something of their normal vitality, so that if at all freely detached from their blood supply they were liable to slough or die. It matters not for our present purpose how far this was due to imperfect sepsis or how far it resulted from the irritant action of chemicals used to prevent the more severe infections: the fact remains that five and twenty years ago most wounds did present some such change and that therefore we could not be certain when we operated that the injury inflicted would be absolutely located to the tissues mechanically divided. To-day this is no longer the case. To-day we know that the injury done by the knife is limited to the track of the knife itself and that it will be repaired with a rapidity so great that after a very few hours the most extensive dissections have left no obvious trace.
It is probably to this fact above all others that we owe the comparatvely recent extension of surgical dissection. The surgeon of to-day must avoid the cutting or bruising of structures essential to life. There are tracts in the brain and in the spinal cord which he must not divide: there are a few—a very few—great blood-vessels which he must not tie or cut. But with these reservations he finds no barrier to his work. He can safely open, divide and restore the great canals and ducts of the body. He can freely expose all its cavities and work in their interior with calm and confidence. He is not limited to a short direct incision giving an imperfect view of what lies beneath; he can fully reveal the parts he has to attack. Imagine to yourselves the study of a picture covered by a sheet, if, on the one hand you must be satisfied to look through a hole in the covering, or on the other hand you can raise or draw aside that covering, exposing the entire picture to view, and then replacing it with accuracy and without injury. It is by raising the covering sheet that we now commence most operations of importance. The tissues of the body
