590 SIGNIFICANCE OF THE NERVOUS SYSTEM 



relaxation and if movements are made, they are feeble and irregular. The 

 face is pale and drawn, the pupils dilated, sweating is often profuse, the reflexes are 

 slight, consciousness is usually present but there is a diminished sensibility and 

 mental activity. The respirations are feeble, irregular and sighing. The pulse is 

 small, frequent and dicrotic, owing to the low blood pressure. The skin is cold and 

 the temperature subnormal. The theories which have been brought forth in ex- 

 planation of this phenomenon, may be grouped as follows : 



(a) Exhaustion of the vasomotor mechanism by a depression of the activities 

 of the center. This theory is not satisfactory because while shock commonly in- 

 duces a fall in blood pressure, the vasomotor center is not exhausted and the blood- 

 vessels may be constricted ; moreover, the heart is not seriously affected, although 

 its output is small. 



(b) Acapnia, or deficiency in CO2, removes a most important stimulus from 

 different nerve centers. The breathing becomes shallow and occasional, the blood- 

 pressure falls and the heart beats more quickly. The objections to this theory are 

 many, chief among which is that shock should then be prevented by artificially 

 supplying CO 2, which is not the case. 



((f) Oligemia, or too little blood, acts by reducing the blood pressure, but allows 

 the cardiac and vasomotor centers to continue their activities. Gravity shock may 

 be classified under this heading, because it is caused by a stagnation of blood in the 

 splanchnic blood-vessels and consequent inadequate filling of the heart. Thus, 

 when a rabbit with a large pendulous abdomen is held vertically with the head up 

 for any length of time, it frequently passes into the condition of shock and may die 

 within 20 to 30 minutes. 



{d) Exhaustion of adrenin, brought about by an initial outpouring of excessive 

 amounts of adrenin in consequence of sensory stimulation. This finally leads 

 to its exhaustion. 



(e) Inhibition of the activity of the nuclei of the spinal cord and midbrain.^ 

 When such an inhibition takes place, the function of the cord is greatly diminished. 

 Consequently, its constituent nuclei cease sending out those impulses which main- 

 tain the tonus of the muscles. The blood pressure falls. Even the respiratory 

 center shares in the paralysis. Eventually a venous engorgement is obtained 

 which makes a proper filling of the heart and arterial channels impossible. Spinal 

 shock, however, possesses a rather local character, because it affects only those 

 parts of the body which lie below the seat of the spinal lesion. Under this heading 

 may be classified the so-called nervous shock or shell shock, as well as the shock 

 accompanying overdoses of anesthetics. In the latter case, however, the reflexes 

 reappear after the discontinuance of the narcosis, while in surgical shock they do 

 not. 



Inhibition in Consequence of the Lessening of the Irritability of the Nervous 

 System. — This condition results during sleep and narcosis. The reflexes which are 

 abolished first, are the abdominal, cremasteric and patellar, while those from the 

 sole of the foot and from the nasal mucosa are more resistant. The reflexes which 

 disappear last are the corneal and retinal. For this reason, sleep and narcosis 

 may be employed as a means to determine whether or no an action is a true reflex, 

 because if it persists during these states of cerebral depression, it must be non- 

 volitional. In infants and children this weakening of the reflexes is less evident 

 than in adults. It need scarcely be mentioned that the intensity of the reflexes 

 may be made to serve as an index of the depth of the narcosis. The reactions 

 usually employed for this purpose are the corneal and pupillar (light) reflexes, the 

 danger line being reached when a mechanical impact upon the cornea very nearly 

 fails to elicit a contraction of the muse, orbicularis palpebrarum and when imme- 

 diately thereafter the pupils become constricted. A weakening and final inhibition 

 of the reflexes also results in coma and depressions of the nervous system resulting 



iPike, Am. Jour, of Physiol., xxx, 1912, 436, and Porter and Muhlberg, ibid., 

 iv, 1900, 334. 



