In pseudo-membranous or croupous bronchitis the bronchial tubes are found lined, and in some cases filled, with a plastic exudate. Usually, only a limited number of the bronchi are affected. The tube-casts that may be expelled are generally in the form of balls, which may be unrolled, and which will then be found to be fragments or complete cylindrical casts of the tubes. They are, when expelled, usually yellowish and often sanious. When washed they are white. There are frequently points of enlargement along the casts which are caused either by the presence of air-bubbles within them, or by a more rapid exudation from that point on the bronchus. The largest casts are usually solid and laminated in structure; the smaller ones more frequently are hollow, containing a greater or less number of air-bubbles; the smallest consist of a single solid thread. Under the microscope the casts seem to be composed of a structureless or fibrinous substance holding numerous mucus and pus-cells, more or less numerous globules of fat, and occasional epithelial cells; seldom red blood-corpuscles, although these may be numerous on the surface. The casts are usually moderately compact, firm, and elastic. Toward the end of the disease, however, they may be less firm. In some cases toward the close of life epithelial cells are abundant in them, but in other cases on post-mortem examination the epithelial lining of the bronchi is found nearly or quite entire. The mucous membrane may be much reddened, or, on the other hand paler than normal. The submucous tissues are also sometimes involved in the swelling, and occasionally infiltrated with serum.[7] [Footnote 7: For a representation of one of the most complete specimens of pseudo-membranous casts from the bronchi the reader is referred to the paper of Glasgow in the _Transactions of the American Medical Association_, already referred to.]