CHAPTER XV. 

 THE SENSES. 



THROUGH the medium of the nervous system man obtains a knowledge 

 of the existence both of the various parts of his body and of the external 

 world. This knowledge is based upon sensations resulting from the stimula- 

 tion of certain centers in the brain by nerve impulses conveyed to them by 

 afferent nerves. Under normal circumstances the following structures, 

 are necessary for the physical development and mental perception of a 

 sensation: a, A peripheral organ for the reception of the impression; b, a 

 nerve for conducting it; c, a nerve center for feeling or perceiving it. 



The senses may be conveniently classified according to the sensation which 

 is experienced. Each sense organ when stimulated is supposed to lead 

 to a sensation of distinctive character. Yet many of the sensations are 

 vaguely defined, such as thirst, fatigue, etc. Other sensations have a very 

 definite and readily identified quality such as sight, taste, etc. Sensations, 

 whether definite or vague, are referred by us to some source or origin either 

 in the body, i, the internal or so-called common sensations, or outside the 

 body, 2, the external or special senses. No sharp line can be drawn in 

 this classification. 



Internal Senses. Under this head fall all those senses which produce 

 sensations that are referred to an origin within the body, such as fatigue, 

 discomfort, faintness, satiety, nausea, together with hunger, thirst, the muscle 

 sense, and pain. In hunger and thirst there is a general bodily discomfort, 

 but in many persons also a distinct sensation referred to the stomach or to 

 the fauces. In this class must also be placed the various stimulations of 

 the mucous membrane of the bronchi, which give rise to coughing, and also the 

 sensations derived from various viscera. It is by means of the muscle sense 

 that we become aware of the condition of the muscles, and thus obtain the 

 information necessary for their adjustment to various purposes standing, 

 walking, grasping, etc. This muscular sensibility is shown in our power to 

 estimate the differences between weights by the different muscular efforts 

 necessary to raise them. It must be carefully distinguished from the sense 

 of contact or pressure, of which the skin is the organ. When standing erect, 

 we can feel the ground contact, and there is a sense of pressure, due to our 

 feet being pressed against the ground by the weight of the body. Both 

 these are derived from the skin of the sole of the foot. If now we raise the 

 body on the toes, we are conscious, through the muscular sense, of a muscular 



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