THE NERVOUS SYSTEM 231 



tion of the spinal cord within the skull has been previously 

 spoken of as the medulla oblongata. It resembles the cord, in 

 being composed of both -white and gray matter, and in con- 

 ducting sensory and motor influences. It likewise gives rise 

 to certain nerves, which are here called cranial nerves (from 

 cranium, the skull). All except two of these important nerves 

 spring from the medulla, or the parts immediately adjoining 

 it ; the exceptions are the two nerves taking part in the special 

 senses of sight and smell, which nerves have their origin at 

 the base of the cerebrum. 



48. The decussation, or crossing of the motor columns, has 

 been previously described, when treating of the direction of 

 the nerve-fibres of the cord; and the singular fact has been 

 alluded to, that when one side of the brain is injured, its 

 effects are limited to the opposite side of the body. One more 

 fact remains to be observed in this connection, namely, this 

 cross action does not usually take place in the cranial nerves. 

 Accordingly, when apoplexy, or the rupture of a blood-vessel, 

 occurs in the right hemisphere of the cerebrum, the left side 

 of the body is paralyzed, but the right side of the face is 

 affected ; this is because that part of the body is supplied by 

 the cranial nerves. 



49. A portion of the medulla presides over the important 

 function of respiration, and from it arises the pneumogastric 

 nerve, so called because its branches serve both the lungs and 

 stomach. The feelings of hunger, thirst, and the desire for 

 air are aroused by means of this nerve. The wounding of the 

 gray matter of the medulla, even of a small portion of it, 

 near the origin of the pneumogastric nerve, at once stops the 

 action of the lungs and causes death. In consequence of the 

 importance of this part, it has been termed the " vital knot." 

 We find, also, that its location within the skull is exceedingly 

 well protected, it being quite beyond the reach of any ordinary 

 form of harm from without. 



43. What final fact is observed in the crossing of the motor columns ? 



49. The pneumogastric nerve ? The feelings aroused by it ? The " vital knot ? " 



