224 CENTRAL NERVOUS SYSTEM 



A general rise of arterial pressure causes the blood to flow 

 more rapidly in the cerebral vessels, raises the venous pressure 

 and also the intracranial pressure. To some extent, the latter 

 is compensated for by a flow of cerebrospinal fluid through the 

 foramen magnum into the vertebral canal. If the pressure 

 continues to rise, the distention of the brain shuts off that 

 outlet. Increase of pressure from now on impedes the circu- 

 lation through the brain. It follows, therefore, that in patho- 

 logical cases where trephining of the skull results beneficially 

 the explanation may lie in a better blood flow. 



The metabolism of the central nervous system is not very 

 intense. This is readily understood when it is known that the 

 cell bodies equal only 2 per cent, of the entire mass of the brain. 

 The relative metabolism is less than that of muscles. During 

 mental activity blood passes from the limbs to the head, as 

 shown in cases where a defect of the cranial wall exists. During 

 fatigue the brain becomes anemic, coinciding with a decrease 

 in the force of the heart beat and of the tone of the abdominal 

 vessels. 



Sleep. This phenomenon is one of many instances of the 

 rhythmic activities of the central nervous system. From time 

 to time all animals with a well-developed nervous system go 

 to sleep, during which psychical activity is at its lowest point. 

 To reach this condition, the most important favoring factor is 

 an exclusion of all or most of the impulses from the central 

 nervous system. In a well-known case of Striimpell, in which, 

 from a complicated anesthesia, all sensory impulses were limited 

 in their entrance to a single eye and a single ear, the patient 

 could be put to sleep at will by closing the eye and stopping 

 the ear. In addition, sleep has been attributed to the following 

 influences : 



1. Chemical influences. 



2. Circulatory influences. 



3. Histological influences. 



Those who hold to chemical influences in the production of 

 sleep, maintain that during normal activity of the body various 

 substances are formed which are circulated in the blood and 

 directly lessen tht activity of the nerve cells or indirectly dimin- 

 ish the supply of blood in the brain. In the theories of circu- 

 latory influences a fatigue of the vasomotor centre is looked upon 



