216 SPECIAL PHYSIOLOGY 



cord messages or impressions of stimuli. There is no conscious 

 sensation in the cord. Motor centers in the cord may send out 

 impulses to muscles, thus producing the reflex responses. 



If the brain is intact the impression travels through the cord to 

 the sensorium, where it becomes a sensation. In the mean time the 

 cord may have returned motor impulses of reflex action. These 

 motor impulses pass away from the central nervous system and 

 therefore never reach the sensorium never give rise to sensations. 

 The efferent reflex impulses cause action of end-organs e. g., 

 muscles. One is unconscious of the impulse, but he is conscious of 

 the action through sensory impressions coming to the brain from the 

 organs in activity. The relation between sensation and reflex action 

 has been set forth. 



The relation between sensation and voluntary action is somewhat 

 less direct. When an animal takes food into the digestive tract it 

 is in response to the sensation of hunger. When he seeks shelter it 

 is in response to sensations of uncomfortable exposure. These are 

 direct voluntary response to sensation. 



When one prepares food for a future meal or builds a shelter, he 

 anticipates the coming sensation of hunger or exposure and forestalls 

 it. Here we have an intervention of reason or of instinct, inducing a 

 series of voluntary actions. 



Voluntary action is, then, either directly or indirectly dependent 

 upon sensation. 



Finally, sensation is the source not only of all activity, but of all 

 knowledge. Its importance in any study of the nervous system then 

 becomes paramount. 



Besides the auto-objective sensations hunger, thirst, suffocation, 

 fatigue, pain, shivering, and tickling there are the distinctively 

 objective sensations: touch, posture, temperature, smell, taste, hearing, 

 and vision. 



1. Appliances. Dividers; millimetre rule; two beakers; two 20d 

 spikes, and towel. 



2. Observations, a. Tactile Sensation. (1) To test the acuteness 

 of the tactile sense as well as the power of localization. Take a pair 

 of dividers whose points may be approximated to a millimetre or 

 less. Let the subject of the observations be blindfolded. Apply the 

 points to the tip of the ring finger so as to bring the line between 

 the two points transverse to the axis of the finger. Press the points 

 gently and draw them over the skin for a distance of 1 mm. If the 

 subject feels two points, bring them nearer together and repeat the 

 experiment; presently the points will have been brought so near 

 together that they can no longer be distinguished as two, but the 

 sensations are merged into one. The crucial point has been passed. 

 The greater the acuteness of tactile sense, the nearer the points may 

 be brought together and yet be felt as two points. What is the limit 



