and secreted in certain cells, but here again the blood must be looked upon as its ultimate source. This is presumed to occur especially in growths connected with tissues which normally contain much pigment, such as the choroid coat of the eye.
Allusion may here be made to the so-called brown alrophy of the heart, in which condition granules of brownish-yellow or blackish pigment form, the muscular fibres being at the same time atrophied, as well as often the seat of fatty change.
2. An important source of pigmentation in connection with the lungs and bronchial glands is the inhalation of certain substances along with the air breathed. All persons are liable to inhale small particles of carbon, the product of imperfect combustion, and this occurs especially in large towns and manufacturing districts. Hence it is found that the lungs become darker as age advances. It is, however, in the lungs of those who, owing to their occupation, are exposed to the constant breathing of air containing various minute particles suspended in it, that the most marked alterations are observed. Colliers inhale fragments of coal, and their lungs become ultimately perfectly black. Miners, stone-masons, and others are also subject to changes in the colour of these organs, due to the inhalation of solid particles.
The carbon or coal exists in the form of minute granules, which resist all chemical change, and these after entering the small bronchi and air-cells, somehow penetrate the tissues, passing into the epithelium-cells, as well as into the tissue between the lobules and around the bronchi, where they lie either free or enclosed in the connective-tissue corpuscles. They are taken up by the lymphatics and conveyed to the bronchial glands, which soon become quite black also. In the matter which is expectorated, abundant pigment is seen within the cells which it contains, and the sputa may be perfectly black. The change of colour in these cases is, however, not entirely due to the direct deposit of material from without, but partly to the irritation caused by this material setting up inflammation, with stagnation of blood, the pigment of which undergoes the usual alterations.
V. Mucoid Degeneration.
Some tissues occasionally undergo a process of softening or liquefaction, to which this term is applied. They become changed into a mucilaginous substance, colourless and homogeneous, which yields mucin. In fact they seem to return to their original fœtal condition. The change may take place extensively or in limited spots, in which, being surrounded by healthy tissue, it may give rise to an appearance of cysts. The intercellular tissue is most affected, but sometimes the cellular elements become involved in the degeneration. This degeneration is met with in cartilage, bone, serous membranes, and in the choroid plexuses of the brain. Some tumours are of a mucoid structure when first formed, and many others may undergo this change to a greater or less degree.
