CHAPTER XIX. 



THE SENSES. 



THROUGH the medium of the Nervous system the mind obtains a 

 knowledge of the existence both of the various parts of the body, and of 

 the external world. This knowledge is based upon sensations resulting 

 from the stimulation of certain centres in the brain, by irritations con- 

 veyed to them by afferent (sensory) nerves. Under normal circumstances, 

 the following structures are necessary for sensation: (a) A peripheral 

 organ for the reception of the impression; (b) a nerve for conducting it; 

 (c) a nerve-centre for feeling or perceiving it. 



Classification of Sensations. Sensations may be conveniently 

 classed as (1) common, and (2) special. 



(1.) 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, 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 irritations of the mucous mem- 

 brane of the bronchi, which give rise to coughing, and also the sensations 

 derived from various viscera indicating the necessity of expelling their 

 contents; e.g., the desire to defsecate, to urinate, and, in the female, the 

 sensations which precede the expulsion of the f oatus. We must also include 

 such sensations as itching, creeping, tickling, tingling, burning, aching, 

 etc. , some of which come under the head of pain: they will be again referred 

 to in describing the sense of Touch. It is impossible to draw a very clear 

 line of demarcation between many of the common sensations above men- 

 tioned, and the sense of Touch, whicn forms the connecting link between 

 the general and special sensations. Touch is, indeed, usually classed with 

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

 yet it differs from them in being common to many nerves, e.g., all the 

 sensory spinal nerves, the vagus, glosso-pharyngeal, and fifth cerebral 

 nerves, and in its impressions being communicable through many organs. 

 Among common sensations must also be ranked the muscular sense, 

 which has been already alluded to. It is by means of this sense that we 

 become aware of the condition of contraction or relaxation of the various 

 muscles and groups of muscles, and thus obtain the information necessary 



