Predominance of endothelial cells, few cells of any kind being present, indicates a transudate (Fig. 119). Endothelial cells generally predominate in carcinoma, but are accompanied by considerable numbers of lymphocytes and red blood-corpuscles. CEREBROSPINAL FLUID Examination of the fluid obtained by lumbar puncture is of value in diagnosis of certain forms of meningitis. _Tubercle bacilli_ can be found in the majority of cases of tuberculous meningitis. The sediment, obtained by thorough centrifugalization or by coagulation and digestion (p. 128) is spread upon slides and stained by one of the methods already given. A considerable number of smears should be examined. In doubtful cases, inoculation of guinea-pigs must be resorted to. The _Diplococcus intracellularis meningitidis_ is recognized as the cause of epidemic cerebrospinal fever, and can be detected in the cerebrospinal fluid of most cases, especially those which run an acute course. Cover-glass smears from the sediment should be stained by the method for the gonococcus. The meningococcus is an intracellular diplococcus which often cannot be distinguished {269} from the gonococcus in stained smears (Fig. 120). It, also, decolorizes by Gram's method. The presence of such a diplococcus in meningeal exudates is, however, sufficient for its identification. [Illustration: FIG. 120.--Diplococcus intracellularis meningitidis in leukocytes. X2000 (Wright and Brown).] Various organisms have been found in other forms of meningitis--the pneumococcus most frequently. In some cases no micro-organisms can be detected even by culture methods. ANIMAL INOCULATION Inoculation of animals is one of the most reliable means of verifying the presence of certain micro-organisms in fluids and other material. Clinically, it is applied almost exclusively to demonstration of the tubercle bacillus when other means have {270} failed or are uncertain. The guinea-pig is the most suitable animal for this purpose. When the suspected material is fluid and contains pus, it should be well centrifugalized, and one or two cubic centimeters of the sediment injected by means of a large hypodermic needle into the peritoneal cavity or underneath the loose skin of the groin. Fluids from which no sediment can be obtained must be injected directly into the peritoneal cavity, since at least 10 c.c. are required, which is too great an amount to inject hypodermically. Solid material should be placed in a pocket made by snipping the skin of the groin with scissors, and freeing it from the underlying tissues for a short distance around the opening. When the intraperitoneal method is selected, several animals must be inoculated, since some are likely to die from peritonitis caused by other organisms before the tubercle bacillus has had time to produce its characteristic lesions.