THE CEREBRUM 613 



the cutaneous and splanchnic vessels and the quantity of blood flowing into 

 the brain is increased. Thus in an indirect way the vaso-motor center, by 

 bringing about a rise or a fall in the general arterial pressure, regulates the 

 blood-supply to the brain, and controls its amount in accordance with its 

 needs. 



Brain Activity. Brain activity is characterized by an active conscious- 

 ness, the development of sensations, ideas, feelings, and the exercise of 

 volitional power (which manifests in muscle movement) and is the result of a 

 physiologic condition of the body at large. For the manifestation of brain 

 activity it is essential that the irritability of the brain cells and more especially 

 of those composing in large measure the cerebral cortex be maintained at a 

 normal physiologic level, so that they may respond in the manner peculiar 

 to them to the action of nerve impulses transmitted through afferent nerves 

 from all regions of the body. Here as elsewhere throughout the body, the 

 irritability depends on, and is maintained by, the presence of blood flowing 

 into and out of the brain in varying quantity from moment to moment, with 

 a given velocity and under a definite pressure. So long as these conditions 

 are maintained in the strictly physiologic condition, so long will the brain 

 respond to stimuli by the development of sensations. The avenues through 

 which nerve impulses pass to the cortical cells are those beginning in the 

 special and general sense organs of the body in contact with the external 

 world, viz.: the eyes, ears, nose, tongue, and skin. The maintenance of 

 these structures in a strictly physiologic condition is also one of the essential 

 conditions for brain activity. 



Judging from the changes in the character and composition of the blood 

 which occur during its passage through the brain capillaries, there is coin- 

 cidently with brain activity an active metabolism, which eventuates, at the 

 end of a variable number of hours, in the decline of the irritability, a reduc- 

 tion of functional activity, and the establishment of the condition of fatigue. 

 The irritability of the sense organs, especially of the eyes and ears, in all 

 probability declines in a similar manner. These structures pass into the 

 condition of fatigue and become less responsive to external stimuli. The 

 results of all these conditions is a less active stimulation of the brain cells, 

 which in connection with other factors predisposes to 



Brain Repose or Sleep. Brain repose or sleep is characterized by a 

 greater or less degree of unconsciousness, the non-development of sensations, 

 ideas, feelings and volitional acts, and is the result of a diminution in the 

 physiologic activities of the body at large and more especially of the brain, 

 sense organs, and spinal cord. Coincident with the cessation of brain activity 

 and the onset of sleep, there is a diminution in the rate and force of the heart- 

 beat, and in the frequency and depth of the respiratory movements, and a 

 relaxation of the skeletal muscles, especially those employed in voluntary 

 movements. 



The sense-organs are in part protected from the action of external stimuli. 

 The eyeball is so turned that its anterior pole is directed far upward under 

 the eyelid, while the pupil is markedly diminished in size, and in consequence 

 the entrance of light largely prevented. The ear is protected against the 

 reception of sounds of ordinary pitch by an increased tension of the tympanic 

 membrane. The nose and mouth are less responsive to various stimuli 

 because of the dryness of their mucous membranes from diminished secre- 



