34 AN INTRODUCTION TO ZOOLOGY. 



and to senH out messages of action (motor im- 

 pulses). It is the seat of sensation and the source 

 of movement : the organ of feeling and of will. Its 

 two functions are accomplished through nerves which 

 in the vertebrata can be distinguished as sensory 

 nerves (afferent nerves) and motor nerves (efferent 

 nerves).^ If we ask how far the terms feeling, and 

 will, and consciousness are applicable to forms of life 

 in which the nervous system is developed only in 

 rudimentj or, as in the unicellular and some other 

 organisms, not developed at all, we open up a wide 

 field for discussion. Many of our own actions, such 

 as winking the eyes and breathing, are performed in- 

 voluntarily and unconsciously, while vital processes, 

 such as the beating of the heart, are not only per- 

 formed involuntarily and unconsciously, but indepen- 

 dently of our control. Actions of this sort are called 

 reflex actions ; and it is believed that they may 

 possibly have come to be eventually performed in this 

 unconscious way by having been performed so very 

 often, not only by ourselves but by an untold number 

 of our ancestors. This may be supposed possible 

 when we consider that anything we do very often 



' Afi'erent (Latin, ad, to; and fero, carry), carrying in- 

 ward-tending, or centripetal impulses, because the sensory 

 impulses are carried from the outside to the centre of con- 

 sciousness. Efferent (Latin, ex, out of ; and fero), carrying 

 outward-tending, or centrifugal impulses, because impulses 

 which produce movement are carried from the brain outward 

 to the various movable parts of the body. 



