SHOCK 311 



that participated in the fatigue can still be called into activity by afferent 

 stimuli transmitted to its nerve cell through other sensory pathways 

 (see page 825). It is thought that in shock the frequent afferent stimula- 

 tion produces synaptic fatigue and therefore dulls the sensory responses 

 of the animal. The researches of Mann, in which he shows that shock 

 may occur without any demonstrable afferent stimuli in the brain stem, 

 would seem, however, to negative the above hypothesis. 



The raised threshold of sensory stimulation is no doubt an effect of the 

 low blood pressure. It has been shown, for example, by E. L. Porter 36 

 that when the arterial blood pressure is maintained at a uniform level, 

 the threshold stimulus for spinal cord reflexes remains practically uni- 

 form, but becomes promptly increased when the arterial blood pressure 

 is made to fall. Why a lower blood pressure should have this effect is, 

 however, difficult to understand in the light of the researches of Stewart 

 and his coworkers, who, as remarked above, found that the cells of the 

 central nervous system may endure total anemia for many minutes and 

 still recover their physiological condition. It may be, however, that the 

 low blood pressure affects the conductivity of the synapsis. 



The muscular weakness is probably also dependent on low blood 

 pressure, for it has been found in animals that, when the arterial blood 

 pressure is lowered to about 90 mm. Hg, the muscles contract much less 

 efficiently than ordinarily. The fall in body temperature is dependent 

 upon the muscular inefficiency. 



In conclusion, it should be pointed out that W. T. Porter, in the inves- 

 tigation of acute shock met with at the front, has found that, in many 

 cases at least, the circulatory disturbance is due to a condition of fat 

 embolism. The fat is derived from the marrow of long bones, such as 

 the femur, by injuries which smash the bones. Porter's observations 

 are at least very suggestive. In any case that fat embolism may be a con- 

 tributory factor is made probable by the fact that fat emboli have been 

 observed by Mott in the medulla and the cortex of the brain. 



The Treatment and Prognosis of Shack 



It remains for us to show that the foregoing conclusions, drawn from 

 observations made on laboratory animals, are applicable to the clinical 

 condition known as surgical shock. It will then be advantageous to con- 

 sider the principles which determine successful treatment. The unusual 

 opportunity afforded at the front to study shock has led to a furtherance 

 of our knowledge of its causes, which might have taken many years of 

 investigation in time of peace, and by far the most important contribu- 

 tions have come from those who have been intimately familiar with the 

 experimental as well as the clinical aspect of the problem. N. M. Keith 39 



