Physicians who inject nerve blocking agents, either to produce local anesthesia or to alleviate pain, are aware of the possible complications associated with these procedures. However, few seem to be cognizant of the potential significance of the relationship of the perineural spaces to the subarachnoid space and the spinal cord as the determining factor in the development of some of the more serious complications. This relationship was bluntly called to our attention by recent experiences after the use of Efocaine (a solution of 1% procaine, 0.25% procaine hydrochloride, and 5% butyl-p-aminobenzoate in a solvent composed of 2% polyethylene glycol 300, 78% propylene glycol, and water). It was originally advertised as a safe, long-acting (two to four weeks' duration), local anesthetic mixture unassociated with encapsulation, abscesses, foreign body reactions, tissue sloughs, or other such adverse effects encountered after the use of oil solvents.1 Early published reports seemed to support the claim of its manufacturer that its prolonged action was effected by the precipitation of procaine crystals in the body tissues.-However, later pub¬ lished reports indicated that the long action was actually due to tissue damage, particularly to nervous tissue, from the propylene and polyethylene glycol that are the principal solvents in the Efocaine mixture. These pub¬ lications reported sloughs, cellulitis, transverse myelitis, and deaths.3In reviewing two deaths and 11 cases of transverse myelitis following the use of Efocaine, it was noted ;i0 that all of the patients received paravertebral injections of the peripheral somatic nerves except one, who re¬ ceived a lumbar sympathetic block. The maximum dis¬ tance of the injection site from the intervertebral foramen was 8 cm., by actual postmortem measurement. Four of these patients had percutaneous injections-in two the intercostal nerves were blocked, and in one the lum¬ bar sympathetic nerves were injected. In the remaining nine patients, the intercostal nerves were injected intraneurally under direct vision for the relief of postopera¬ tive pain prior to closure of thoracotomies. In most instances, the physician blamed himself for having in¬ advertently injected the anesthetic agent into the subarachnoid space through unusually long cuffs of dura. This is exemplified by the following statement from a recently published paper by Brittingham, Berlin, and Wolff *: "An almost unavoidable hazard of paravertebral nerve block is inadvertent injection of the agent used into the subarachnoid space. This probably occurs be¬ cause of an outward prolongation of the subarachnoid space or in other instances because of movement of the patient and tearing of the tissues after the needle is in place." In spite of this possibility, it is unlikely that durai cuffs extend 6 to 8 cm. past the intervertebral foramina; thus, direct injection into the subarachnoid space by this route is doubtful.5If long durai cuffs are not the route of entrance into the spinal subarachnoid space, what other rou...