In the absorption of the substance from the intestine, it may go direct into the blood-stream in a similar fashion through the lacteals along the lymph channels, and so into the thoracic duct, and finally into the general circulation. On the other hand, a certain amount, probably not an inconsiderable portion, is taken up by the portal circulation and transferred direct to the liver itself. Chemical analysis of the liver supports this view, as does also the considerable amount of stress thrown upon the liver when poisoning has taken place from the intestinal canal on administration of massive doses of a highly soluble lead compound. According to Steinberg[16], excretion of lead takes place partly from the liver by the bile. This is probable, but there is no experimental evidence at the present time to support the view. If such an excretion does take place, the form in which the lead is excreted is probably one in which it is no longer soluble by digestive action. On the other hand, it may be in so soluble a form as to become reabsorbed from the intestine, thus setting up a constant cycle. But such a theory is one that would require a considerable amount of experimental evidence to support it before it could be relied on.
There is no doubt that, however absorbed, lead remains stored up in the body in minute quantities in many places, and the close analogy to arsenic is met with in the curious elimination of the metal by the fæces. Cloetta[17], quoted by Dixon Mann, discovered that, although dogs were unable to take a larger dose of arsenic than 0·0035 gramme per day without exhibiting toxic results, they could nevertheless take arsenic in much larger doses if it were given in the solid form, and he was able to increase the dose to as much as 2 grammes per diem without showing any toxic symptoms. Examination of the urine and fæces showed that as the amount of urinary excretion of arsenic diminished, so that in the fæces increased, and in lead poisoning, even in massive doses swallowed in error, the amount of lead excreted by the urine rapidly diminishes in quantity, although the patient may be still suffering from the effects of lead poisoning. The experiments, also, quoted on [p. 100] constantly pointed to the elimination of lead by way of the intestine, and in practically all the animals that had suffered from chronic poisoning well-marked dark staining of the upper part of the cæcum due to lead was invariably present. This staining and excretion of lead of the large intestine undoubtedly takes place in man. In a case described by Little[18], where diachylon had been administered, the administration of a large enema containing sulphate of magnesium came away black. A more detailed result of the experiments and a consideration of the elimination of lead are reserved for another chapter, but it is impossible to consider the ætiology of the disease without some reference to the general histological channels of absorption and excretion.
Cutaneous Absorption of Lead.
—A considerable amount of controversy has centred on the question of the absorption of lead through the unbroken skin. It has been shown that such drugs as belladonna applied to the skin alone may produce dilatation of the pupil; an ointment containing salicylic acid spread upon the skin and thoroughly rubbed in is followed by the appearance of derivatives of salicylic acid in the urine; mercury may be applied to the skin, and rubbed in, in sufficient quantities to produce salivation; and a very large number of other drugs may be cited, all of which when applied to the unbroken epidermis with friction produce the physiological action of the drug.
There is no reason to exclude lead from the category of drugs which may be absorbed through the medium of the skin, and, as several observers have shown, animals may be poisoned by lead on applying a plaster of lead acetate to the skin. Amongst these experiments may be quoted those of Canuet[19] and Drouet[20] on rabbits. Some observers, among whom may be mentioned Manouvrier[21], have attempted to prove that paralysis of the hands occurs more often in the right hand in right-handed people, in the left hand with left-handed people, and from the various experiments showing absorption of lead through the unbroken skin they seek to connect the lesion of the nerve with absorption direct through the skin of the hands.
Many objections can be urged against acceptance of this theory. Lead workers who are constantly manipulating lead in a state of solution with bare hands do not appear as a class to be more subject to wrist-drop than do persons who are exposed to inhalation of fumes or dust of lead; in fact, incidence of paralysis and of nerve lesions generally is more severe among persons exposed to prolonged inhalation of minute quantities of lead through the respiratory tract. The greater the exposure to dust, the greater the number of cases of anæmia and colic, whilst in other industries, as has already been stated, where lead exists as an oleate on the hands of the workers day in and day out for many years, paralysis and even colic are of rare occurrence; in other words, persons especially exposed to the absorption of lead through their hands show a much smaller incidence of lead poisoning of all types than do those exposed to lead dust. Further, the pathology of wrist-drop and similar forms of paresis tends to show that the nerve supplying the affected muscles is not affected primarily, but that the initial cause is hæmorrhage into the sheath of the nerve, producing ultimate degenerative change. The hæmorrhage, however, is the primary lesion.
REFERENCES.
[ [1] Goadby, K. W.: A Note on Experimental Lead Poisoning. Journal of Hygiene, vol. ix., No. 1, April, 1909.
[ [2] Legge, T. M.: Report on the Manufacture of Paints and Colours containing Lead (Cd. 2466). 1905.
[ [3] Duckering, G. E.: Journal of Hygiene, vol. viii., No. 4, September.