The physiological action of lead is exerted on the nervous system, so that lead poisoning may be appropriately enough treated of as a disease of the nervous system.
Sources. The sources of lead as a poison for animals are extremely varied. In England in the vicinity of lead mines and smelting furnaces it is deposited from the air in a fine powder, and consumed with the vegetation. Herapath found that the deposit, in dangerous amount, began half a mile from the chimney of the smelter and extended about half a mile further.
A second source is in lead paints used about farms and the scrapings of paint pots thrown out with manure and spread upon the fields. These lead combinations will last for years in the soil or on the surface, being plowed under one year and turned up again the next when the occasion of their presence has been completely forgotten. In one case I found the red lead paint marked by the tongues of cattle at the back of an abandoned cottage the fence around which had been broken down. In another the scrapings were found in an orchard which had been near and convenient for throwing them out. In a third case a paint can hung on the branch of an apple tree, well out of the way of the stock as the owner fondly supposed, showed in its contents the marking of the barbed tongues of the cattle. In a fourth case a barrel of paint was set under the barn where there was not height enough to admit the matured cattle, but it bore the marks of licking by the young stock, and they alone died but in such numbers that the owner concluded it must be the “Rinderpest.”
The lead packing from the joints of pumps, engines and other machinery, thrown away around works and mines, is a common source of the trouble. I once found large quantities in the gastric contents of cows that had died around a coal mine in Ayrshire, Scotland.
Sheet lead—tea-chest lead—is another common source of the poison. This is thrown out, scattered with the manure on the field, and will resist the elements for years but dissolves when taken into the acid stomach of the animal.
The spray from bullets in the vicinity of rifle butts is another common cause of the poisoning.
In one instance I have seen a cow poisoned by eating some lead-impregnated wall paper which had been carelessly left in the stable.
Less frequently the poisoning comes from drinking water carried in leaden pipes, or left to stand in a leaden cistern. The softest waters—rain, snow, distilled water—are the most liable to this impregnation. The hard waters containing carbonates, sulphates or phosphates, tend to be decomposed, the acid uniting with the lead to form comparatively insoluble carbonates, sulphates or phosphates of lead, which protect the subjacent lead against solution. The hardness of the water is not, however, a sufficient safeguard, as iron, solder, and other agents present in the lead as an impurity or merely resting upon it, are sufficient to set up a galvanic action resulting in solution.
The salts of lead may find direct access to the animal, as in the case reported by Gamgee in which a farmer used a barrel which had contained acetate of lead for mixing the feed given to his stock. A somewhat similar source of poisoning is found in the use of buckets or silos which have been painted inside, and scale off in contact with hot water, etc.
Blythe enumerates the following compounds of lead as employed in the arts: