234 Physiology. 



nervous than of muscular origin. When one is completely 

 "tired out," as he would say, if his mind can be aroused, 

 as by some excitement, he will be found able to expend a 

 good deal more muscular energy. So, too, many persons 

 of slight muscular build, but of great "will power," are 

 able to do more work with the muscles than others with 

 larger muscles and less will. During fatigue the cell 

 bodies are found to decrease in size, but there is no per- 

 ceptible change in nerve fibers as a result of fatigue. 



Blood Supply of the Brain. Blood is supplied to the 

 brain through four arteries : the right and left internal 

 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. 



Cause of Fainting. If the supply of blood to the brain 

 is shut off, unconsciousness quickly follows. In the ordi- 

 nary faint the blood supply to the brain has been reduced. 

 It is due to checking the action 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. 



Apoplexy. Apoplexy is caused by rupture of a blood 

 tube and 1 the formation of a clot that presses on the brain. 



Meningitis. Meningitis is an inflammation of the mem- 

 branes immediately surrounding the brain or spinal cord 

 or both. 



