The spacer serves for limiting the depth of insertion of a shaft of a medical instrument into a body of a patient. The spacer has a distal abutment for bearing on a body of a patient. The spacer has a spacer element extending along a length section of the shaft of the medical instrument. A device for releasably mounting the spacer on the shaft has an aperture via which the spacer can be mounted laterally to the shaft. For it the aperture is designed as a lateral slit aperture.
