Practically all cases of water poisoning and of swallowing of lead compounds have developed colic. Further, colic is cited in all the early recorded cases, even in the very earliest cases referred to in the historical note, of lead poisoning; and as poisoning in those cases had invariably taken place by swallowing the drug, it may be presumed from this association has arisen the belief that lead must be swallowed to produce gastro-intestinal symptoms. No attention has been paid to the fact that a few cases of definite cutaneous absorption of lead from the use of hair lotions have been followed by colic. Gastro-intestinal symptoms, therefore, can be produced without the direct ingestion of the drug, and colic is a symptom of generalized blood-infection rather than a localized irritative action on the intestinal mucosa. This question, again, is more related to pathology than ætiology, and is dealt with in that [section]. But mention may be made here of the fact that a number of observers, more lately Meillère, have laid it down as an axiom that experimental production of lead poisoning in animals gives no criterion or evidence of lead poisoning produced in man industrially. Very grave exception must be taken at once to such a statement. In the majority of experiments quoted by Meillère the quantity of lead given for experimental purposes has been large—much larger, indeed, than is necessary to produce small and characteristic effects—and instead of chronic poisoning an acute lead poisoning has generally been set up; and even where chronic poisoning has supervened, the condition has as a rule been masked by the severer initial symptoms. On the other hand, the evidence to be derived from comparison of the various observations from animal experiments brings out with remarkable unanimity the similarity of the symptoms to those produced in man, and, as will be seen later in the section devoted to Pathology, experiments by one of us (K. W. G.) have so far confirmed this surmise; in fact, a description of a case of encephalopathy coming on after lead poisoning of a chronic nature, described by Mott, agrees in practically every particular with the train of symptoms as observed in these experimental animals. Certain slight differences as to the muscles first affected are observed, but it is practically always the homologous muscle (the physiological action of which more nearly resembles the human muscle) which is the one to be affected in the animal, not the anatomical homologue. Thus, for instance, in the cat the spinal muscles, and particularly the quadriceps extensor, is the muscle which is first affected through the medium of the anterior crural nerve. This extensor muscle is one which only performs a slight amount of work in extending the knee-joint, the amount of work being, however, disproportionate to the size of the muscle. The extensors of the fore-feet ultimately do become weakened, but it is the hind-limb upon which the stress first falls.
Attention has been given to the solubility of lead salts in gastric juices, the majority of such experiments having been performed with artificial gastric juice. The method at present in use, prescribed by the amended rules of August, 1900, for earthenware and china factories, is based on some, if slight, consideration of the physiology of digestion. The method described by Rule II. states that the estimation of the quantity of lead present in the lead fritt shall be performed as follows:
A weighed quantity of dry material is to be continuously shaken for one hour at room temperature with one thousand times its weight of an aqueous solution of hydrochloric acid, containing 0·25 per cent. of HCl. This solution is thereafter to be allowed to stand for one hour, and to be passed through a filter. The lead salt contained in a portion of the clear filter is then to be precipitated as lead sulphide, and weighed as lead sulphate.
This method has been adopted on the supposition that the solubility of a lead salt in the gastric juices is the chief source of the lead poisoning in the Potteries, and that the hydrochloric acid content of the solution determines, for practical purposes, the quantity of lead dissolved out of a given sample. The temperature, however, at which this estimation is made—namely, room temperature—is one considerably lower than that of the body, and the quantity of lead taken up into solution at this temperature is less than that which occurs at the ordinary temperature of the body—37° C. Practically twice as much lead is dissolved out of fritt at 37° C. for an hour as is rendered soluble at the ordinary temperature of the room—about 15° C. Thomason[11], who made some experiments in this direction, gives a figure of 2·35 lead oxide dissolved at 15° C. and 4·54 at 37° C. In another estimation—a matter, too, of some considerable importance—it was found that acetic acid dissolved 1·97 per cent, at 15° C., and 3·27 at 37° C. In lactic acid the figure was 2·28 at 15° C., and 3·53 at 37° C. It is therefore a low estimation of the solubility of any substance by the gastric juices if the substance is operated on at a temperature below that of the body.
The question of the solubility of a lead salt in the gastric contents is important in view of the small quantities of dust swallowed; and in addition to hydrochloric acid, other substances are also present in the gastric juice, which is by no means a simple aqueous solution of the mineral acid. Further, the gastric juice, except in cases of pathological type, is not acid in periods of gastric rest, unless such acidity may be represented by the presence of fermentative acids—acetic, lactic, and butyric.
The activity of the gastric juice on lead is directly caused by the quantity of organic acids present in addition to the hydrochloric acid, and by the presence of foodstuffs—(1) in the undigested and (2) in the semidigested condition. In considering the absorption of lead products from the gastro-intestinal canal, the normal digestive processes should not be lost sight of—that is, the sequence of events which occur during digestion of food. On swallowing food, no definite acidity is present in the stomach for fifteen to twenty minutes, and even after that time the hydrochloric acid is only commencing to be secreted. As digestion proceeds, and the whole mass becomes partially dissolved, such portions as are in a soluble condition are passed through the pyloric opening at intervals, and the whole contents of the stomach do not pass straight through the pyloric opening as through an ordinary straight drain-pipe. As each mass of food passes onwards through the pylorus, it comes into contact in the duodenum with pancreatic juice, and with the bile, these alkaline fluids rapidly change the reaction, and allow the other ferments, trypsin, etc., to become active. As the mass proceeds onwards through the intestine, the succus entericus also exerts its function. Finally the fluid contents of the intestine are passed onwards through the ileo-cæcal valve. During the passage from the pylorus to the ileo-cæcal valve, the reaction of the intestinal contents undergoes variations, from an alkaline in the duodenum or upper parts of the jejunum, to acid at the ileo-cæcal valve. Practically no absorption takes place from the stomach itself; a small quantity of water and such highly volatile fluids as alcohol may be absorbed, but the main absorption is not commenced until the food has left the stomach; in fact, the stomach contains no mechanism for food absorption. The work of absorption of the products of digestion is carried on actively through the small intestine until finally the materials have reached the large intestine through the ileo-cæcal valve; water is then mainly absorbed, and albuminous fluids and substances in solution to some extent, but the amount of absorption which takes place is infinitesimal as compared with that of the small intestine.
These points in the physiology of digestion require to be taken into account when discussing the absorption of lead salts in the gastro-intestinal canal.
When human gastric juice is obtained direct from the stomach in man, and lead is submitted to its action, definite quantities of lead pass into solution; and, curiously enough, in the normal gastric juice lead sulphate is as soluble as both white lead and litharge. The following two tables give the results of the estimation of the direct action of human gastric juice upon lead. The particular point is that the juice was obtained by the stomach tube from persons who had been given a simple test meal preceded by a twelve hours’ fast; the juice was therefore in a normal condition. The tests gave the following results in the normal stomach:
| Lead sulphate | 0·080 | per cent. |
| White lead | 0·048 | „ |
| Litharge | 0·040 | „ |
In the second digestion, in which the analysis of the contents showed the patient to be suffering from the condition known as “hyperhydrochloridia,” the results were—