674 THE NERVOUS SYSTEM 



and immediately following, there is a sharp dilatation of the blood vessels of 

 the arm, probably chiefly of the skin, as shown by the increase in volume. 

 The vessels remain dilated until the individual begins to awaken, then there 

 is a rapid constriction with decrease of volume of the organ. 



The dilatation of the general blood vessels draws off the supply of blood 

 from the brain, and the resulting partial anemia contributes to loss of con- 

 sciousness. The blood supply is ample for the growth, repair and rest of the 

 nervous system. How efficient this rest period is for the rejuvenation of the 

 nervous tissue is indicated even by the relatively gross means, figure 357, 

 shown in the histological preparations of nerve cells. 



Somnambulism and Dreams. What we term sleep occurs often in very 

 different degrees in different parts of the nervous system; and in reference to 

 some parts the expression cannot be used in the ordinary sense. For exam- 

 ple, during the most profound cerebral sleep the medulla is discharging 

 rhythmic nerve impulses to maintain respiratory movements. 



The phenomena of dreams and somnambulism are examples of differing 

 degrees of sleep in different parts of the cerebro-spinal nervous system. In 

 the former case the cerebrum is still partially active; but the activity is no 

 longer corrected by the reception, on the part of the sleeping sensorium, of 

 impressions of objects belonging to the outer world. Neither can the cere- 

 brum in this half-awake condition control the centers of reflex action of 

 the voluntary muscles so as to cause the latter to contract in close co- 

 ordination with the changing cerebral reactions as during waking hours 

 a fact within the painful experience of all who have suffered from 

 nightmare. 



In somnambulism the higher centers are capable of co-ordinating that 

 train of reflex nervous action which is necessary for progression, while the 

 nerve center of the muscular equilibrium sense (in the cerebellum) is, pre- 

 sumably, fully awake; but the sensorium is still asleep, and impressions made 

 on it are not sufficiently felt to rouse the cerebrum to a comparison of the 

 difference between mere ideas or memories and sensations derived from 

 external objects. 





