SUMMARY, MAMMALIAN NERVOUS SYSTEM; SPINAL SHOCK 969 



tone of the extensor muscles holds the resting limbs in slight extension. 

 Responses comparable to the extensor thrust and the mark time reflex 

 of the spinal animal occur. 



The Cause of Spinal Shock 



It would seem natural to suppose that the cause of the depression in 

 reflex activity which follows transection of the spinal cord, and which 

 is known as spinal shock, is the irritation set up directly in the tissues 

 injured by the lesion. This irritation might be thought to have an in- 

 hibitory influence on spinal reflexes. That this supposition is incorrect 

 is shown by the fact that after the shock induced by a cervical tran- 

 section of the cord has worn off, a second transection at a lower level 

 does not cause its reappearance. The abnormal character of reflexes 

 recovering from spinal shock does not resemble that of reflexes which 

 are being inhibited, but rather those which have experienced fatigue. 



Spinal shock might be thought to be due to the disturbance in the cir- 

 culation which follows the spinal transection, but this cannot be the case 

 because all parts of the body would suffer alike from the fall in blood 

 pressure, whereas only those parts of the nervous system below the lesion 

 exhibit shock. This aboral incidence of shock is a very striking character. 

 So slight is the effect of a spinal transection upon the higher centers that 

 men who have been shot through the spine may not even lose conscious- 

 ness. They are aware that sensations from the lower limbs have sud- 

 denly been cut off, and their first impression is that they have been blown 

 in two. Sherrington has described a monkey the cord of which was 

 cut below the cervical region, and which immediately after the opera- 

 tion amused itself by catching flies with the anterior extremities, whereas 

 the posterior extremities were in a condition of the profoundest shock. 



The most probable explanation of shock, which is in accord with the 

 preceding facts, is that it is due to cutting off from the spinal reflex 

 arcs of some influence normally exerted by fibers descending from the 

 brain. Just what this influence is, or how it facilitates reflex activity 

 is impossible to state, but it would appear that once this influence is .cut 

 off, some time is required before the cord can acquire the power of 

 carrying out its reflex functions without its assistance. We have seen 

 that a parallelism exists between the depth and persistence of spinal 

 shock and the development of the nervous system, particularly of the 

 brain, in vertebrate evolution. This fact consequently supports the 

 view that spinal shock is due to isolating the cord from the influence 

 of the higher centers. Moreover those neuromuscular mechanisms are 

 affected which are normally under control of the brain centers. Dis- 

 turbances in the visceral activities controlled by the automatic nerv- 

 ous system are insignificant except in the case of the discharges of 



