278 STUDIES IN PHYSIOLOGY 



of telegraph wires of a great city are simplicity itself when 

 compared with the fibers of the human brain. 



Sensory and Motor Cells and Fibers. If we divide the cells 

 of the gray matter according to their functions, we shall find 

 at least two distinct classes. At the end of the afferent 

 fibers are the cells of the first class. They receive the mes- 

 sages that result in sensations, and are therefore called sen- 

 sory cells. The afferent fibers, too, are commonly known as 

 sensory nerves, because they bring in -these messages. 



In the second class are the cells from which originate the 

 efferent fibers. The function of these cells is that of dis- 

 patching the messages which control the work of the muscles 

 and other organs. To these cells and to their fibers is 

 therefore given the name motor. 



The Cranial Nerves. Twelve pairs of cranial nerves arise 

 from the base of the human brain. They leave the cranium 

 through holes in the base of the skull, and are distributed 

 to the muscles, skin, and sense organs of the head. The first 

 ten pairs correspond more or less closely to the ten cranial 

 nerves of the frog; true eleventh and tivelfth nerves are 

 wanting in the amphibia. In the table on the following 

 page will be found a statement of the origin, distribution, 

 and function of each of these twelve pairs. 



7. PHYSIOLOGY OF THE BRAIN 



The principal functions of the brain may for convenience 

 be divided into (1) reflex activities, (2) conscious activities, 

 and (3) habits or automatic activities. 



Reflex Activities. The machinery of reflex action through 

 the spinal cord has already been explained. Similar reflex 

 processes constitute one of the most important functions of 

 the brain. Suppose I inhale some pepper. A message goes 

 up the first nerves to the cells in my brain. This mes- 

 sage is then reflected or switched off to cells which send 

 impulses down the nerves which control the muscles of my 



