TUBEECULOUS MENINGITIS. - 161 



sacral vertebra. Hence at all ages lumbar puncture may be 

 effected between the laminae of the third and fourth or fourth 

 and fifth lumbar vertebras without risk of injury to the spinal 

 cord. The drainage is usually facilitated by flexion of the head 

 during the tapping since this movement opens up the angle 

 between the lower aspect of the cerebellum and the fourth 

 ventricle, and so allows free exit of the fluid. The subdural 

 space has no communication with the subarachnoid space and 

 the ventricular cavities, but in the treatment of hydrocephalus 

 an artificial communication is sometimes made in the hope that 

 the vessels of the dura may dispose of the excess of fluid which 

 has accumulated in the ventricles and subarachnoid space. 



Tuberculous meningitis is in most instances a secondary 

 infection, the bacilli being carried to the leptomeninges in 

 the blood stream. A primary focus is commonly found in the 

 mediastinal glands, but sometimes the infection is derived from 

 the mesenteric glands, the lungs, bones, joints, or generative 

 organs. Occasionally direct infection of the brain substance can 

 be traced from the petrous bone, or the bones of the nasal cavity. 

 The meningeal tubercles are found in the pial sheaths of the 

 cerebral vessels ; they chiefly accumulate in the walls of the basal 

 cistern of the subarachnoid space and in its Sylvian prolongations, 

 and so follow the course of the middle cerebral arteries. A 

 moderate degree of hydrocephalus with consequent flattening of 

 the convolutions co-exists, the ventricles being invaded along the 

 velum interpositum which is invaginated into the brain at the 

 great transverse fissure. The inflammation is carried from 

 the meninges to the surface of the brain along the perivascular 

 sheaths of the small vessels passing into the cortex from the pia, 

 and a moderate amount of diffuse softening is thus produced. 

 Similar inflammatory softening is also present around the 

 ventricular cavities. In some instances the inflammation of 

 the pia and arachnoid is sufficiently intense to cause occlusion 

 of larger arterial branches which lie in the subarachnoid spaces 

 and then definite areas of softening may be found in the central 

 ganglia and sometimes in the cortex. In such cases the ganglia 

 C.A.A. 11 



