Pavlov’s two curves for the amount of gastric secretion after a meal of 100 grams of meat. Lectures on the Work of the Main Digestive Glands LRGPZ, 43; LWMDG, 22–23)
Pavlov’s Lectures on the Work of the Main Digestive Glands (1897) was a masterful, conceptually and rhetorically powerful synthesis of six years of research. He promised the reader “a single idea, increasingly embodied in tenable and harmoniously linked experiments”—and provided just that.
The book practically wrote itself. Originating as a series of lectures to a medical audience at the IEM in 1895, it was then refined through presentations at the Military-Medical Academy. The form and content of Lectures owed much to its origins as a lecture series for an audience of nonspecialists, primarily physicians, rather than as a physiological treatise. Pavlov’s voice is lively, informal, and authoritative as he draws upon his specialized knowledge to explain both the fundamentals of digestive physiology and, using his research as exemplar, the proper relationship between lab and clinic. In his lectures at the Academy, only a few students in the front rows could actually see the results of experiments, so Pavlov chose a student to “witness” them for the rest of the class. The reader of Lectures is also dependent on a witness—Pavlov himself, who selects data from his vast “storehouse of information,” asks the reader to believe these are typical, and presents them according to the lab’s “single idea.”
The six central lectures on the functioning of the digestive glands are framed by two others, Lectures 1 and 8, which address the nature of physiology and its relationship to daily life and medical practice. In Lecture 1, “A General Overview of the Subject—Methodology,” Pavlov introduces the factory metaphor, explains the cardinal importance of methodology, explicates the unique advantages of physiological surgery and the chronic experiment, describes his dog technologies, and presents his own physiology division as a model research institution.
He synthesizes the lab’s findings confidently in the next six lectures. Introducing in Lecture 2 the characteristic secretory curves for gastric and pancreatic secretion as the empirical embodiment of the “laboratory view,” he then devotes two lectures to the nervous apparatus governing the precise, purposive work of the glands, another to the role of the psyche, and two to miscellaneous issues.
The text, then, moves confidently from the general to the specific—from a discussion of methodology and Pavlov’s single idea to the characteristic secretory curves and the psychic and nervous mechanisms that generate them, and then outward to the relationship of physiology to medicine and daily life.
The unifying “single idea” was provided by the factory metaphor,1 As he had when introducing it in 1894, Pavlov carefully preserves Mendeleev’s distinction between a manufactory—for Pavlov, the glands; and a factory—the digestive canal (and, with its outlying parts, the digestive system as a whole). Only in the factory is the food matter actually transformed by “mainly chemical” processes. He amplifies his discussion of 1894 by reference to kustarnyi lad, a common arrangement between Russian factories and cottage industries by which the former ordered precisely what they needed from the latter. For Pavlov, that is precisely the relationship between the digestive canal and the glands. Both the gastric glands (“built into the very walls of the factory”) and the pancreatic gland (one of the “more distant specialized organs, great chemical manufactories, which communicate with the factory through tubes that transmit reagents”) respond to specific, timely orders from the digestive tract, which thereby acquires precisely what it needs to process various foods.2 As in an ideal factory, the work of the digestive system is purposive (or adaptive) and therefore characteristic (or, to use one of his favorite words, “stereotypical”) for each food, responding precisely and subtly to the requirements for optimal digestion.3
Pavlov draws constantly upon experiments conducted by his coworkers to provide a detailed and unified picture of the coordinated work of this digestive factory.4 Food first encounters the salivary glands, which supply reagents that prepare its descent through the digestive canal. Along the way, it is processed by ferments supplied by the gastric and pancreatic glands.
This process is coordinated exclusively by the nervous system. First, appetite is aroused by the act of eating. The “passionate desire for food and the sensation of satisfaction, pleasure, derived from eating” is “the first and strongest stimulant of the secretory nerves of the gastric glands.” This initial excitation is relayed through the vagus nerves to the gastric glands, where it generates a “psychic secretion.” The resultant “appetite juice” flows rapidly and has great proteolytic power. The second phase of gastric digestion begins five or ten minutes later, when the food excites the specialized nerves in the mucous membrane of the stomach, eliciting a “chemical secretion” of the optimal amount and ferment content for digestion of the ingested substance. The work of the glands is well coordinated—the moisture produced by the salivary glands excites the gastric glands, and the hydrochloric acid produced by the gastric glands excites the pancreas. As food proceeds from the stomach into the duodenum, it stimulates specialized nervous exciters in the mucous membrane, eliciting another precise secretory response. This system produces exactly the amount and quality of secretions necessary to process any food—it is as if, as in any well-run factory, the glands “possess a mind.”5
This purposive “mind” resides not in the psyche but rather in the specific excitability of the nervous system. That the sensitivity and precision of the digestive factory could be the result of any other mechanism was, for the instinctive nervist Pavlov, inconceivable. “The basis of the adaptive capacity of the glands must of course first be sought in the nervous relations of these organs. One must turn to any other explanation only in the event of the complete failure of this one.” Nowhere in Lectures does he find this necessary.6
The psyche is here a complex actor—pravil’nyi in its general outlines, but capricious in any individual experiment. On the one hand, Pavlov’s lab revealed it to be a constant, objectively measurable participant in the digestive process, producing a predictable, characteristic curve of ferment-rich “psychic juice” in response to meat, bread, and milk. But in any specific trial, the activity of this “first and strongest” exciter of secretion depended upon the personality, food tastes, and mood of the dog.
Pavlov introduces the psyche by relating the results of sham-feeding experiments. If these are properly conducted, the act of eating elicits a strong response from the gastric glands. Furthermore, one can observe the relationship between the “greediness” with which a dog devours a meal and the quantity and quality of this secretory response. The “psychic moment,” then, has “acquired a physiological character, that is, it has become compulsory, repeating itself without fail under defined conditions, like any fully investigated physiological phenomenon.” If one regards this phenomenon from “a purely physiological point of view, one can say that it is a complex reflex.” Behaving predictably in experiments, the psychic moment, commonly called appetite, “takes form as scientific flesh and blood, transformed from a subjective sensation into a precise laboratory fact.” 7
Acquisition of that laboratory fact, however, depended upon proper experimental technique, including the effective recognition and management of the dog’s character and preferences. For example, most dogs prefer meat to bread and some exhibit a marked preference for raw over boiled meat, but “sometimes one encounters dogs who incline more, with better appetite, to bread rather than meat, and in such dogs, contrary to the rule, sham-feeding with bread elicits more and stronger juice.” A dog that has eaten within fifteen to twenty hours will perhaps react with a psychic secretion only to its favorite food, but once “prepared” by a two- or three-day fast, it will respond to any food with “a copious secretion of gastric juice.” The results of teasing experiments (and, by extension, the action of the psyche in normal feeding) depend on “the degree of the desire to eat, and this depends on how much and how long ago the dog has eaten, and with what it is teased: is the food really interesting to it, or does it regard it with indifference?”
One must, in sum, understand and handle the experimental animal as if it were a person:
It is known that dogs have no less varied tastes than do people.... Among dogs, too, one encounters more positive and cold-blooded types, who are not in the habit of being teased by a dream, by that which is distant from their mouth. Consequently, the experiment requires greedier dogs and dreamier animals.... An extraordinarily important moment with which one must contend in these experiments is the cunning and touchiness of the animals. We rather frequently come upon dogs who quickly notice that they are being teased with food and become angry, stubbornly turning away from that which you are doing before them. It is always better, therefore, to conduct an experiment with teasing as if you are not even thinking of teasing the animal, but are simply preparing its food.8
The intimate involvement of the psyche in the digestive process challenged the experimenter’s observational powers and manipulative skills. The lack of vigilant management easily contaminated experiments on all aspects of glandular work. “When one is long occupied with the work of the gastric glands under various conditions, one becomes convinced of the great danger posed to all experiments by the psychic secretion of juice. You must constantly, so to speak, conduct a struggle with this factor, constantly reckon with it.”
As presented in Lectures, then, the psyche is both a central actor in digestion and a source of authority for Pavlov’s conclusions. In other labs, its capriciousness led investigators astray, but in Pavlov’s it was expertly managed, acquiring “a physiological character” and “repeating itself without fail under defined conditions, like any fully investigated physiological phenomenon.”9
What evidence did Pavlov provide that this digestive factory worked in a truly purposive manner? He introduces this notion in Lecture 1 as a commonsensical truth: “Upon reflection, one must a priori recognize that each food, that is, each mixture of substances subject to processing, should encounter its own combination of reagents and their properties.”10 Yet he knew from his experience at the Society of Russian Physicians that it was not self-evident to physicians that the glands produced the optimal juice for each food. Indeed, a number of tensions existed between this view and his experimental data. How could this presumed purposiveness be reconciled with the variations in psychic secretion from dog to dog and from day to day, or with the fact that most foodstuffs failed to excite nervous-chemical secretion when placed directly in the stomach? What was the digestive purpose of the relatively small quantity of high-ferment juice elicited by a meal of bread and the relatively large quantity of low-ferment juice elicited by a meal of meat?
In Lectures, Pavlov speculates about these and other issues but defers their resolution to the distant future. He acknowledges that he could not demonstrate biochemically the purposiveness of any of the characteristic curves—nor could he (or any other physiologist of the day) even identify the precise substances in foods that excited the nerves in the mucous membrane of the stomach and duodenum. His argument, he acknowledged, rested “mainly on general considerations, and only in part on separate, more or less clear and indisputable, cases of it.”11
His argument, then, rested on the regularities he perceived at the organ level—on the identification of laws “in the complete apparatus.” These laws were embodied in the standard secretory curves, which displayed a pravil’nost’ and precision that testified to the purposiveness of digestive work. As Pavlov puts it, “Their lawfulness testifies to their importance.”12
He makes the same assertion when introducing the first set of characteristic curves. Each food generates its specific curve, he informs the reader. “In view of the exactness of this curve and its stereotypicity, one must recognize that one or another course of secretion does not exist by chance, but is necessary, useful for the most successful processing of food and the greatest good for the organism.” The very existence of these recognizable curves, then—and their repetition in experiment after experiment— testifies to the glands’ “astonishing exactitude,” to the “great accuracy and precision” with which they provide “just enough” juice of precisely the proteolytic power necessary to digest each food.13
The conceptual and rhetorical importance of these curves is clear from their appearance at critical junctures in Pavlov’s text. They first appear early in Lecture 2, where Pavlov converts into curves the experimental data for two experiments each on the gastric and pancreatic response to the same quantity of the same food.14 These convincingly illustrate the stereotypicity, and hence purposiveness, of the course of digestive secretion. The next two pairs of curves demonstrate this same point with respect to the proteolytic power of gastric and pancreatic secretions.15 Pavlov closes Lecture 2 with a discussion of the distinctive characteristic curves for gastric and pancreatic secretions in response to meat, bread, and milk.16
He begins the next lecture by reviewing what the reader has learned “in the boring form of curves”: that the glands “poured their juice, with regard both to its quantity and quality, in correspondence to the mass and type of food, providing specifically that which was most advantageous for the processing of a given sort.”17 This sets up his discussion of the psychic and nervous-chemical mechanisms that generate these curves. Later in his text, he uses one set of curves to prove that the Pavlov isolated sac faithfully reproduces the secretions of the large stomach, and a final set to demonstrate that the ordinary course of gastric secretion is precisely equal to the sum of the psychic and chemical phases of digestion.18
These curves were not the simple empirical products of experiments that produced precisely the same results every time. Pavlov is surely overstating his case when he writes that, as a result of his lab’s precise techniques, “the course of secretion in identical conditions has become truly stereotypical.”19 How, then, did he present these varied results to demonstrate that glandular secretion was stereotypical and thus purposive?
Here Pavlov confronted the tension between the developed factory metaphor, his Bernardian notion of determinism, and the nature of his data. On the one hand, the factory metaphor invoked the determined precision and regularity of glandular responses—qualities that Pavlov indeed discerned in his data and sought to portray convincingly for his readers. On the other hand, however impressive they might be, the results of chronic experiments with complex and intact dog technologies were never precisely the same from one trial or one dog to the next. His secretory curves ultimately rested upon precisely the kind of average data that Bernard rejected, and these averages concealed substantial differences between the results of various trials.
Pavlov’s resolution of this problem is evident, for example, in the first two curves in Lectures (see below), which he deploys to introduce the reader to the “stereotypical” precision of the glands. He reproduces data from two of Khizhin’s experiments on the quantity of gastric secretion elicited by the digestion of 100 grams of meat and two of Val’ter’s on the quantity of pancreatic secretion elicited by the digestion of 600 cubic centimeters of milk. For rhetorical effect, these data are converted to curves, confronting the reader with two virtually identical pairs.
Pavlov’s two curves for the amount of gastric secretion after a meal of 100 grams of meat. Lectures on the Work of the Main Digestive Glands LRGPZ, 43; LWMDG, 22–23)
Pavlov’s two curves for the amount of pancreatic secretion (in cubic centimeters) after a meal of 600 cubic centimeters of milk. Lectures on the Work of the Main Digestive Glands (LRGPZ, 43; LWMDG, 24)
In his thesis, Khizhin reported a total of five experiments on the amount of gastric secretion elicited by the digestion of 100 grams of meat. He does not provide complete data, but his reported results make clear that Pavlov chose the two experiments whose results best illustrated stereotypical precision. For example, the total amount of secretion in the five experiments was 18.5, 23.9, 25.4, 30.2, and 34.2 cubic centimeters. Pavlov uses the second and third experiments, making much of their striking similarity. Furthermore, Khizhin’s thesis reveals that in two of the other three experiments, a greater quantity of juice was secreted in the second hour than in the first, yielding a curve different from the stereotypical one presented to readers of Lectures.20
Val’ter’s thesis provides more complete data, permitting a closer look at Pavlov’s choices regarding pancreatic secretion (Figure 3). Val’ter conducted thirty-two experiments on Zhuchka’s pancreatic response to the ingestion of 600 cubic centimeters of milk. He openly acknowledged the great variations in his results. For example, the total quantity of pancreatic secretion in these trials varied by almost 100 percent—from 37.25 to 72.25 cubic centimeters. Val’ter ascribed these variations to the uncontrolled variables in his chronic experiments. For reasons he did not explain, he discarded eight of his thirty-two trials. His data for the remaining twenty-four are presented in Chart 3.
Figure 3: This chart reproduces minor computational errors in calculations of total secretion and omits information on the duration and ferment content of pancreatic secretion. Val’ter, Otdelitel’naia, 180
In Lectures, Pavlov chooses trials 1 and 5, converts them into curves, and offers these to his readers as examples of the “truly stereotypical” work of the glands. “The powerful impression of such an almost physical precision in a complex organic process,” he confides, “is one of the pleasant compensations for sitting many hours in front of the glands at work.”21
Pondering Val’ter’s data, one can understand Pavlov’s choice of precisely these two trials. Their results resembled one another in two important ways: yielding much the same total amount and course of secretion. The amount of secretion in each began modestly in the first hour, fell slightly in the second hour, rose about threefold in the third hour, declined sharply in the fourth hour, and fell to almost zero in the fifth hour.
No other pair of trials fulfills these two criteria so well. Assume that Pavlov first chose trial 1 as his “template curve.” Which other trials provide a good match? The total amount of secretion in trial 2 is too low. That in trial 3 is very close indeed, but here the amount of secretion rises much less sharply in the third hour and declines much less sharply in the fourth hour—hardly stereotypical. In trial 4, the total amount of secretion is, again, very close to that in trial 1, but the sharp increase in secretion comes only in the fourth hour. Trial 5 is a good fit, and Pavlov used it accordingly. The total amount of secretion in trials 6 and 8 is too low, and the slope of these curves diverges markedly at several points from that in trial 1. Trial 9 fits trial 1 more snugly than does trial 5 in terms of total secretion, but the amount of secretion more than doubles in the second hour, contrasting sharply with the slight decline in trial 1. Trial 10 is again a good fit in terms of total amount of secretion, but the amount of secretion rises inappropriately in the fourth hour. Proceeding through the available data in this manner, we see that trial 5 provides the best fit with trial 1, and of the others, only trials 19, 22, 23, and 32 offer plausible (if less convincing) alternatives. (Pavlov constructed and presented in the same way the curves for the three different ferments secreted by the pancreatic gland in response to different foods—a claim that would become controversial.)22
Pavlov was no doubt thinking about the selectivity of his choices when he interrupted his argument to concede, “Of course, not all experiments are so similar as those given, but if such a similarity is encountered in two experiments out of five, or about that, this cannot, in all justice, but be considered clear proof of the strict lawfulness of glandular work.” The phrase “two experiments out of five” refers, as we have seen, to his selection among Khizhin’s experiments; the less exact “or about that” refers to his choice of two experiments (or, one might argue, of six experiments) from Val’ter’s twenty-four.23
We can easily imagine Pavlov as he writes this passage—searching through Valter’s data, finding just the right two experiments, and reflecting a bit about what he has done. He was, of course, engaging in a certain sleight of hand: assuring readers that he was presenting typical results while actually choosing those that made his case most convincingly. In my view, however, he signaled this to his readers, however elliptically, not only because it seemed the honest thing to do but also because he was essentially comfortable with his interpretation of experimental data. He was, after all, following Bernard’s dictum to present one’s “most perfect experiment as a type”—that is, to choose the experiment that has been most effectively stripped of the “numberless factors” concealing the determinism of physiological processes. Pavlov was no doubt confident that, were he to show all his results to an open-minded and experienced physiologist and were he to have the opportunity to explain the complexity and difficulty of chronic experiments, the variations in mood and temperament from dog to day and day to day, and the other uncontrolled variables that obscured experiments’ results, such a physiologist would accept his choice of “model experiments.” This was especially true because, although his raw data did not compel belief in a precise digestive factory, they did cluster in a manner that allowed one, if so inclined, to discern the contours of a factory concealed within.
* * *
In Lectures, Pavlov uses his own lab and research findings to offer his vision, not just of the digestive factory, but of modern experimental physiology and its relationship to clinical medicine. The “gentlemen” whom he addresses at the beginning of each lecture were the physicians, medical students, and medical bureaucrats of St. Petersburg, who comprised the most important constituency for laboratory physiology. By 1897 he had accumulated more than twenty years of experience as an intermediary between the lab and medical practitioners—as a medical student, a member of the faculty of the Military-Medical Academy, an active participant in the Society of Russian Physicians, and the manager of three labs that were largely populated by physicians. These experiences explain much about his book’s form, tone, and rhetorical qualities.
Pavlov describes in Lectures a close relationship between the nature of the lab and the quality of its products. Only a scientific enterprise that was itself purposive, regular, and precise could divine these same qualities in the digestive system. Previous investigators, bound by clumsy methodologies and vulgar generalizations, had inevitably failed to discern its subtlety and precision. Themselves working crudely, they perceived glands that responded bluntly and indiscriminately to mechanical, chemical, or thermal agents. The precision of Pavlov’s lab techniques had allowed him to “banish—one hopes forever—[this] crude and fruitless idea” by revealing “the contours of an artistic mechanism imbued, like everything in nature, with subtlety and internal purposiveness.”24
Addressing himself to medical practitioners, he emphasizes the necessarily symbiotic relationship of lab and clinic, and offers a physician-friendly physiology that is respectful of traditional clinical wisdom. Effectively triangulating between mechanistic physiologists and physicians, he defends the empirical wisdom of the latter against the vulgarisms of the former while insisting on the indispensable role of the right kind of experimental physiology. The physician, Pavlov explains—using a popular metaphor of the time—was the “mechanic of the human body.” Only by gradually incorporating physiological facts could medicine eventually achieve “the ability to fix the damaged mechanism of the human organism.” Yet the scientific resolution of therapeutic questions still lay far in the future; the lab could not yet bring within its purview the complex issues that confronted the physician, whose “laboratory” was mankind itself. “Physiology, of course, cannot presume to forcefully guide the physician, because, lacking complete knowledge, it turns out constantly to be narrower than clinical reality.” It could, however, greatly elucidate the mechanism of ailments and “the inherent sense of useful empirical therapeutic practices.” Pavlov’s specific examples of his physiology’s contribution to medical practice, then, are almost always cases in which his lab has confirmed and refined accepted clinical (and even folk) practices. He adduces many cases in which, as adjudicated by his lab, the “instinct” of common people and their physicians enjoyed a “victory in the court of physiology.”25
These cases revolve largely around his reaffirmation of the importance of appetite. Peoples and medical practitioners throughout the world had long understood that “food should be eaten with attention and satisfaction.” The use of alcohol, spices, and condiments; the provision of a separate room for meals; and the rules of etiquette that forbade serious discussions at the table all attested to an understanding that it was important to facilitate “attention, interest, and pleasure in food.” Unfortunately, scientifically minded physicians had been led astray by physiologists in recent times. Unable to transform this psychic moment into a reliable experimental phenomenon and guided by crude mechanistic conceptions, physiologists had ignored and denigrated appetite—until Pavlov himself had restored it to its rightful place.26
His findings also validated other common medical practices and supported a broad conception of the physician’s authority. Physicians had long fed milk to grievously ill patients—and Pavlov now demonstrated that, in the absence of appetite, milk, with its rich chemical exciters, was indeed the most easily digested of foods. Since Hippocratic times, physicians had claimed that part of their art was to adapt treatments to a patient’s experiences and inclinations—and Pavlov had proven the importance of individuality to the digestive process. Physicians had long emphasized the importance of proper regimen—and Pavlov now underlined the significance of managing the idiosyncratic psyche.
In his closing peroration, he invites physicians to engage in the necessary dialogue between lab and clinic by alerting him to deficiencies in his own work.
If the physiological data collected here help the physician to understand anything in the sphere of his activity and facilitate a more correct and successful approach to treatment, then the physician can secure for himself still greater advantage by informing the physiologist of any corrections...that, from his point of view, need to be made to the explanations offered here, and by indicating those new dimensions of the subject that have been revealed to the physician in the broad world of clinical observations but still remain outside the physiologist’s field of sight. I believe profoundly that only by such a lively exchange of views between the physiologist and the physician will the goals of physiology as knowledge, and medicine as an applied science, be quickly and truly attained.27
The style and content of Pavlov’s research allowed him to assume this posture with great authority. Working with intact animals, he was able to make specific recommendations about the treatment of intact patients and to appreciate the individual idiosyncrasies with which physicians contended in their daily practice. For their part, one suspects, physicians would have been sympathetic to the inherent difficulties of obtaining precisely the same results in different experimental trials, as this was often the case in their own therapeutic experience.
Here, too, resided the great appeal of Pavlov’s grand synthesis of the two basic moments at the heart of his scientific vision: his effort both to identify precise, quantitative, determined physiological laws and to encompass the intact, functioning animal. The joining of these two moments in Lectures, embodied thematically in the characteristic secretory curves and the powerful idiosyncratic psyche, presented physiologist and physician alike with an impressive example of the union between experimental science and the daily complexities of medical practice.