CHAPTER XXIII 

 THE SPECIAL SENSES 



GENERAL SENSATION 



In studying the structure and functions of the nerve system, 

 we learn that sensory stimuli are received in every part of the body 

 by afferent nerves, and conducted to sensory cells in the spinal 

 cord; there they either evoke a muscle response of reflex character, 

 or are transmitted by connecting tracts to the brain, where the 

 result is conscious sensation of some sort: as, for example, of tem- 

 perature whether of the surrounding air, or of bodies which we 

 touch; or of other conditions whether an object is hard or soft, 

 wet or dry, rough or smooth, etc., etc. These are common and 

 definite sensations of external things and by these external sensa- 

 tions we gain knowledge of the world about us. 1 



Others there are which are definable in general terms only, 

 and are not definitely located, although plainly felt. For in- 

 stance, we are hungry, or thirsty, or tired; after pain we have a 

 sense of relief, etc., the route for stimulus and response in these 

 matters is through visceral and vaso-motor nerves and their 

 spinal and cerebral connections, and by these internal sensations 

 we gain acquaintance with our individual selves. For example, 

 hunger is the recognition of a lack of food in the tissues; thirst, 

 of a lack of fluid. Fatigue is the sensation caused by over- 

 loading the tissues with waste products of metabolism (fatigue 

 poisons) . 



Other mechanisms in the body are adapted to a more definite 

 class of sensations, by which we learn to know still more exten- 

 sively, the world in which we live; these are called the organs of 

 the special senses. 



1 We do not'now refer to cranial nerves in which the arrangement is similar but 

 more intricate. 



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