248 PHYSIOLOGY. 



carotid arteries, and the right and left vertebral arteries. 

 These arteries are so connected by cross-branches that 

 if any three of them should be compressed, or the blood 

 flow in them otherwise stopped, the fourth would still be 

 able to give the brain blood enough for its work. When 

 the brain is more active it receives a larger supply of 

 blood. During sleep it is paler. 



Fainting. If the supply of blood to the brain is shut 

 off, unconsciousness quickly follows. In the ordinary 

 faint the blood supply has been reduced, owing to the 

 diminution of the blood pressure or heart's force. It may 

 be due to inhibition of the heart from some emotion, or 

 bad odor, as in a close room ; severe pain may be the 

 cause ; a blow over the pit of the stomach may stop the 

 heart by reflex action. Fresh air should be supplied, 

 and the body laid flat on the back. This position makes 

 it easier for the blood to reach the brain and restore 

 consciousness. Smelling salts (or ammonia) may stimu- 

 late respiration and circulation. Sprinkling a little cold 

 water on the face may have the same effect, but it is 

 not necessary to pour a large quantity of water over the 

 person. Rubbing the limbs toward the heart promotes 

 the flow of blood, and tends to start the heart to activity. 



Apoplexy. Apoplexy is caused by rupture of a blood 

 tube and the formation of a clot that presses on the brain. 



Meningitis. Meningitis is an inflammation of the 

 membranes immediately surrounding the brain or spinal 

 cord or both. 



The Water Cushion of the Brain. Between the coats 

 surrounding the brain and spinal cord there is a layer of 

 liquid, comparable to that around the heart or lungs. 

 When an undue amount of blood is sent to the brain, 



