ, the parthenogenetic female lays its eggs while still in the cocoon and then dies without ever leaving the cocoon. The imago of the ephemera leaves the water in the evening, copulates, lets its eggs fall into the water, and is dead the next morning. The imperfect condition of their mandibles and alimentary canal makes them unfit for a long duration of life. The males of the rotifers which are devoid of organs of digestion live but a few days.
In the Zoölogical Station at Naples in 1906, an actinian, Actinia equina, was alive after having been in captivity fifteen years, and another one, Cerianthus, had been observed for twenty-four years. Korschelt kept earthworms for as long as ten years. The fresh-water mussel may reach the age of sixty years or more and crayfish may live for over twenty years. The differences in the duration of life of mammals are too well known to need discussion. If the cells and tissues are immortal, how does it happen that the duration of life is so characteristic for each species?
Metchnikoff[305] has recently investigated the cause of “natural” death in the butterfly of the silkworm. The butterfly in this species lacks the organs necessary for taking up food, like the male rotifer or the ephemeridæ and hence is already, by this fact, condemned to a short life. Metchnikoff observed that these butterflies could live twenty-three days, but the average duration of life was 15.6 for the males and 16.6 days for the females; and that seventy-five per cent. of them contained no parasitic fauna or flora in their intestine. They lose considerably in weight during their lives, but the males still contain the fat body at the time of death. None of the changes accompanying “old age” in man are found in the tissues of these butterflies before death. Metchnikoff is inclined to believe that the animal is poisoned by some excretion retained in the body; namely, the urine, and that this poison also causes the symptoms of weakness which characterize the animal. He could prove the toxic character of their urine on other animals. This combined with starvation could sufficiently account for the short duration of life. The facts of the case show that it is due to an imperfection in the construction of the organism such as one would expect to find more or less in each animal if one discards the idea of purposefulness and divine wisdom in nature. Only a slight, perhaps an infinitesimal, fraction, of those species which are theoretically possible and which at one time or another arise can survive. Those which are durable show all transitions from the grossest disharmonies to an apparent lack of such shortcomings.
5. Minot had tried to prove that the death of metazoa is due to the greater differentiation and specialization of their tissues. Admitting the immortality of the unicellular organisms he argues that death is the price metazoa pay for the higher differentiation of their cells. This is of course purely metaphorical, but we may put it into a form in which it is capable of discussion in physicochemical terms, by assuming that death is a necessary stage in the development of a species. We are inclined, however, to follow Metchnikoff and suspect some poison accidentally or unavoidably formed in the body or some structural shortcoming as the cause of “natural” death.
An unusually favourable object for the study of natural death is the animal egg. The egg of the starfish Asterias forbesii when taken out of the body is usually immature, but in the spawning season it ripens in sea water. The writer[306] observed that eggs which ripen disintegrate very rapidly when not fertilized. This disintegration may be due to a process of autolysis, which sets in only after the egg has extruded the two polar bodies. The writer found that by preventing the maturation of the egg either by withdrawing the oxygen or by replacing the alkaline sea water by a neutral solution or by exposing the eggs for some time to acidulated sea water, the disintegration could also be prevented.
Further experiments showed that even in the mature egg rapid disintegration could be prevented by lack of oxygen, and similar results were obtained by Mathews. When the egg is fertilized it does not disintegrate in the presence of oxygen but it gradually dies in the absence of oxygen. One is almost tempted to say that while the fertilized egg is a strict aërobe the mature unfertilized egg is an anaërobe. This latter statement, however, becomes doubtful since the presence of oxygen may help the disintegration only indirectly by allowing certain changes to go on in the egg. The important points for us are that duration of life in the mature unfertilized egg is comparatively short and that the entrance of a spermatozoön or the process of artificial parthenogenesis saves the life of the egg. Loeb and Lewis found that the life of the unfertilized sea-urchin egg (which is usually mature when removed from the ovaries) can also be prolonged when its oxidations are suppressed. The decay of the unfertilized egg seems to be due to the fact that those alterations in the cortical layer which underlie the membrane formation and which are responsible for the starting of development gradually take place. In such a condition the egg will die quickly unless deprived of oxygen. This view is supported by the observation of Wasteneys that unfertilized eggs of Arbacia show an increased rate of oxidations when allowed to remain for some time in sea water; we have seen in Chapter V that such an increase also accompanies artificial membrane formation.
6. If the limited duration of life of an organism is determined by one or more definite harmful chemical processes, we should expect to find a temperature coefficient for the duration of life or at least be able to show that if all other conditions are the same the duration of life is for a given organism a function of temperature. The writer[307] investigated the duration of life of fertilized and unfertilized eggs of Strongylocentrotus purpuratus for the upper temperature limits.
TABLE XX
| Temperature | Duration of life of the eggs of S. purpuratus | |||
|---|---|---|---|---|
| Unfertilized | Fertilized | |||
| °C. | Minutes | Minutes | ||
| 32 |  | > 11⁄6 | 11⁄2 | |
| < 2 | ||||
| 31 | | > 21⁄4 | ||
| < 3 | ||||
| 30 | | > 3 | | > 4 |
| < 5 | < 5 | |||
| 29 | | > 6 | ||
| < 7 | ||||
| 28 | | > 8 | | > 11 |
| < 10 | < 13 | |||
| 27 | about 18 | | > 20 | |
| < 22 | ||||
| 26 | | > 35 | | > 35 |
| < 40 | < 40 | |||
| 25 | | > 76 | ||
| < 81 | ||||
| 24 | | > 168 | | > 192 |
| < 200 | < 209 | |||
| Hours | ||||
| 22 | 101⁄5 | |||
| 21 | 24 | |||
| 20 | 72 | |||
These observations show a very high temperature coefficient near the upper temperature limit, and this may account at least partly for the fact that in tropical seas the pelagic fauna is so much more limited than in polar seas.[308] It is quite probable that the high temperature coefficients at the utmost limits are only an expression of the coagulation time of certain proteins.
P. and N. Rau state that in the cold certain butterflies live longer, and similar statements exist for the silkworm, but these statements are not based on exact experiments, which are difficult. Dr. Northrop and the writer have started experiments on the influence of temperature on the duration of life of the fly Drosophila. Newly hatched flies were kept first without food except water and air at 34°, 28°, 24°, 19°, 14°, and 10°, and second with cane sugar. The average duration of life was as follows:
| With water | days | With cane sugar | days | |
| 34° | ....... | 2.1 | .......... | 6.2 |
| 28° | ....... | 2.4 | .......... | 7.2 |
| 24° | ....... | 2.4 | .......... | 9.4 |
| 19° | ....... | 4.1 | .......... | 12.3 |
| 14° | ....... | 8.3 | ||
| 10° | ....... | 11.9 |
These experiments show that there is a definite temperature coefficient for the duration of life and that this coefficient is of the order of magnitude of that of a chemical reaction. We are continuing these experiments with animals in the presence of food. It should, however, be remembered that the fly carries with it a good deal of reserve material from the larval period. We have carried on simultaneously determinations of the temperature coefficients of the duration of the larval and pupa stage of these organisms at the same temperatures and found ratios similar to those given above for the duration of life with water only.
7. Metchnikoff[309] has furnished the scientific facts for our understanding of senescence. He has demonstrated that the changes in tissue which give rise to phenomena of senility are due to the action of phagocytes. Thus the ganglion cells are altered (digested?) and destroyed by “neuronophags” and this is the main cause of mental senility. Definite phagocytic cells, the osteoclasts, slowly dissolve the bones (by the excretion of an acid?) and this leads to the known fragility of the bones in old age. The whiteness of the hair is due to the action of phagocytes; in the muscles in old age the contractile elements are destroyed by the sarcoplasm, and so on. It agrees with these facts that where organs are absorbed in the embryonic development of an animal, as e. g., the tail of the tadpole in metamorphosis, the phenomenon is due to a process of phagocytosis (and autolysis). We have mentioned the fact that in the larva of the Amblystoma the absorption of the gills and of the tail occurs simultaneously and that both must be caused by a constituent of the blood. Such a constituent may be responsible for phagocytosis and autolysis in the organs undergoing absorption. Metchnikoff calls attention to the fact that certain infectious diseases, e. g., syphilis, may bring about precocious senility; and he mentions also the senile appearance of young cretins which is due to the diseased thyroid. “It is no mere analogy to suppose that human senescence is the result of a slow but chronic poisoning of the organism.” He assumes that in man this poisoning is caused by the products of fermentation in the large intestine and that the micro-organisms responsible for these fermentations may therefore be regarded as the real cause of senility in man. Parrots which are long-lived birds have a limited flora of microbes in their intestine, while cows and horses which are short-lived in comparison with man possess an extraordinary richness of the intestinal flora. But, needless to say, it is not the quantity of microbes alone which is to be considered, the nature of the microbes is of much greater importance.
Certain plants like the Californian Sequoia gigantea may be considered as practically immortal since they live several thousands of years; other plants, the annuals, die after fructification. Metchnikoff quotes from a letter by de Vries that this author prolonged the life of Œnotheras by cutting the flowers before fertilization.
Under ordinary conditions the stem dies after producing from forty to fifty flowers, but if cutting be practised new flowers are produced until the winter cold intervenes. By cutting the stem sufficiently early the plants are induced to develop new buds at the base and these buds survive winter and resume growth in the following spring.
Metchnikoff suggests that it is a poison formed in the plant (in connection with fructification?) which kills the annuals, while it is not formed or is less harmful in the perennials. He compares the situation to the death of the lactic acid bacilli if the lactic acid is allowed to accumulate. This hypothesis is certainly worthy of consideration, and it is quite possible that in addition to structural shortcomings poisons formed by certain organs of the body as well as poisons formed by bacteria account for the phenomenon of death in metazoa.