referring to the drawings , the cranial bolt comprises a generally cylindrical shank 1 having a threaded lower end 2 for securing the bolt into a preformed hole in the skull 30 , see fig3 . the distal end 3 of the shank has a smaller diameter than the threaded portion 2 . this is to prevent the passageways within the shank being occluded by uncut portions of the skull at the inner end of the hole which has been cut to receive the threaded end . this arrangement may also avoid the need to ream the hole in the skull after cutting a hole of appropriate size to receive the shank and therefore simplify installation of the bolt . the shank 1 is formed at its upper end with a shaped profile 4 adopted to engage in a hole 5 in a plastic body member 6 . body member 6 is formed with wing - like projections 7 to facilitate screwing the threaded part 2 of the shank into a preformed hole in the skull . body member 6 may be moulded separately from the shank or moulded onto the upper part 4 . at its upper end , the body member 6 is formed with a part 8 which is connectible with a flexible collar 9 . collar 9 may be manufactured from a flexible , resilient plastics material and is a push fit onto the end part 8 of the shank or moulded in situ onto the end of the shank . the collar 9 helps to prevent chafing of the tubes 21 , 22 . shank 1 is formed internally with a plurality of passageways . as seen best in fig2 a , one of the passageways 10 is larger than the other two 11 . passageway 10 preferably extends axially of the shank and is sized to house a drainage catheter . the passageways 11 may be disposed around the major passageways 10 and are designed to accommodate small catheters or probes for sensing parameters , such as oxygen concentration , carbon dioxide concentration , ph and temperature . fig1 illustrates one way in which tubes 20 , 21 , 22 are grasped together and anchored to the bolt fixed into the extension piece 9 , by fixing with adhesive into the shank portion . the distal ends of the tubes 20 , 21 , 22 may be sealed into the passageways 10 , 11 . the tubes 20 , 21 , 22 may constitute tubular guides through which sensors or smaller catheters are fed to the desired site . this is illustrated in fig1 in which a catheter 23 is fed through tube 22 , passes through the respective passageway in the shank and exits through the distal end of the shank . a luer lock 24 is provided to maintain the catheter in the desired position after it has been located in the brain . a further alternative method of feeding probes or catheters into the bolt is to provide a multi - lumen tube which is fixed into the shank . individual lumens are divided from the multi - lumen tube and aligned with passageways in the shank . probes or catheters having diameters smaller than a lumen can then be fed into a respective lumen and passageway and into the brain through the distal end of the bolt . the tubes , 20 , 21 and 22 are preferably kink - resistant , at least over the portion from the shank 1 to the luer lock or locks 24 . because the tubes or catheters are grouped together in the flexible collar 9 and extend axially of the shank , the bolt has a minimum lateral spread . this is advantageous because a bolt which is bulky and has in - feed catheters extending from the bolt in various directions is more likely to be knocked or displaced accidentally during nursing or handling the patient . fig2 b and 2c show alternate arrangements for the location of the passageways through the shank . in practice , it may be desirable to form the outer passageways 11 so that they are inclined at a small acute angle to the longitudinal axis of the shank . preferably the angle is 3 ° to 15 °, e . g . 4 ° to 8 °. fig3 shows one method of fitting the cranial bolt into the skull 30 . the scalp 31 is cut away for a sufficient distance to gain access to the skull 30 and a hole is cut in the skull 30 of a size which is just less than the diameter of the threaded part 2 . the bolt can then be secured to the hole in the skull 30 , by securing the threaded part 2 into the skull 30 . once the bolt has been secured into the skull 30 , catheters are passed through the guide tubes 20 , 21 , and 22 into the bolt and through the internal passageways in the bolt into the desired positions in the brain . the guide tubes 21 and 22 may be fixed e . g . by adhesive into the upper part of the bolt . when the bolt is to be removed , the catheters are first withdrawn and the bolt is then removed by unscrewing the shank 1 . the bolt in accordance with the invention can be used to install single or multi - lumen catheters , sensors or drainage or sampling tubes into various parts of the brain , including the ventricles , sub - dural , epidural or parenchymal areas of the brain . sampling tubes may include microdialysis catheters in which a saline solution is passed down one lumen and samples of chemicals in the brain are extracted through a second lumen via a membrane , and the extracted fluid analysed .