To find elastic fibers when not abundant boil the sputum with a 10 per cent. solution of caustic soda until it becomes fluid, add several times its bulk of water, and centrifugalize, or allow to stand for twenty-four hours in a conical glass. Examine the sediment microscopically. {29} The fibers will be pale and swollen. Too long boiling will destroy them entirely. [Illustration: FIG. 5.--Curschmann's spirals: I., Natural size; II. and III., enlarged: _a_, central fiber (after Curschmann).] 2. Curschmann's Spirals.--These peculiar structures are found most frequently in bronchial asthma, of which they are fairly characteristic. They may occasionally be met with in chronic bronchitis and other conditions. Their nature has not been definitely determined. Macroscopically, they are whitish or yellow, twisted threads, frequently coiled into little balls (Fig. 5, I.). Their length is rarely over half an inch, though it sometimes exceeds two inches. Under a two-thirds objective they appear as mucous threads having a clear central fiber, about which are wound many fine fibrils (Fig. 5, II. and III.). Leukocytes are usually present within them, and sometimes Charcot-Leyden crystals. The central fiber is not always present. [Illustration: FIG. 6.--Charcot-Leyden crystals (after Riegel).] {30} 3. Charcot-Leyden Crystals.--Of the crystals which may be found in the sputum, the most interesting are the Charcot-Leyden crystals. They are rarely found except in cases of bronchial asthma, and were at one time thought to be the cause of the disease. They frequently adhere to Curschmann spirals. Their exact nature is unknown. They are colorless, pointed, often needle-like, octahedral crystals (Fig. 6). Their size varies greatly, the average length being about three or four times the diameter of a red blood-corpuscle. Other crystals--hematoidin, cholesterin, and, most frequently, fat needles--are common in sputum which has remained in the body for a considerable time. [Illustration: FIG. 7.--Fibrinous bronchial cast (Sahli).] 4. Fibrinous Casts.--These are fibrinous molds of the smaller bronchi. Their size varies with that of the bronchi in which they are formed. They may, rarely, {31} be three or more inches in length. When large, they can be recognized with the naked eye by floating them out in water; when small, a low power of the microscope must be used. They are easily recognized from their branching, tree-like structure (Fig. 7). Fibrinous casts are characteristic of fibrinous bronchitis, but may also be found in diphtheria of the smaller bronchi. Very small casts are often seen in croupous pneumonia. [Illustration: FIG. 8.--Sputum from a case of actinomycosis; stained (Jakob).]