We may condense the results of much work on this subject under the following headings:-- (_a_) Most stimuli from the field of operation cause a sharp rise of blood pressure, followed by a sharp fall. (_b_) Successive stimuli delivered quickly one after another add their effects together, the total result being considerably greater than from one severe trauma. (_c_) After a time, the pressor effect of stimuli begins to lessen: the animal or patient “wears out,” and finally no pressor result can be obtained by the most massive stimulation: the curve of B.P. steadily falls: the condition of full surgical shock is produced. (_d_) The tearing or pulling of tissues produces more powerful stimuli than the use of a sharp knife, and, therefore, brings on the full condition of shock more rapidly. (_e_) Stimuli from some tissues cause much more reflex effect upon the organism than from other less sensitive structures. This is well exemplified when an abdominal section is in progress. Incision of skin causes immediate response in deepened respiration and higher B.P.: division of the fascia very little effect. If the muscle is divided by the knife, again little reflex effect is noticeable, but if it be stretched and split by the fingers, the response is powerful. The parietal peritoneum, however delicately handled, is one of the most sensitive structures in the body, and, unless the patient is fully under at the stage either of opening or closing this layer, actual breath-holding or straining will occur. On the other hand, incision or suture of the hollow viscera will cause practically no response however light the anæsthesia, provided these structures, and their connections with the parietes, are not pulled upon. (_f_) Stimulation of certain selected areas, of which the spermatic cord is a well-known but by no means the only example, results in an almost immediate fall of blood pressure with little or no preliminary rise. In the operating theatre, we sometimes see faintness or syncope arising quite suddenly during operations in such regions. This subject is explained more fully in Chapter XVI. under the term “Reflex Syncope.” Illustration: FIG. 2.--Combined blood pressure chart showing the average of a number of experiments.--A--Under nitrous oxide and oxygen. B--Under ether. At each spot marked x, a trauma (burning of the paw) was inflicted. (After CRILE.)] (_g_) While no general anæsthetic protects absolutely from shock stimuli, some anæsthetics give more protection than others. Nitrous oxide is the most effective in this respect, its powers being two and a half times greater than that of ether: chloroform is even less effective than ether (_see_ Fig. 2).