Bichat supposes the black matter in the lungs "to be owing to small bronchial glands extending along the surface of the pleura." Breschet believes that it is formed by the blood exhaled into the cellular tissue, stating that its chemical composition leads him to that conclusion. Trousseau says that it is produced by a misdirection of the natural pigments of the body, resulting from age, climate, or disease. Andral says, that the black appearances are the result of secretion, and that it is more manifest as the individual advances in life. Heasinger's opinion is, that it is analogous to pigment, and therefore he agrees with Trousseau. Laennec was doubtful as to the real origin of black pulmonary matter. He makes a distinction between melanotic and pulmonary matter. He found that the melanotic matter was composed almost entirely of albumen, while the black pulmonary matter found in the bronchial glands contains a great quantity of carbon and hydrogen, and also that these colouring matters have other distinguishing characters. The melanotic matter is easily effaced by washing, while the other is removed with difficulty. Laennec further says, that he suspected that this pulmonary matter might arise, at least in part, from the smoke of lamps, and other combustible bodies which are used for heat and light; for some old men are to be met with whose lungs contain very little black matter, and whose bronchial glands are but partially tinged with this colour; and it has struck him that he observed this amongst villagers who had never been accustomed to watch.[23] Mons. Guillot, physician to the hospital for the aged at Paris, has undertaken a series of researches in regard to the black matter found in the lungs of old men of very considerable age. These investigations are published in the January, February, and March numbers of the _Archives Generales de Medecine_.[24] It is his belief that death in such cases is owing, in all appearance, more or less to a suppression of the circulation of air and blood by the black substance. His impression is, "that the carbon is not procured from without, but naturally deposited, as life advances, in the substance of the respiratory organs; and that this deposit of carbon causes death, by rendering the lungs irrespirable, while, at the same time, it has much influence in modifying the progress of _tubercular_ disease; so that, if the tubercular affection was not cured, its progress was so far checked, that life has been very long preserved." The black matter envelopes completely both the pulmonary tubercles which have undergone a transformation, and the caverns which no longer contain tuberculous matter. He, while regarding these as the results of black matter in the lungs, throws no light on the cause of the deposit of the particles of carbon within the lungs.