In acute meningitis the fluid is turbid, and contains an excess of albumin. Organisms also are present, such as the diplococcus intracellularis in acute cerebro-spinal meningitis; staphylococci, streptococci, and pneumococci, particularly in the intra-cranial complications of middle ear disease. In all cases of acute microbic infection, and especially in the suppurative forms, polynuclear leucocytes are found in the fluid; while in chronic affections, such as tubercle and syphilis, there is an excess of lymphocytes (Purves Stewart). The detection of the tubercle bacillus is confirmatory of a diagnosis of tuberculous meningitis, but, as it is often difficult to find, its absence does not negative this diagnosis. In tuberculous meningitis the clot which forms floats in the centre of the fluid, and is translucent, grey, and flaky; in the pyogenic forms it is yellow, and sticks to the side of the vessel. In a few cases of malignant tumour of the spinal cord and its membranes, characteristic cells have been found in the fluid after centrifugalising. In uræmia there is a diminution of chlorides, and an increase of phosphates and sulphates. The Wasserman test is sometimes positive in the cerebro-spinal fluid, when it is negative in the blood. _Therapeutic Puncture._--In certain cases of cerebral tumour, and of tuberculous meningitis associated with an excessive quantity of fluid in the arachno-pial space, temporary relief of such symptoms of increased intra-cranial tension as headache, vertigo, blindness, or coma, has followed the withdrawal of from 30 to 40 c.cm. of the fluid. Terrier and others have found this measure useful in relieving pain in the head, delirium, and even coma, in cases of basal fracture. Carrière has found it beneficial in some cases of uræmia. The quantity withdrawn must not exceed 40 c.cm., lest the ventricles be emptied and pressure be exerted directly on the basal ganglia (Tuffier). In a number of cases sudden death has followed the withdrawal of cerebro-spinal fluid. This route is sometimes selected for the induction of spinal anæsthesia, and for the injection of antitoxin in cases of tetanus. HEAD INJURIES The brain is protected from injury by moderate degrees of violence applied to the head, by the dense and mobile scalp, the dome-like shape of the skull, the elasticity of its outer table and the buffer-like sutural membrane between the numerous bones of which it is composed, and the various internal osseous projections with the membranes attached to them, all of which tend to diminish vibrations and to disperse forces so that they expend themselves before they reach the brain. Further protection is provided by the water-bed of cerebro-spinal fluid, and by the external buttresses formed by the zygomatic arch and the thick muscular pads related to it, as well as by the mobility of the skull upon the spine.