266 NERVE STIMULI. 



Apoplexy is caused by rupture of a blood tube and the 

 formation of a clot that presses on the brain. In meningitis 

 there is inflammation of the membranes immediately sur- 

 rounding the brain or spinal cord or both. 



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 it is supposed that part of the cerebro-spinal fluid is 

 pressed out into the spinal cavity, thus relieving the pressure 

 in the brain cavity. 



The gray matter is physiologically more active than the 

 white, and in keeping with this is the fact that the capillary 

 network is closer in the gray matter than in the white. This 

 is true of the spinal cord as well as of the brain. 



GENERAL CONSIDERATIONS CONCERNING THE NERVOUS 



SYSTEM. 



Nerve Stimuli. Natural nerve impulses that run out- 

 ward are ordinarily started by the action of some nerve cell 

 or cells, as from the gray matter of the brain or of the spinal 

 cord. 



Nerve impulses coming inward may be started in several 

 ways. Ordinarily by some one of a few forces that are capa- 

 ble of affecting the nerve endings. 



Mechanical force, as pressure, acts on the nerve endings of 

 the skin, and starts nerve impulses which are carried to the 

 brain and rouse certain cells to activity, and give us the sen- 

 sation of touch. The vibrations known as light excite the 

 special nerve endings in the retina,* but affect no other nerve 

 endings. Sound is appreciated only by the endings of the 

 auditory nerve. Certain gases or fine particles affect the 

 olfactory nerve endings, and certain substances may give 



