Spinal Anæsthesia or _Analgesia_, consists in the production of analgesia in the lower extremities and in the lower part of the trunk by the injection into the subarachnoid space of an anæsthetic drug which blocks the spinal nerves as they enter and leave the spinal cord. The cord ends at the lower border of the first lumbar vertebra and the subarachnoid space at the second sacral vertebra so that there is a considerable area into which the injection may be made without risk of injury to the cord. It is, in reality, a special variety of regional analgesia, the anæsthetic being injected into that part of the subarachnoid space which is occupied by the cauda equina. The subarachnoid space of the medulla spinalis contains the cerebro-spinal fluid and communicates above with the subarachnoid space inside the skull and through the foramen of Magendie, with the ventricular system of the brain. The subdural space of the medulla spinalis is merely a capillary interval. At the upper end of the cauda equina the nerve trunks of the two sides are separated by a median interval--containing only the filum terminale--which has been termed the cysterna terminalis. It is into this median space that the injection is made, in order to avoid wounding the nerve trunks and to procure equal diffusion of the anæsthetic to both sides of the middle line. If the injection is made among the nerve trunks on one side, a unilateral anæsthesia may result, the drug being prevented from diffusing freely to the other side by the presence of the numerous nerves. The ligamentum denticulatum forms an imperfect scalloped septum between the posterior and the anterior nerve roots, passing from the surface of the cord to the dura mater. The presence of this septum probably explains the fact that the motor nerves are not affected with the same constancy and to the same extent as the sensory roots. Technique. The drug which is most commonly employed in the Edinburgh school is Tropacocaine, and the results of its use with proper technique are eminently satisfactory. The dose of the drug for most purposes is ·07 gramme. Smaller doses are sometimes used but the larger dose gives more constant anæsthesia and appears to be well within the limits of safety. The dose is dissolved in 1 c.cm. of distilled water and sufficient sodium chloride added to make a solution isotonic with the cerebro-spinal fluid. A convenient method of obtaining the drug is in glass ampoules, each ampoule containing one dose, which has been carefully sterilised. [Illustration: FIG. 53.--Needle and syringe for spinal analgesia. Note the short oblique character of the point of the needle.] The syringe and needle employed are illustrated in Fig. 53.