Principles of General Physiology

It might be supposed that such complex processes as those discussed in the present chapter would be impossible of attack on mathematical lines. There are, however, some encouraging results which require brief reference. Slator (1913) has shown that the growth of yeast can be expressed by a logarithmic formula. If a culture medium be inoculated with N cells of yeast per cubic centimetre, the rate of growth at a given moment of time is proportional to the number of cells present at that time, that is N + n, where n is the increase in number during the time which has elapsed since the inoculation. This is clearly a case of the " compound interest " law, which was explained on page 36 above, and the simplest assumption that can be made is that the increase in number is in direct linear proportion to N -f n, that is : —

It might be supposed, not unnaturally, that this simplest formula would be found to be insufficient, but Slator has shown by four different methods that it does actually express the results until nearly the end of the period of growth, at which time the food supply becomes restricted and the products of the reaction, metabolites, commence to inhibit. The four methods used were : — (1) counting the cells directly, (2) the rate of fermentation by measurement of the rate of formation of carbon dioxide, (3) the growth constant, K, is estimated by counting the number of cells and the rate of fermenmtion while the time is eliminated, and (4) by comparing the time taken for two cultures, inoculated in a known ratio, to arrive at some definite stage. In malt extract, with a small amount of hops, it was found that the time taken to double the number of cells was 2*9 hours (see also the work of Horace Brown, 1914).

Even in the metabolic processes of the higher animals, we have already seen reason to regard the operation of the law of mass action as being uninterfered with, more especially where we know the reaction to be reversible. An interesting instance of rate of reaction being proportional to the concentration of the substances reacting is to be found in a paper by Hoesslin and Lesser (1911, p. 356). If a fasting dog is fed with meat, the nitrogen excretion is 14 to 15 per cent, greater if it is given all at once, than if the same total amount is given in six separate doses, at intervals of three to four hours.

The Mendelian Laws of Heredity will be referred to presently. The fundamental processes of life in all organisms are, no doubt, similar, so that it appears to be held by certain investigators that, on account of the comparative simplicity of structure of the " lower " organisms, we are more likely to be able to discover what is the essential nature of these processes, if we devote our attention to the apparently simpler creatures. Without denying the great value of the comparative method in eliminating merely incidental phenomena, it must be pointed out that this very simplicity is, in the majority of cases, a disadvantage. The same organ, or even cell, fulfils a variety of purposes, which in the higher organisms are relegated to distinct groups of cells. Moreover, the size of the organism is of much importance, as will have been sufficiently obvious in the present chapter. The science of nutrition would be almost impossible without the larger, warm-blooded animals. The advantage of the increased rate of reactions, owing to the higher temperature, is not to be undervalued.

The physiology of unicellular organisms, although of considerable importance in special aspects, is not to be regarded as a "general physiology." Indeed, if the choice had to be made between the investigation of simple or complex organisms alone, there is no doubt that a much more general and fundamental body of doctrine would be obtained from the latter. The following remarks of Claude Bernard (1866, p. 100) may be read with interest : "II ne faudrait pas croire, en effet, que 1'animal inferieur est plus simple ou que ses fonctions sont moins compliquees ou moins nombreuses ; et qu'on pourrait les prendre pour ainsi dire a leur naissance, pour suivre ensuite leur developpement dans les animaux superieurs, qui auraient ainsi des proprietes nouvelles se surajoutant aux premieres. L'animal inferieur possede toutes les proprietes essentielles qu'on retrouve aux degres les plus eleves de 1'echelle des etres ; mais il les possede a 1'etat confus, et pour ainsi dire repandues dans toutes les parties du corps. Ainsi 1'infusoire, qui s'agite et se dirige dans le liquide ou il a pris naissance, possede evid«m- ment la propriete de se mouvoir ; il doit etre doue de sensibility pour determiner ses mouvements ; enfin, il peut se reproduire, puisque 1'espeee ne perit pas. Voila done la vie a son degre le plus infime, avec toutes les fonctions qu'elle manifeste chez les animaux eleves. Mais quand on cherche les organes de chacune de ces fonctions, on ne peut plus rien distinguer, et c'est & ce point de vue seulement qu'on doit parler de la pretendue simplicite des animaux inferieurs."

In the most primitive state it frequently happens that the whole cell contents of an organism divide into a number of smaller parts, each of which gives rise to a new organism. In such a process the new organisms are endowed only with the qualities of the one cell. Since the powers of adaptation of any two organisms or cells are not, as a rule, identical, it is clear that if the new organisms could "inherit" the characteristics of more than one it would be to its advantage. Accordingly, we find, very early in the course of evolution, arrangements by which two cells join their forces by fusion or conjugation. At first, the two cells are similar, as in Spirogyra, but almost at once we find a differentiation by which a large cell, called the female cell or gamete, is incapable of further development without fusion with another smaller, usually motile, cell, the male gamete. The great variety of arrangements by which this " fertilisation " is effected or facilitated are beyond the scope of this book and will be found in the textbooks of botany and zoology. The main point to be kept in mind is the incapability of either the male or female cell alone to grow to a new organism. Since it is the female cell which remains more or less stationary and is, as it were, sought out by the male cell, while the new organism grows from the fertilised female cell, the obvious effect of the male cell is to set into activity the dormant powers of segmentation and growth of the female cell. It is easy thus to lose sight of tlie fact that the male cell also brings with it the capacities of the organism from which it has arisen.

The mysterious power of the male has from the earliest times excited wonder and has, not unnaturally, become the object of religious worship. It is indeed greatly to be regretted that the sexual process should have become the subject of unseemly jesting. Of course, incidents of real humour may arise in any connection, "without detriment to its essential solemnity, as witness the great art of Shakespeare. But I feel compelled to state my belief that much mischief is done by the habit of looking upon anything related to sex as, in itself, a

matter for jesting, apart from any real humour. Possibly, the excessive secrecy and reticence maintained on the question are much to blame, and there is no doubt that the wider teaching of a proper physiology in schools will have a good effect in this direction. The almost universal ignorance of matters of the most vital importance to the community, as well as to the individual, is scarcely less than amazing. It is much to be hoped that in the future the sexual process will be looked upon as something essentially beautiful and good, in fact as /cuAos in the old Greek sense, if I may be allowed to use the word again in this connection. The reader will surely not need to be reminded that the love of man and woman has been the motive force of many of the greatest and noblest deeds in the world's history.

Owing to the very urgency of the impulse for the sake of the preservation of the race, charitable excuse may be made for those who offend ; but condemnation must be unsparingly given to those who tempt others to sin. For the reasons given above, I feel unable to agree with the view taken by Sir Thomas Browne (" Religio Medici," vol. i. p. 100 of Sayle's edition, 1904). After this apparent digression, for which I offer no apology, we may continue what may perhaps be regarded as the proper subject of our book. In reference to the, as yet, mysterious power of the male cell to excite the process of development in the female cell, the work of Loeb (1900) should be mentioned. This investigator showed that " artificial parthenogenesis " can be produced to a certain extent by treatment of the eggs of sea urchins in various ways.

The following is the most effective of these. The eggs are first placed for 1'5 to three minutes in a mixture of 50 c.c. of sea water with 2'8 c.c. of O'l molar butyric acid. They are then removed to 200 c.c. of sea water. Fertilisation membranes are formed, but nothing more happens. The next step after the eggs have remained for twenty minutes or more in the natural sea water is to remove them to hypertonic sea water, made by adding 8 c.c. of '2'.") molar sodium chloride to 50 c.c. of sea water. The actual time they require to remain in this solution can only be found by trial, so that samples are withdrawn every five minutes, after they have remained for fifteen minutes. This taking of samples is continued for sixty minutes. Those that have remained for the correct time develop into normal larvae on removal to natural sea water. Further details may be found in the book by Loeb (1909).

In rare instances, as the well-known one of the bee, where unfertilised eggs develop into drones, natural parthenogenesis is to be met with. It will be obvious, however, that the advantages of mixing the qualities of two individuals is absent in all these cases of parthenogenesis. What we learn from the experiments of Loeb is that it is only under a combination of chemical and physical influences, such as is very unlikely to occur in natural conditions, that the female cell, except in such rare cases as that of the bee, is able to develop without the co-operation of the male cell. In this way the advantage of sexual reproduction, the union of two individuals, is ensured. At the same time, we see that the female cell actually does possess the power of development apart from the entrance of the male cell.

The work of Przibram on "Embryogeny" (1908) may \te consulted for the laws governing development. The facts of heredity have, of recent years, become more or less amenable to scientific treatment, mainly by the work arising from that of Mendel, abbot of Briinn. This work was published in 1865, but did not become known until its discovery by De Vries in 1900. It is only possible in the limits of space permissible here to give but the merest outline of the fundamental facts. The reader is referred for further details to the book by Bateson (1913).

In order to be able to follow the process of inheritance from generation to generation, Mendel directed his attention to some single character ; in the Sweet Pea, for example, he took the quality of tallness and dwarfness. Suppose that a tall individual was crossed with a dwarf one, it was found that the next generation consisted entirely of tall individuals. The quality of tallness was called, therefore, "dominant," while that of dwarfness was called "recessive," since

it was found to be merely dormant, as it were, because it reappeared in the next following generation. The tall individuals of the first generation were allowed to fertilise amongst themselves only and, in the whole number of individuals produced, there was found to be one dwarf to every three tails. The remarkable fact is that the dwarfs are found to be of pure breed, that is, all of their posterity are dwarfs. Of the tails, one-third are pure breed, the other two-thirds will give the same mixture of one dwarf to three tails in the next generation and so on. We shall see presently how this proportionality is to be explained. For the present, we note that, although they may be the offspring of two

Fio. 78. PHOTOGRAPH OF SWEET PEA PLANTS.— Five tall and three dwarf, grown from the same pod from a cross between a tall and a dwarf variety. The dwarf plants on the left are pure breed, recessive, although one of the parents was tall. The tall character is the dominant one, and some of the tall individuals are mixed breed. The whole family corresponds to the stage F2 of Fig. 79. tall individuals, the dwarfs are pure breed (Fig. 78, from Bateson's Address, 1906, illustrates some of these facts).

Now, in the sexual cells, or gametes, male and female, the chromosomes, arising from the nucleus by mitotic division, are only half the number of those formed in the somatic or general body cells ; so that, when the sexual cells unite to form a zygote, the normal number is again attained. Taking, for simplicity, the characters of blackness and whiteness whose heredity is to be traced, the scheme of Fig. 79 will represent matters. It is to be remembered that blackness and whiteness do not always obey simple Mendelian laws, so that any character taken at haphazard cannot be used to trace the laws of heredity.

We have just seen that, compared with the germ cells, the body cells are double structures, so that, for the present purpose, we may represent a pure black individual by two black symbols, say rectangles, each rectangle representing a germ cell. A pure white individual is represented by two white rectangles. Further, when a black and white individual are crossed, the hybrid contains both characters, but, when it forms germ cells, the qualities are separated again, so that each germ is either black or white, not a mixture of the two, and an equal number of black and white cells are formed. When one of the characters is dominant, that is, when it is such as to overpower the manifestation of the other, which is recessive (in our case, let us call blackness, dominant, and whiteness, recessive), the result will be as in the diagram of Fig. 79 (Bateson, 1906). The gametes are represented by the single letters and rectangles, the zygotes by each pair of these. To show that blackness is dominant, in the zygotes the black rectangles are placed on the top of the white ones. The possible combinations are as shown and, since black is dominant, individuals composed of black and white will appear to be black and indistinguishable from those composed of all black. In this way, as can easily be seen, there will be three black to one white, or three dominants to one recessive. In reality, two of

the three dominants are impure. If one of these, DR, is crossed with RR, an equal number of blacks and whites will result (see the second diagram of Fig. 79) ; while, if DR is crossed with DD, all blacks will appear (third diagram). We see also white, recessive germ meets another white germ, the result is pure white, and blackness is thrown out from that family altogether. Similarly for the pure black germs. It follows that, when we know that a particular character is dominant and another is recessive, we can say with confidence that when a recessive D individual has once made its | D appearance, all its descendants will be recessive, assuming, of course, that it is not crossed

with a dominant individual. For example, the original Chinese primrose has a palmate leaf. About 1860 a vai'iety appeared with a pinnate leaf. The first is dominant, the second recessive, so that whenever the latter appears it continues to breed true. Naturally, the matter is not always quite so simple as this, even when we have found out which character is the dominant. There are interactions, in some cases, between the factors, and the manner in which, say, a colour is produced has to be taken into account. Colour is not necessarily a single character, and the factors which produce it are separately transmitted, giving rise to the variety of colours met with, especially in flowers. The complex inter-relationships, nevertheless, are doubtless capable of resolution.

Another interesting case is when the same character, as that of bearing horns in sheep, may be dominant in the one sex, recessive in the other. The case of horned and hornless sheep was investigated by T. B. Wood (1905). This very short account may serve to indicate the kind of problems that may be attacked from the Mendelian point of view. In certain unicellular animals, such as Euglena, we find chlorophyll grains, as if in a plant cell. Some species of multicellular animals, also, such as Hydra viridis, and the planarian worm, Convoluta Roscoffensis, take in algae and carry on a partnership in nutrition, as it were. Keeble (1910) shows that this Convoluta, after it has taken up the green algae, is able to live and grow in sea water which contains no solid particles, and only traces of organic matter of any kind. The algse are able to decompose carbon dioxide and form carbohydrate, as under normal conditions. It appears that the starch, thus formed, is changed into fatty substances by the vegetable cells and passed on to the animal cells in this form. Starch itself cannot be attacked by the cells of the animal. Rows of fatty particles are seen apparently passing from the green cells to the neighbouring animal cells. It is, of course, possible that sugar also may pass, as such, to the latter cells.

Without the green cells, Convoluta Roscoffensis fails to grow, and the same thing happens in darkness. At a certain stage of its existence, the organism ceases to take in solid food and depends entirely on its vegetable partners. Keeble has shown further (1910, p. 123) that the infecting algae are capable of independent existence. . An interesting question is why the infecting algae grow so rapidly as they do inside the animal organism. It is obvious that they must obtain nitrogen and it is very significant that Convoluta Roscoffensis and C. paradoxa, which also contains symbiotic algae, are peculiar amongst the Turbellarian worms in possessing no excretory system for the waste products of their nitrogenous metabolism. The conclusion is clear ; these waste products are utilised by the algae. In fact, it is actually found that these particular algee grow better when supplied with their nitrogen as uric acid than as nitrate. There is, moreover, evidence that, not only do the cells of the algae supply the animal cells with fat and carbohydrate, but also with nitrogenous food, which they are able to hand on after having converted that obtained from the sea water into a form with which animal cells can deal.

The reader is recommended to consult the fascinating little manual by Keeble (1910) for further information. The use of food in the growing organism is to supply material for construction of body substance, to replace that lost in wear and tear, and to give energy for the performance of muscular movements as well as for the bringing about of endothermic reactions. In the adult, of course, the necessity of a supply for growth is absent. The amount required for the replacement of wear and tear, or maintenance, is small.

In growth, it is obvious that all the chemical elements which are constituents of the organism must be supplied in some form or other, but, while very simple chemical compounds suffice for the lower organisms, the capability of dealing with such is, to a large extent, lost by the animal, even at a comparatively early stage of the protozoa. These require food in the form of complex organic compounds already prepared by other animals or plants. As carbon, nothing less complex than glucose ; as nitrogen, nothing less complex than amino-acids suffice for these animal organisms.

In addition to the known organic compounds, the presence of traces of some substances, whose constitution is as yet unknown, is necessary, not only for growth, but also for maintenance. It appears, however, that there are some of these which are absolutely indispensable for growth, but unnecessary for maintenance. These "accessory factors" do not act by forming part of the constitution of definite chemical compounds such as the proteins of the protoplasm, but as " hormones " or catalysts ; although the possibility of their forming some essential part of the cell mechanism, such as the surface membrane, has not yet been definitely excluded.

For the purpose of energy production, the supply of due amounts of carbon and hydrogen alone is, in theory, sufficient. Nitrogen is absolutely necessary merely for the purpose of replacing that which is lost from the structure of the machine in wear and tear. At the same time, there appear to be certain advantages in taking a larger proportion of nitrogen food, at all events, for most people. The protein content of most standard dietaries is certainly, however, unnecessarily high.

There are some organisms which have very special requirements as to food materials. As naturally follows from the facts given in preceding chapters, the presence of inorganic salts in food is essential. The method by which atmospheric nitrogen is made available for the food of plants and animals is described in the text. The stages are, briefly, bacteria of the soil, and in the root-nodules of leguminous plants, proteins of plants and animals, ammonia, nitrites, nitrates. The latter are again utilised by plants, part being lost as nitrogen gas, and if not used up, by a reverse process back to ammonia.

With regard to the " accessory factors," or hormones, it will be clear that, if a certain chemical grouping is required for a special purpose in the organism, such as an internal secretion, and if the organism is unable to synthesise it for itself, then it must be given in the diet. Such a substance is tryptophane for the growing rat. But there is something else needful. Rats will not grow on a diet of pure protein, fat, carbohydrate, and salts, even when containing all the known chemical constituents of food, including tryptophane. There is a substance present in a minute amount of milk, or boiled extracts of fresh vegetables or meat, which is absolutely necessary. Moreover, when these factors are present, aniin.iK are able to preserve their tissue nitrogen on pure amino-acids.

Certain diseases, such as beri-beri and scurvy, have been shown to be caused by the absence of similar "accessory factors " in diet. There is some evidence that even yeast and bacteria are dependent for growth on similar "accessory factors." The chemical constitution of proteins, as condensations of amino-acids of different kinds, is described briefly in the text. These substances, when taken as food, are first hydrolysed in the organism, by means of the digestive enzymes, to their constituent amino-acids and related substances. The greater part of these amino-acids, passing into the blood, is de-aminated, mainly, or perhaps exclusively, in the liver ; the resulting ammonia is converted to urea, again mainly, or perhaps exclusively, in the liver ; the hydro xy or ketonic fatty acids produced are burnt up for energy purposes. The small part of the amino-acids not de-aminated is used by the tissues for growth or for replacement of loss by wear and tear.

There are, then, two more or less distinct forms of protein metabolism, one for energy purposes, "exogenous," in which the nitrogen is lost, the other for replacement of loss or for growth, " endogenoits," in which the nitrogen is retained. The question of the minimum nitrogen requirement is discussed in the text. The amount absolutely necessary for a healthy man, doing the ordinary amount of work, has been reduced to 3-5 g. per day, equivalent to 21 g. of protein. The total energy value of the diet, expressed in heat units, must not be less than 4,000 calories, made up with carbohydrate and fat.

There is evidence that the presence of carbohydrate is essential for the synthesis of protein, both in the animal and in the plant. In the wear and tear of the protoplasmic mechanism only a certain part requires replacement, not the whole of a complex molecule. The importance of creatine and purine metabolism is pointed out. In muscular work, so long as it is not excessive, no evidence of increased nitrogen excretion, due to wear and tear, or otherwise, is to be obtained. It appears that a complex substance, containing nitrogen and of a high energy content, is broken down to give the energy of the contraction process. The nitrogenous constituent is normally used again for resynthesis of the " inogen," while the carbon and hydrogen are burnt up.