The A.B.-Z. of our own nutrition

by Horace Fletcher

In order to avoid repetition, and at the same time impart the utmost clearness to our representation, it may be useful to bring before your minds at once the plan of innervation of a given organ, all the more since this scheme is seldom completely followed out or adequately described in physiological text-books. Consequently, it is not borne in mind with sufficient precision by the majority of medical men.

A complete innervation mechanism consists of the peripheral endings of the centripetal (afferent) nerves, the centripetal nerves themselves, the nerve cells (a group of nerve cells connected with each other is termed a “nerve centre”), the centrifugal (efferent) nerves, and, lastly, their peripheral terminations. Physiology now accepts it as a settled fact, that nerve fibres serve only as conductors of nervous impulses, which come in from contiguous links of the nervous chain. Only the peripheral endings of nerves and the nerve cells themselves have the power of transforming the external stimulus[29] into a nervous impulse. In other words, in the intact organism these alone constitute the normal receiving apparatus of the nervous system. Whether the peripheral ends of centrifugal (efferent) nerves are likewise able to function as normal sites for the application of external stimuli has still to be answered. Consequently, when any external agency excites the peripheral terminations—the receiving stations—of centripetal nerves in this or that organ, the effect of the stimulus will be conveyed through the centripetal nerves, as through a receiving wire, to the central station—the nerve cells. Here it becomes changed into a definite impulse and now comes back along the centrifugal nerves—the outgoing wires.

The utmost importance is to be attached to the fact that only the peripheral endings of centripetal (afferent) nerves, in contrast to nerve fibres themselves, respond to specific stimuli; that is to say, are able to transform definite kinds of external stimuli into nervous impulses. The function of the end organs with which they are connected is therefore of a purposive nature; in other words, these organs are only called into play by certain definite conditions, and impart the idea of being aware of their purpose, of being conscious of their duty. We have long known that the peripheral endings of sensory nerves are possessed of a high degree of speciality, and cannot therefore have any doubt regarding the specific nature of the end organs of other centripetal nerves. This is a sore point in present-day physiology. But, notwithstanding our knowledge of the separate parts of the animal body, we shall only be able to form a true conception of the motive agencies of the whole complicated machine, when we have established the specific excitability of the end apparatus of every centripetal nerve, and have discovered all the mechanical, chemical, and other factors which throw this or that end apparatus into an active condition. I always look upon it as a period of scientific inadequacy so long as the effects of the most diverse external agencies upon any normal physiological process are admitted to be indistinguishable. As the work of the digestive canal is now represented in the majority of text-books, and consequently presented to the mind of the physician, it bears the impress of this period. To impart to the physician a more correct conception of this matter was my chief object in giving these lectures. I hope, indeed, to furnish you with evidence sufficiently convincing, that the alimentary canal is endowed not with mere general excitability; that is to say, does not respond to every conceivable form of agency, but only to special conditions which are different for the different portions of its length. Just as men and animals in the world are only able to maintain their existence and constantly adapt themselves to changing circumstances by aid of the peripheral endings of their sensory nerves, so every organ, indeed every cell of every organ, can only maintain its place in the animal microcosm, and adapt itself to the activity of innumerable associates, as well as to the general life of the whole, by virtue of the fact that the peripheral end apparatus of its centripetal nerves possesses a specific excitability.

The same applies to the nerve cells: obviously they are endowed with specific sensibility. Irrespective of the excitations which are communicated to them from centripetal nerves, they respond, as originators of nervous impulses, only or at least mainly to definite forms of mechanical, chemical, or other stimuli arising in the organism. This follows not alone from a number of physiological facts but also from various pharmacological data. Thus we learn that various drugs excite or annul the activity of definite portions of the nervous system, at least in the earlier phases of their effects. This specific excitability of nerve cells, just as much as the same property of peripheral end organs, lies at the bottom of the purposive action of these organs.

Hence, our next duty is to endeavour to discover the normal exciting conditions of the centripetal nerves belonging to the glands which we had under consideration in our last lecture, or, more correctly, to find out the conditions which excite the centres, as well as the peripheral endings of the different nerves, which form parts of the nervous apparatus of these glands. We have, therefore, for each phase of the work of secretion, to find out that portion of the nervous mechanism which is for the time being under excitation, and to discover the primary agency by which this condition is elicited. This would include an exact analysis of the stimulating influence which mastication and food exert upon the nervous mechanism of these glands. We shall also be able more fully to comprehend the inner mechanism underlying the facts which formed the subject of the second lecture. This, of course, is an ideal programme which we can only follow out as far as the present state of physiology permits. It may now be instructive, and, for our further conclusions, advantageous, to glance shortly at the nervous control of the salivary glands.

The salivary glands, whose innervation has long ago been investigated, have generally been accepted as types of the deeper-lying digestive glands, and when it became necessary to form a conception of the mode of activity of the latter, medical science resorted to a bold analogy and thought of the nervous apparatus of the salivary glands. But the attempts of investigators to apply rigidly to others the scheme of innervation which holds good for the salivary glands, have done considerable harm to the usefulness of the analogy and have prevented our arriving at a correct idea of the plan of innervation of the abdominal glands. We have already had an example of this nature before us. In the salivary glands we have no clearly marked indications of nervous inhibition, and this circumstance has decidedly retarded the due development of our knowledge of the nervous control of the abdominal glands. Authors naturally expected to see a simple and prompt stimulation-effect from the same conditions of experiment which sufficed for the salivary glands, and the failure of this gave them, as they thought, the right to deny the existence of any extrinsic nervous influence upon the abdominal glands. The error is now obvious; the abdominal glands behave in some ways different from the salivary glands, and for their successful investigation, other conditions of experiment are necessary than those which held good for the former. In the working of the abdominal glands nervous inhibitory processes play a large part, but they are almost wholly absent in the case of the salivary glands. This is an additional warning that one must never push the conclusions drawn from analogy too far, but must constantly bear in mind that the life-functions of all organs are extremely complicated, and that the work of even the most apparently similar organs should be submitted to separate and careful observation. To me it appears that the unjustified analogy drawn between the abdominal and salivary glands has to be credited with another important misapprehension. And precisely for this reason I think it desirable to bring under consideration, if only in brief fashion, the conditions of work of the salivary glands, especially since Dr. Glinski has instituted in the laboratory some easily performed experiments which bear upon the matter.

The experiences of daily life teach us from the outset, that the activity of the salivary glands begins even before the introduction of food into the mouth. With an empty stomach, the sight of food or even the thought of it is sufficient to set the salivary glands at once into activity; indeed, the well-known expression, “to make one’s mouth water,” is based upon this fact. Hence a psychic event, the eager longing for food, must be accepted as an undoubted excitant of the nervous centre for the salivary glands. On the other hand, the same every-day experience, as well as numerous experiments upon animals, teach us that a number of substances, when brought into contact with the mucous membrane of the mouth, are likewise able to call forth a secretion of saliva. One even acquires the impression that everything brought into the mouth may reflexly influence these glands, the only difference being a gradual shading off in the effect, dependent upon the strength of the stimulation which the substance introduced is able to exert, and it appears to me that it is precisely this impression which has driven the idea into the background, that the peripheral end apparatus of the centripetal nerves of the digestive canal are specifically excitable. The facts were here correctly observed, but their indications erroneously interpreted.

The great multiplicity of excitants of salivary secretion, has without doubt, some connection with the complicated physiological functions of the saliva. This is the first fluid encountered by everything which enters the alimentary canal. It must, therefore, in a sense play the part of host to every substance taken in—moisten the dry, dissolve the soluble, envelop the hard and bulky with mucus in order to facilitate its passage down the narrow œsophagus; and submit certain forms of food material, such as starch, to a process of chemical elaboration. Nor is its duty by any means ended here. The saliva is secreted in the first compartment of the alimentary canal, which is at the same time the sorting-room of the organism. Much of what enters the mouth may prove in the testing process to be useless, or even noxious, and must either have its deleterious properties neutralised or be completely rejected. The saliva is secreted in the first instance to obviate injurious effects in some way; thus, for example, a strong acid is to a certain degree neutralised, while other corroding substances may be simply diluted, and by mere reduction of concentration have their harmfulness diminished.

In the second place, when the injurious substances have to be wholly removed, the saliva plays the rôle of a washing-out fluid; otherwise the material, by clinging to the mucous membrane of the mouth, might in longer or shorter time gain entry into the blood and there develop its noxious influence. This last function of the fluid is hardly taken into account at all in physiology, and yet it is evident that the saliva, as a cleansing fluid, must have a wide importance. If you only think of how often we are impelled to expectorate, that is, to wash out the mouth with saliva after something unpleasant, this will be clear. Such a view finds additional support when we reflect that a feeling of disgust produces almost as strong a flow of saliva as the sight of a tasty meal. In both cases the secretion performs the office of forerunner: in the first it prepares for the washing out of the mouth, in the second for the requisite elaboration of the food. Think how often, when something disagreeable enters the mouth, with what rapidity the saliva is poured out, even after the unpleasant substance has been for a considerable time removed, and not a trace more is apparent to the sense of taste. Indeed, long afterwards one has only to recall the circumstances to mind in order to bring on anew the secretion of saliva. Apparently the psychic excitation of the nerves of salivary secretion also ushers in the act of vomiting, which, as is well known, can be called forth by mental influence. Further, the function of the saliva just mentioned is probably the true physiological explanation of the feeling of disgust which many persons experience at the sight of the secretion itself.

Hence I hold that substances which obtain entry to the mouth set up a secretion of saliva only because we have here the seat of a definite physiological sense, and not because the peripheral terminations of the buccal nerves are devoid of specific excitability, and capable of being thrown into action by every desired form of stimulus. In other words, the specific excitability of the peripheral endings of the salivary nerves is very comprehensive and widely extended. This is no picture of the imagination, for it can be supported by facts. To say nothing of the testimony of earlier authors, that the salivary glands have each particular exciting agencies to which they specially respond, we can demonstrate the following facts from the material collected in our laboratory.