THE REGULATING MECHANISM 193 



the other side. The grey matter in the pons 

 and bulb gives origin to fibres which run 

 into the great cranial nerves, analogous 

 to, but much modified from, the pairs of 

 spinal nerves. We may shortly indicate 

 the functions of such nerves. Some of the 

 cranial nerves are entirely motor, convey- 

 ing impulses to the muscles of the face on 

 the opposite side, such as the seventh nerve, 

 that innervates the muscles of expression ; 

 others are entirely sensory, such as the optic 

 and the auditory ; while a third class are 

 sensori-motor, containing both sensory and 

 motor fibres, such as the fifth, the sensory 

 nerve of the face, but which also contains 

 motor fibres for the muscles of the tongue. 



109. The bulb or medulla contains centres 

 connected with respiration, the action of 

 the heart, and the blood vessels. The latter 

 is the vaso-motor centre already referred 

 to. In the bulb also originate the roots of 

 some of the cranial nerves. Passing through 

 it we also find the motor and sensory 

 paths connecting the brain with the spinal 

 system of nerves. It is important to notice 



