THE PHYSIOLOGY OF AN ANIMAL 



213 



these impulses after reaching the centers are complex in the 

 extreme, and quite beyond our power to follow. They are ac- 

 companied by sensations and by whatever of consciousness 

 the animal possesses, and they 



control the life and motions of 

 the animal. The simplest of 

 these connections may produce 

 motion without any conscious- 

 ness on the part of the animal. 

 This is shown diagrammat- 

 ically in Figure 103. Some ex- 

 ternal stimulus excites one of 

 the sense organs in the skin, s, 

 and starts an impulse in the 

 nerve fiber, which then travels 

 quickly through the axon to its 

 inner end, c, in the cord. The 

 impulse then passes out of the 

 axon through the arborizations, 

 at c, to the neighboring den- 

 drites of other neurons. These 

 neurons may be motor cells, m, 

 by which is meant that their 

 axons, e t extend outward and 

 terminate in muscle fibers, as 

 at mu. Hence the impulse that 

 enters them, after passing out 

 over the axon, eventually 

 reaches a muscle fiber, and 

 causes the muscle to contract. 

 Such an action may take place 



without the impulse going to the brain, and would therefore 

 not involve any consciousness or any sensation, for these 

 latter functions occur in the brain only. Hence the animal 

 might move if touched by an irritating object, without any 



FlG. 103. A DIAGRAM ILLUSTRAT- 

 ING A REFLEX ACTION 



An impulse that starts from the sense 

 organs in the skin, s, passes to the spinal 

 cord through the afferent nerve, a. Upon 

 reaching the center, at c, the impulse may 

 pass over to the motor cell, m, from whence 

 it passes downward through the efferent 

 nerve, e, to the muscle fibers, mu. Part 

 of the impulse from the c may pass up 

 through the fiber, a, to the brain and pro- 

 duce sensation. 



