Among the powerful antiseptic agents in common use are the soluble preparations of _mercury_, ordinarily _corrosive sublimate_, in solutions of varying strength, which are used for irrigation, douching, etc., and for preparation of dressings. An intense eczema may follow its local use, and symptoms of mercurial poisoning may appear in individuals of peculiar susceptibility to this drug. Salivation, intestinal irritation, and other phenomena of mercurial poisoning have been produced, with the result that the solutions and preparations of corrosive sublimate are much weaker than those which were used at first. The drug eczema produced by _corrosive sublimate_ interferes with one of the essentials of ideal wound healing--_i. e._, physiological rest. The area involved should be protected with a sterilized powder or by anointing it with sterilized ointment. PART III. SURGICAL PRINCIPLES, METHODS AND MINOR PROCEDURES. CHAPTER XVIII. DISTURBANCES OF BLOOD PRESSURE; SHOCK AND COLLAPSE. The maintenance of the normal pressure of the blood is a material factor in the welfare of surgical cases. Deviations in the direction of lowered pressure constitute the most important features of _shock_ and _collapse_. Prevention of loss of blood is but one of several complex indications in prophylaxis and after-treatment. Blood pressure is maintained in large part by the vasomotor system of nerves, whose prime centre is in the gray matter of the fourth ventricle, with subsidiary centres in the spinal cord and great ganglia. Stimulation of these centres causes contraction of the peripheral arterioles and increases intravascular pressure. If, however, it be long-continued or excessive, these centres become exhausted, vasomotor paralysis results, the arterioles dilate, and pressure is lowered. Three factors coöperate to maintain this pressure: 1. Regular and normal rhythmic heart action; 2. Normal vascular contractility; 3. Normal quantity of suitable fluid in the vascular system. Departure from the normal in any one of these factors causes perceptible disturbance, but when in all three of them it may prove fatal. Whether this be caused by emotion, accidental injury, or deliberate operation is of slight concern, as the effect is essentially the same. The pulse will usually tell its own story to the experienced observer, but scientific accuracy in measuring blood pressure can only be obtained by certain instruments of precision, such as the _tonometer_ or the _sphygmomanometer_, consisting of an air-containing armlet which encircles the arm, a bulb by which the pressure of air can be regulated, and the whole connected with a manometric gauge and mercury tube. These instruments can be procured of the dealers, and their employment during an operation gives the operator a continuous record of the blood pressure, by which he may judge at any moment of the degree of shock.