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 sensation: a, A peripheral organ for the reception of the im- 

 pression; b, a nerve for conducting it; c, a nerve center for feeling or per- 

 ceiving it. 



Sensations may be conveniently classed as, i, common, and 2, special 

 sensations. 



Common Sensations. Under this head fall all those general sen- 

 sations which cannot be distinctly localized in any particular part of the body, 

 such as fatigue, discomfort, faintness, satiety, nausea, together with hunger 

 and thirst, in which, in addition to a general discomfort, there is in many 

 persons a distinct sensation referred to the stomach or 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, such as the desire to defecate, to urinate, and in the female 

 the sensations which precede the expulsion of the fetus. It is impossible to 

 draw a very clear line of demarcation between many of the common sensa- 

 tions above mentioned and the sense of touch, which forms the connecting 

 link between the general and special sensations. Touch is classed with the 

 special senses, and will be considered in the same group with them; yet it 

 differs from them in its wide distribution over the body. Among common 

 sensations some would rank the muscle sense, which has been already 

 alluded to. It is by means of this sense that we become aware of the con- 

 dition 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 and of 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 con- 



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