852 CENTRAL NERVOUS SYSTEM 



lack or 30 per cent of C0 2 . The significance of the difference in sensi- 

 tivity of the spinal and bulbar centers to changes in the composition 

 or flow of the blood lies in the fact that stimulation of the latter tends 

 to bring about automatically reactions which restore the blood to its 

 proper condition. As a result the spinal centers are rarely confronted 

 in healthy life with a circulatory condition which might modify their 

 activity. 



After all activity of the central nervous system has been suppressed by 

 anemia, or asphyxia, complete recovery may result if the circulation is 

 restored to a normal condition soon enough. The respiratory reflexes are 

 the first to reappear, then the excitability of spinal reflexes is regained, 

 and finally cerebral function is restored. Less prompt recovery occurs 

 if the circulation remains inadequate for a longer period, manifesting 

 itself chiefly in a failure of the cerebrum to regain its normal function 

 completely. A pregnant cat, which had been subjected to anemia of 

 the brain and upper cord for 10 minutes, regained her ability to walk, 

 to clean her paws, and to lap up water or milk, but her movements were 

 poorly controlled. On the twelfth day she gave birth to kittens. To these 

 she paid no attention unless one of them came in contact with her 

 nose when she would lick it with her tongue. She allowed the kittens 

 to suckle and fondled them with her paws when nursing very much as 

 a normal cat would do, but if one of them wandered away she would 

 make no attempt to bring it back. The picture was one of the automatic, 

 reflex aspects of motherhood deprived of the discriminative attributes 

 which depended on cerebral processes. 



The central nervous system exhibits a difference in the nutritive re- 

 quirements of its different parts. It is interesting to see how long dif- 

 ferent groups of nerve cells will resist complete anemia without losing 

 their ability to revive. Considerable variations occur in different ani- 

 mals, but the following figures may be taken as typical. They represent 

 the time beyond which anemia cannot be extended without producing 

 changes in the nerve cells which cannot be recovered from. 



Cerebrum, small pyramidal cells 8 minutes 



Cerebellum, Purkinje cells 13 minutes 



Medullary centers 20-30 minutes 



Spinal Cord 45-60 minutes 



Sympathetic ganglia 3-3% hours 



Myenteric plexus 7-8 hours 



The great susceptibility of the cerebral cells explains the ease with 

 which consciousness is lost as the result of circulatory failure or asyphyxia. 

 A prolonged condition of low blood pressure may result in the failure 

 of the medullary centers because of the insufficiency of the blood supply. 



